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FHWA Home / Safety / Roadway Departure / Frequently Asked Questions: Barriers, Terminals, Transitions, Attenuators, and Bridge Railings
FHWA barrier guidance is contained in the AASHTO Roadside Design Guide. However, FHWA field offices often raise numerous issues that involve interpretations, extrapolations, device selection, hardware deployment, or simply trying to fit safety devices into real world conditions.
These questions and answers offer clarification on the use of roadside hardware for issues not covered by FHWA policy or topics that simply need additional explanation. They are the considered opinions of engineers in the FHWA Office of Safety Design and the FHWA Resource Center with helpful input from members of the American Traffic Safety Services Association's Guardrail Committee.
In general the questions relate to rigid and semi-rigid barrier systems. The Office of Safety's July 20, 2007, memorandum on Cable Barrier Considerations dealt with numerous issues of cable barrier design, selection, and placement. Additional guidance on cable barrier selection and placement on sloping terrains and adjacent to median ditches will be provided in conjunction with National Cooperative Highway Research Project 22-25, scheduled for completion in early 2010. A similar project (NCHRP 20-7(257)) synthesizing information on portable concrete barrier shapes, connections, anchorages, and other considerations will also be completed soon.
As noted at the end of the FAQ list, we expect to develop additional guidance in this format. Please contact Mr. Nicholas Artimovich at nick.artimovich@dot.gov if you have a special need for guidance in any of those areas, or to suggest others.
Is it OK to use Weathering Steel (sometimes called Cor-Ten, A-588, or Rusting Steel) in longitudinal barriers?
A. No, the use of weathering steel guardrail should be limited. Where aesthetic concerns are primary, weathering steel guardrail may be used if the owner agency adopts a frequent periodic inspection and replacement schedule.
Roadside barriers and bridge rails are usually close enough to the travelled way that they can be sprayed with water from passing traffic. In most parts of the country this water contains deicing chemicals during winter months. In seaside locations in warmer climates the salt laden air deposits corrosive chemicals on barriers. In northern climates plows can throw snow onto the rail and the abrasive action of the snow can erode the protective layer. When exposed to these environments, weathering steel never develops the 'patina' that slows corrosion as in other less aggressive environments. Within a few years significant section loss may result. The interior of box beam barriers and the lap splice of w-beams can corrode rapidly to the point where the barrier may become more hazardous than the feature it was meant to shield.
Weathering steel may continue to be used on the backside of the Steel Backed Timber rail as the steel thickness is significantly greater than the typical 12 gage w-beam section.
One accommodation that has been tried is using zinc foil at the w-beam overlap where the zinc's galvanic action slows the corrosion. Use of thicker sections (exclusive of the terminal) may also prolong the life, but maintenance should still include inspection of the sections and joints. Powder coating of galvanized guardrail is an acceptable aesthetic option.
Barrier terminals are also subject to section loss at rail splices, but pendulum tests have been conducted on highly weathered barrier rails using galvanized extruder-type terminals and crash-test performance has been satisfactory. Questions on aesthetic treatments of barrier terminals should be addressed to the manufacturer.
Can 6x8 inch timber, W6x9 steel, and W6x8.5 steel posts be used interchangeably in the length-of-need section of guardrail?
A. Yes. Crash testing under NCHRP Report 350 has shown that these posts may be substituted when not in a barrier terminal. For short stretches of damaged barrier it is probably better to use the same type posts as in the existing installation, but where longer sections must be repaired substituting posts is acceptable. Some states use 8-inch round posts for w-beam guardrail, but there is not sufficient performance information to offer a recommendation on whether they may be substituted for steel or rectangular wood posts. Some proprietary guardrail and cable barrier posts have also been shown to be interchangeable with the generic posts. Recent crash testing [see NCHRP Project 22-14(3)] under the AASHTO Manual for Assessing Safety Hardware (MASH) has shown that there may be a difference in performance between steel post systems and wood post systems, especially when the top of the rail is less than 27 ¾ inches high.
Can I use a water-filled barrier on my project instead of concrete barrier?
A. Yes, but only if it includes a steel framework that has been accepted as crashworthy. To explain why, these definitions apply:
A "barrier" is a device that safely redirects, slows, or stops an errant vehicle and prevents a more severe crash or prevents vehicles from entering the work area. A "barricade" is a lightweight channelizing device that warns motorists of a hazardous situation and offers little or no resistance when hit. For example, a barrier offers "positive protection" to shield workers in a work zone from being hit by an errant motorist while a barricade does not. A "channelizer" is a line of traffic control devices used to delineate the traveled way.
Barriers include w-beam guardrail, jersey barriers ("K-rail" in California), steel barriers, bridge railings, weak post cable barriers, certain water-ballasted plastic units, and crash cushions. They must be crash tested at 100 km/hr using a small car and a pickup truck to assess occupant risk and barrier integrity. The test vehicle may not penetrate or vault over a barrier. When put in place each unit must be physically connected to the next unit per the state standard or per the manufacturer's instructions. If the units are merely butted end to end, or if the connection hardware is missing a hazard exists that is dangerous to both the traveling public and the workers.
Barricades must have orange and white reflectorized striping in accordance with Part 6 of the MUTCD, and include Type I and II "sawhorse" barricades, Type III Road Closure barricades, and some large plastic units that accept water ballast, among others. Barricades must be crash tested at 100 km/hr with a small car to ensure that they do not cause harm to occupants of the impacting vehicle when they are struck.
A hybrid device called a "longitudinal channelizing device" or "longitudinal channelizer" consists of large plastic units linked together, end to end, forming a wall. They are useful for controlling pedestrian traffic, guiding vehicles through confusing work zones, discouraging the use of median crossovers, and providing more delineation when only a line of cones or drums are typically used. A longitudinal channelizer is not a barrier because, upon impact by a vehicle, the plastic units rupture and the vehicle penetrates the wall. Some longitudinal channelizers can be converted into crashworthy barriers with the addition of continuous steel rails or by virtue of an internal steel framework.
Now the answer to the question: Concrete "New Jersey" Barrier or "K-rail" that is properly installed and connected will redirect most impacting vehicles. Certain "water filled barriers," namely those with internal or external steel rails or frames, can also contain and redirect vehicles. Without these external steel rails or the internal steel framework, water filled longitudinal channelizers do not have the capability to redirect vehicles and may not be substituted when a barrier is specified. Because of the confusion over water filled barriers and channelizers that look alike, the FHWA, the AASHTO/AGC/ARTBA Task Force 13, and the American Traffic Safety Services Association (ATSSA) support the use of clear labels on each water-filled unit that explain its purpose as a channelizing device or as a barrier unit. A discussion and a sample label, will be posted on the Task Force 13 web site (see www.aashtotf13.org)
Please note that barrier deflection should be carefully considered. Precast concrete barriers have lower deflection and can also be pinned in place to severely limit deflection upon impact.
Which concrete barrier shape should we use – Jersey Barrier, "F-Shape," Constant-slope, Single Slope, or vertical?
A. All these shapes are acceptable. Generally, the F-Shape or the 9.1 degree constant slope are preferred, since the F-shape design was specifically engineered to limit the potential roll over and the 9.1-degree constant slope reasonably mimics that performance. Another consideration may be the nature of the traffic using the facility or future overlays.
An explanation of the differences in the shapes may be useful. The Jersey- and F-shape barriers are both "safety-shape" barriers that begin with a 3 inch vertical face at the pavement level. Then they break to a sloped face that goes up to 13 inches above the pavement on the Jersey barrier, but only up to a height of 10 inches in the case of the F-Shape. Both then transition to a nearly vertical face to the top of the barrier.
The Texas Constant-Slope Barrier is 1070 mm (42 in) high and has a constant-slope face that makes an angle of 10.8 degrees with respect to the vertical. California developed a Single Slope profile that makes an angle of 9.1 degrees with respect to the vertical. The crash tests indicate that the performance of the Texas Constant-Slope Barrier is comparable to that of the Jersey-shape and the performance of the California Single-Slope Barrier is comparable to that of the F-shape.
A vehicle impacting one of the safety shape designs will have a significant portion of its energy absorbed in the climbing or lifting action that occurs when the tires roll up the lower sloping face. In low speed impacts this may result in the vehicle's redirection with no sheet metal contact with the face of the concrete wall. In medium speed impacts there will be damage to the vehicle but the occupants will experience minimum forces. In high speed impacts to safety shaped walls there will be significant vehicle damage and minor to moderate injury potential to the occupants. For the Jersey barrier there is a much greater likelihood that a small car will be rolled by the "safety shape" profile. The F-shape design was specifically engineered to limit the potential for small cars to roll over upon impact.
Vehicles impacting the single slope barrier or vertical wall will experience little potential for roll-over. However, the barrier will absorb none of the crash energy by lifting the vehicle – there is always sheet metal damage and the occupants get the full force of hitting a concrete wall. The vertical wall has similar impact parameters, with the added potential for an occupant's head to hit the wall if the wall is high enough.
A benefit of the constant slope, single slope, or vertical barriers is that multiple overlays can be applied without affecting the shape, and therefore the performance, as long as the total height remains adequate. Both "safety shapes" allow for no more than three inches of overlay.
In general, for high speed highways the single slope barrier is most appropriate to limit rollovers, since much of the fleet now has side airbags to absorb the impact to the occupants. The side impact airbags improve the safety of the occupants. For lower speed roads, the F-shape would be better for the majority of impacts the barrier would be expected to handle.
Do portable concrete barriers need to be tied down?
A. It depends. If you are placing the barrier near the edge of a bridge deck a catastrophic failure could occur if a vehicle caused the barrier to deflect enough to push it over the edge. If the barrier were placed on pavement with a work area on the other side, then more deflection can be tolerated and bolting it down usually is not necessary. Barrier deflection in this case may, indeed, push the concrete into the work area, but there appears to be little if any data relating to workers injured when the barrier is deflected and caused it to slide into the work area.
Can I attach a channel shape or some other device to the pavement behind portable concrete barrier to keep the barrier from sliding?
A. No. If the barrier is struck by a vehicle tall enough to push it across the deck the barrier could 'trip' over the channel shape and tip over, allowing the vehicle to intrude into the work area. The only acceptable location to secure a barrier is in front so that the anchors will resist the overturning moment.
Do cable barriers pose an extraordinary safety risk for motorcyclists?
A. Motorcyclists worldwide have raised this concern. First, the unprotected motorcyclist is at great risk anytime he or she goes off the roadway at speed and contacts a barrier or any other object. Second, there is not sufficient evidence that the cables cause the severe injuries. Reviews show that the barrier posts cause the greatest number of injuries (other than the cyclist going completely over the barrier and impacting the ground or some other unforgiving hazard.) Since the post spacing on cable systems is typically two to three times greater than the post spacing on steel beam systems, cable systems allow a greater potential for the rider to avoid striking the posts.
We also note that some European installations (notably in Sweden and the UK) place cable systems in the paved roadway where there had been no median (known in England as the "central reserve"). The cable barrier separates traffic on "two plus one" roads that have three lanes, two lanes in one direction and one in the opposite direction. This puts traffic very close to the barrier and allows very little room for error for motorcyclists or other vehicle drivers. The proximity of the barrier to traffic also results in an increase in the number of impacts, but motorcyclists are much more vulnerable and have more reported crashes. Cable barrier installations of this sort are not anticipated in the U.S.
The European community addressed this question in "Barriers to Change: Designing Safe Roads for Motorcyclists" where it states "The Panel concludes that, despite the amount of high profile coverage that wire rope barriers have attracted, limited research does not warrant the inference that they are more or less dangerous than other types of barrier on the market."
What guidance is available on the timeliness of guardrail repair?
A. It is important that each agency develop its own guidance for when to make repairs. While severely damaged roadside barriers need to be repaired within a reasonable amount of time, the FHWA cannot recommend a specific response time. Each agency must make a risk assessment about the timing of repair for each different category of damage and establish specific response times. The assessment would include, among other factors, agency resources (within its overall mission), hazard exposure (how likely is it that the guardrail will be hit again), and hazard severity. Vagueness on the timeliness of repairs does not prevent liability. Timing of repairs should be based on providing the safest facility, not on liability concerns, or on the State recovering damages from insurance companies before proceeding with the work.
The performance of damaged guardrail was assessed in the NCHRP Project 22-23 "Criteria for Restoration of Longitudinal Barriers." Information on that study may be found at: http://144.171.11.40/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=696.
When repair work is done under contract, the state should notify the contractor promptly when damage is discovered. The time that the contractor is given to respond should consider utility coordination (ie "Miss Utility" or "One Call" to avoid damaging subsurface utilities) and the fact that additional terminal grading or lengths of barrier may be needed to bring the device up to current standards. Special events and weather factors should also be considered when establishing mandatory response times.
The FHWA has updated the publication "W-Beam Guardrail Repair" (Publication # FHWA-SA-08-002) which is available on line at: http://safety.fhwa.dot.gov/local_rural/training/fhwasa08002/fhwasa08002.pdf.
Information on the eligibility of Federal funding for replacement parts of safety features may be found at http://www.fhwa.dot.gov/federalaid/080610.cfm.
What is "guardrail?" Our agency only uses "guiderail."
A. These terms are synonymous. A few states are required by judicial interpretation to refer to steel beam barriers as "guiderail" because the barriers are not seen as devices that can guard motorists from all injuries. Rather, the steel beam system can only "guide" the car and its occupants. (In Europe, "guard fence" and "road restraint systems" are the common names for roadside barriers.)
The Office of Safety's July 10, 2008 memorandum, "Consideration and Implementation of Proven Safety Countermeasures," asks states to review their policy on placement of barriers in medians up to 50 feet wide. Does this include divided highways without access control such as rural expressways and divided urban arterials?
A. Criteria for installing median barriers on these facilities must consider much more than just median width and ADT. Chapter 6, "Median Barriers," of the2006 AASHTO Roadside Design Guide (RDG) includes the following discussion relating to non-access controlled facilities.
Median barriers are sometimes used on high-volume facilities, which do not have fully controlled access. As indicated in Figure 6.1 ["Suggested guidelines for median barriers on high-speed roadways"], these median barrier guidelines were developed for use on high-speed, fully controlled-access roadways. Utilizing these guidelines on roadways that do not have full access control requires the need for engineering analyses and judgment, taking into consideration such items as, right-of-way constraints, property access needs, number of intersections and driveway openings, adjacent commercial development, sight distance at intersections, barrier end termination, etc. Therefore, trying to apply these guidelines to roadways that do not have full access control can be rather complex in many locations.
While the FHWA believes each state should have policies that deal with median barriers on all divided facilities, the agency is not ready to issue national guidance for highways without full access control. During its 2009 meeting the AASHTO Technical Committee on Roadside Safety (TCRS) considered further refinement to the RDG discussion of median barriers for non-access controlled divided highways. The TCRS concluded that warrants for median barriers on these facilities were not feasible because of the lack of research on the topic. Moreover, valid crash data are limited as many states do not even have a separate coding element for crashes involving a vehicle that crosses the median of a divided highway. In addition, some agencies avoid calling for median barriers on lower speed facilities because they believe the barrier may encourage higher speeds. For the foreseeable future, each state should assess its own need to have a policy dealing with median barriers on highways without full access control.
Finally, it's important to note that median bridge piers, culvert headwalls, non-traversable ditches, trees, and other fixed-object hazards in medians are to be removed, redesigned, or shielded with barrier just like hazardous elements on any roadside. When barriers are placed to shield these features, the potential for impact from vehicles coming from the opposite side of the highway should be considered.
Where Do I Measure the height of the guardrail from?
A. There are a number of different scenarios for guardrail height measurement. Only the first one is easy:
1) Guardrail is located above pavement: Measure the height from the pavement to the top of the w-beam rail.
2) Guardrail is located 2 feet off of the edge of the pavement: Use a 10-foot straightedge to extend the pavement/shoulder slope to the back of the rail. Measure from the bottom of the straightedge to the top of the rail.
3) Guardrail is located 2 feet off a recent pavement overlay: Follow the guidance in #2 above. You may have to re-set the barrier to achieve proper height. The gap between the pavement edge and the guardrail posts should be backed up with fill material to accommodate low-speed or shallow angle incursions.
4) Guardrail is located down a 1V:10H slope: Measure from the nominal terrain. Good contractors can get fairly even grading, but it will rarely be perfect enough to always be spot on the design height. Use a string line or straight edge to even out terrain variations.
5) Guardrail is located down a 1V:4H slope: If located more than 2 feet beyond the slope break point, remove the guardrail. Guardrail may not be placed on a steep slope. The Roadside Design Guide specifies where you may place guardrail on slopes as steep as 1V:6H, but guardrail must remain about two feet off the edge of the shoulder when there is a 1:4 slope behind it. (If steeper than 1:3 you may need longer posts, or additional posts, but that is yet another forthcoming FAQ pending the completion of ongoing research.)
WHY IS THE W-BEAM CONSTRUCTION TOLERANCE NOW ONLY ONE INCH?
A. Crash testing has shown that the standard strong post w-beam guardrail without rub rail is acceptable in the range from 27-3/4 inches to 30 inches above the ground. When the rail was tested at a lower height the pickup truck vaulted over the rail. A taller rail without rub rail can cause significant wheel snagging on small cars. This leaves a very narrow range of installation heights, and FHWA recommended 29 inches +/- one inch.
The Midwest Guardrail System (MGS) tolerance is greater at plus one to minus three inches. The MGS was initially tested at its design height of 31 inches with 12-inch blockout with no rub rail. It was known that the performance would be acceptable down to 27-3/4 inch just like the G4(1S) but we wanted to encourage the taller initial height so we recommended a construction tolerance of just one inch from the design height of 31 inches. A subsequent crash test (in July 2010) of the MGS at a height of 34 inches using the small passenger car was successful (which is generally considered the worst case scenario for tall guardrail), and now testing with the MASH pickup truck at 34 inches (or above) is pending to confirm an installation tolerance of +/- 3 inches.
HOW DO WE HANDLE THE HEIGHT TRANSITION BETWEEN G4(1S) AND MGS AND THEIR TERMINALS?
A. You should transition from a 27-3/4 inch tall barrier or terminal to a 31-inch tall barrier over the span of two 12-foot, 6-inch pieces of w-beam rail. When replacing or repairing long portions of a damaged rail the new rail should be installed at the proper design height, transitioning down to the existing rail over the length of two 12 foot, six inch, pieces of rail at either end. W-Beam to Thrie-Beam bridge transitions may need to use the non-symmetric W-to-Thrie connector that keeps the top height of the entire rail at approximately 31 inches. In addition, there is no need to transition in height to many 27 ¾-inch high terminals. The SKT, FLEAT, and ET end terminals have all been tested and accepted at the 31-inch rail height and provide the benefits of 31" guardrail without transitioning in height down to a lower system
OUR GUARDRAIL CROSSES A CULVERT AND WE CAN'T DRIVE A POST. CAN WE OMIT THE POST?
A. The Midwest Guardrail System (31-inch rail height) has been successfully tested with three posts omitted, leaving a span of 25 feet. Special posts (CRT, or Controlled Releasing Terminal posts) are used at either end of the gap but the rail does not have to be doubled up, or "nested" over the gap. Standard strong-post w-beam rail (minimum 27-3/4 inch rail height) can also be installed with three CRT posts at either side of a three- post gap, but the rail needs to be nested across the gap as well as up- and down-stream from the gap, for a total length of 100 feet. In addition, the MGS system is allowed to be placed closer to the headwall than the nested W-beam long span system.
CAN WE PAVE A MOW STRIP UNDER OUR GUARDRAIL?
Q: CAN WE PLACE GUARDRAIL POSTS IN A CONCRETE SIDEWALK OR MEDIAN?
A. Concrete or asphalt pavement under the guardrail would have to be constructed with a gap behind the post and backfilled with a loose material to allow the post to move when the rail is struck. An asphalt spray surface treatment would be acceptable as it would not prevent post movement through the soil. There are also various commercial products that can be placed under the W-beam to block weeds. Check with the manufacturer to see that they have designed the product with post deflection in mind. The FHWA also provided guidance on this issue in 2004 in our memorandum "W-Beam Guardrail Installations in Rock and in Mowing strips," http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/barriers/pdf/b64b.pdf
HOW CAN WE DESIGN OUR BARRIERS TO BE "MOTORCYCLE-FRIENDLY?"
A. Any time a motorcyclist leaves the roadway unintentionally there are likely to be severe consequences. Even the "safest" barriers can cause serious injury whether the motorcyclist is still on his/her bike or they are sliding/rolling on the pavement. Although there are many barrier modifications being used in Europe intended to moderate the severity of impact with guardrail posts, FHWA does not yet advocate the use of any such modifications on the NHS. As of Fall, 2010, there are two research projects underway that will analyze motorcycle crashes in depth. One is a general study of motorcycle crash causes, while the other is specifically targeting motorcycle impacts with roadside barriers. When these studies are completed, we hope to have information that will help us to determine the nature of motorcycle impacts with barriers, and whether or not the barriers can be redesigned without adversely affecting the good performance we have experienced with four-wheel passenger vehicle impacts to date.
Q: MANY OF OUR GUARDRAIL TERMINALS HAVE A STEEL BEARING PLATE ON THE FIRST POST THAT SOMETIMES ROTATES UNTIL IT IS UPSIDE-DOWN. IS THIS OK?
A. No. This bearing plate (8 x 8-inch square with an off-center hole) must be installed with the longer dimension upright (5" dimension up and the 3" dimension down). If the cable slackens over time traffic vibrations may allow this plate to rotate downward due to gravity. If this happens the ability of post #1 to fracture in a head-on impact (thus preventing a snag point) is severely compromised. On wood posts, a nail can be driven to prevent this rotation. A solution that works on both wood and steel breakaway posts is to specify that this steel plate be fabricated with tabs on either side that will wrap around the side of the post an inch or so to prevent rotation. This is an acceptable modification to all crashworthy terminals that use this 8 x 8-inch bearing plate. Of course, it is still critical to install the bearing plate with the 5" dimension up and the 3" dimension down. (The function of the bearing plate is to transfer load from the cable to the end anchorage.)
HOW DO WE KNOW THAT DAMAGED BARRIER NEEDS REPAIR?
A. Small dents and dings do not seriously affect the performance of guardrail. Vertical tears and bent posts are a different matter. A recent NCHRP study was completed and published as "Criteria for Restoration of Longitudinal Barriers" and it is available for download at http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_656.pdf. Also, the 2008 FHWA guide "W-Beam Guardrail Repair" is available for download at: http://safety.fhwa.dot.gov/local_rural/training/fhwasa08002.
WHAT GUARDRAIL HARDWARE MUST BE REPLACED OR UPGRADED ON THE NHS?
A. On September 29, 1994, the FHWA Executive Director signed a memorandum "ACTION: Traffic Barrier Safety Policy and Guidance" that identified various items that were to be inventoried and scheduled for replacement or upgrade if found within the clear zone. The FHWA Headquarters did not conduct a formal follow up to that memo. But now, more than 15 years later, it is time all remaining examples of these devices/situations be scheduled for correction as soon as practical. Terminals meeting NCHRP Report 350 or MASH are to be used.
The following terminals/transitions should be upgraded on the NHS:
The following device is to be upgraded whenever encountered within the limits of a project on the NHS:
4. Breakaway Cable Terminal**
*Versions of these terminals may be used on the downstream end to anchor the rail if they are outside the reverse direction clear zone and/or cannot be struck by vehicles crossing the centerline or median, impacting from the opposite direction.
** The BCT may also remain as a downstream anchor if outside the clear zone. It is also acceptable for use within some cable-to-guardrail transition designs. A crash test of the BCT as a Test Level 2 device failed.
WHAT KIND OF FOUNDATION DO WE NEED FOR OUR CONCRETE MEDIAN BARRIER?
A. Many variations exist between highway agencies regarding reinforcing and footing details for concrete median barriers; however, there have been few reported problems with any particular design and a need for a standard detail is not apparent. Concrete median barriers develop loads as a function of the barrier capacity and the foundation capacity. While it is true that some median barrier designs have been show to work with minimal foundation design, this does not suggest that any median barrier design can be installed in this manner. Thus, it falls on the designer to consider the combination of barrier and foundation that meets the design impact loading safely.
What about the national trend to go greener and the growing preference for smaller, lower profile vehicles? Is a higher rail needed for the future?
A. "Green" vehicles such as battery powered cars are larger and heavier than the 820C test vehicle of NCHRP Report 350, so the additional rail height should not be detrimental when considering the future vehicle fleet.
Will there be similar new height requirements for box beam rail?
A. No. The metric version of the box beam is 27 1/6 inches tall, and that is the height at which it was tested. The difference from a 27" high box is negligible, especially since the box beam is a weak post system where the box separates quickly from the post and stays in contact with the vehicle.
Have there been a statistically significant number of crashes and/or fatalities with the lower height rail that is driving the increase in height or is it all based on crash testing and simulation?
A. The recommendations for increased w-beam height are based on crash testing and an increase in the number of high center-of-gravity vehicles currently traveling on highways and roads compared to the 1960's when the 27-inch standard was set.
Would a nominal height of 29" +/- 1" with an 8" blockout be acceptable? At what height is a 12" blockout recommended? Can you adjust the simulation to test heights between 27-3/4 and 31"?
A. Conventional G4 (1S) strong-post guardrail at 29 inches to the top of the rail should have 8-inch blockouts. The size of the blockout is not simply a factor of the height of the guardrail. When the 30- inch maximum height of the G4(1S) guardrail is exceeded an entirely different system is encountered. The generic MGS system with a nominal height of 31" has a blockout that is 12 inches deep, and splices between the posts. The MGS and some of the proprietary 31" systems have the advantage that they will also meet crash test criteria at lower heights, but should be installed at 31" (or slightly higher to accommodate forthcoming overlays). This is also discussed in Chapter 5 of the RDG.
How was the 31" height selected? Did small car performance, testing, or simulation play a role?
A. In the early 2000's the Midwest Roadside Safety Facility (MWRSF) conducted a study to develop a better-performing roadside barrier. That research indicated that the performance of the G4 (1S) strong steel post guardrail improved when the splices were moved to mid-span. Increasing the blockout depth also improved crash test performance, as did raising the height of the rail from 27 3/4" to 31" which reduced the embedment of the post. The MGS represents the combination of these factors. The MGS has also been crash tested with 8" blockouts and Texas DOT is currently (June 2012) preparing to submit a Request for Eligibility.) Other systems were subsequently developed that also have a 31-inch mounting height.
When do you need to raise the guardrail when an overlay has reduced the height?
A. The AASHTO Roadside Design Guide, 4th Edition, states that guardrail that is at least 26 ½ inches high after an overly may remain in place. (See RDG p. 5-17)
How high do you need to raise guardrail that is lower than 26 ½ inches? What are the best ways to do that?
A. The guardrail should be raised to 29 inches, which represents the target height for new installations of strong post w-beam systems. If the pavement work requires the barrier to be moved, then the posts should be carefully extracted and, if in good condition, re-driven at the new location so that the rail will be at 29 inches. If the barrier does not need to be moved, then raising the blockout up to three inches is a common practice. This will require field drilling or punching of a new hole in the guardrail post.
I thought a non-gating terminal captured everything that hit it?
A. Under the criteria of NCHRP 350, capturing every vehicle that impacts the nose of the terminal was not a requirement for a non-gating system. The test with a 2000P vehicle impacting at an angle of 15 degrees on the nose of the system (Test 3-33 for 100 kph) can have the vehicle penetrate the system and proceed a considerable distance behind it. However, under the new MASH criteria, there is a specific requirement for non-gating systems that any hit on the nose or near the nose "should contain and redirect the vehicle or bring the vehicle to a controlled stop; the vehicle should not penetrate . . . ." Therefore, the criteria being used, (NCHRP Report 350 or MASH) dictates whether a system should be called "gating" or "non-gating." So, it is important to know how a system performs and where the impacting vehicle will end up under each of the required impact tests on the nose - zero degrees, at an angle (5-15 degrees depending on the criteria), or on the side.
Regardless of the type of system being used, a safe runout distance is needed beyond the beginning of the system, either for the situation where the vehicle passes through the system, or where the vehicle just misses the end of the system and continues behind it. Likewise, proper grading providing an essentially flat pad and traversable side slopes to ensure stability of the vehicle is needed, regardless of the type of system. The terminal is only re-directive beyond the point of length of need, which should be specified by the manufacturer.
May a decorative texture or graphic design be added to the face of a crashworthy concrete barrier?
A. Yes, but the relief of that texture is subject to certain limitations. These limitations are detailed in the NCHRP Report 554 "Aesthetic Concrete Barrier Design."
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_554.pdf
May computer modeling be used as a substitute for full-scale crash testing of new devices?
A. No. FHWA determination of Federal-aid reimbursement eligibility of roadside hardware is performance-based, which means full-scale crash testing under the AASHTO Manual for Assessing Safety Hardware is needed to establish such eligibility. Manufacturers developing new hardware are encouraged to use Finite Element Analysis (ie: LS-DYNA) to develop their device using the Verification and Validation process as detailed in NCHRP Web-Only Document 179 (Procedures for Verification and Validation of Computer Simulations Used for Roadside Safety Applications (V&V)) procedures in efforts to refine their designs and reduce the cost of developmental crash testing. For existing devices that have already demonstrated that they comply with NCHRP Report 350, certain minor modifications to the system may be evaluated using Finite Element Analysis. In addition, any Finite Element Analyses that are submitted to FHWA as part of the documentation package for determining eligibility for reimbursement under the Federal-aid highway program should be accompanied by a Verification and Validation Report as detailed in NCHRP Report 22-24.
Do we need to have the vertical devices (road tubes, vertical panels, etc.) in place when crash testing a "curbing" system as a work zone device in Category II.
A. Yes. MASH section 3.4.1 states "The test article should be constructed and erected in a manner representative of in-service installations and should conform to specifications and drawings of the manufacturer or designer." The FHWA has issued several letters for curbing systems since 2002 and the tests themselves have evolved to incorporate vertical elements starting in 2004. Over the next several years, more and more tests incorporated "delineator" style vertical elements as this became the in service condition of nearly all curbing systems utilized in this country.
When repairing crash-damaged guardrail terminals or crash cushions, may we use "breakaway posts" or other components that fit if they are supplied by another manufacturer?
A. Barrier terminals and crash cushions are precisely engineered devices that are subjected to a range of crash tests (up to 8 different tests) meant to show proper performance when impacted by errant vehicles. If the substitute parts do not crush, break, bend, or slide the same way as the crash-tested parts, the device's performance will be affected, with the potential for negative performance. (Even if the device's performance in one test may improve with the substitute part in place, it may lead to failure under another test impact condition.) If the component in question is covered by patent and unique to the system, then the overall effect can only be determined by the original manufacturer and/or a crash test laboratory.
Substitutions of components are allowable if any one of these conditions is met:
This guidance applies to the safety performance of barrier terminals, crash cushions, and the barriers themselves when considering the use of substitute components.
Most current guardrail terminals and impact attenuators are patented devices. Where the system, device, or components thereof are patented proprietary products, then the guidance in the January 11, 2006, FHWA Memorandum "Guidance on Patented and Proprietary Product Approvals" [http://www.fhwa.dot.gov/programadmin/contracts/011106.cfm] should be followed. This memo contains a link to additional FAQs on the use of proprietary products in Federal-aid contracts.
Our highways are signed for 75 mph. Shouldn't we use crash cushions that have been crash tested at speeds higher than 100 km/hr (62.5 mph)?
A. No. The FHWA Office of Safety considers that a 100 km/hr test is representative of worst case run-off-road crashes.
Early on in the panel discussions related to the NCHRP project for the updating of NCHRP Report 350, there was much discussion involving the need to increase test speeds over the 100 km/h (62.2 mph) maximum speed now used. Based on data available to the research team, it was concluded that regardless of posted speeds, most impacts with fixed objects occurred at somewhat reduced speeds, probably because most drivers are braking hard as they are about to run off the road or into some fixed object. Historically (from FARS data), crash cushions have been directly responsible for very few fatalities and even fewer of these can be attributed directly to inadequate cushion capacity. Granted, a longer cushion will perform better in some head-on full-speed crashes, but the cost-effectiveness of a 70 mph cushion over a 62 mph design is far from clear. FHWA's position is that highway features tested to Report 350 TL-3 (i.e., 100 km/h) are sufficient, but if any DOT wishes to use longer designs, they may. The best question to ask is whether or not there has been a performance problem with existing installations.
What is the difference between energy absorbing terminals and those that allow the vehicle to break through?
A. All terminals dissipate energy during an impact, some more than others depending on impact conditions. It is agreed that in an end-on impact by a vehicle aligned with the terminal, "energy absorbing" terminals will dissipate more energy than "non-energy absorbing" terminals. However, there are also certain impact conditions in which both types of terminals dissipate essentially the same amount of energy. For these conditions the vehicle can be expected to travel a considerable distance after impact with either terminal type. Additionally, a vehicle that inadvertently leaves the road in advance of a terminal may be just as likely to miss the end as to impact it. Therefore, it seems prudent to require similar run out distances for both energy-absorbing and non-energy-absorbing terminals.
The FHWA memoranda entitled, "Guidelines for the Selection of W-Beam Barrier Terminals," (October 26, 2004) and "Supplementary Guidance for the Selection of W-Beam Barrier Terminals" (November 17, 2005) contain additional considerations beyond those in the Roadside Design Guide (RDG). Guardrail run out distances and length of need requirements, as recommended in the RDG, are dependent on traffic conditions, guardrail layout, and the characteristics of the hazard to be shielded. They are independent of the terminal type, assuming that the point at which the length of need begins is the same for each terminal (normally 12.5 ft from the terminal's beginning).
An energy absorbing terminal may be preferred where narrow right-of-way restricts the width of the clear roadside. An energy absorbing terminal can be installed parallel to the traveled way and can capture a vehicle that impacts it on the nose. However, with narrow rights-of-way come numerous unaddressed hazards including fixed objects and improper grading. It is not uncommon to see barriers used only to shield built hazards like the approach end of a bridge railing or a culvert headwall, but terrain and other natural obstructions such as ditches and trees are not addressed. Trees are the #1 fixed object that is struck in run-off-road crashes. When these hazards remain within the clear zone, the guardrail designer should ensure that the situation is not made worse when locating a guardrail terminal.
When can I use a non-redirective crash cushion?
A. Care must be used in applying a non-redirecting, gating crash cushion. They are designed to decelerate a vehicle impacting head-on on the nose. Vehicle penetration is likely to occur for angle hits from the nose to near the mid-point of the array. Vehicle penetration / override of the system is possible for high speed, high angle impacts near the rear of the device.
All gating, non-redirective crash cushions should be applied to hazards that are not likely to be impacted at an angle on the side at any significant velocity. They are appropriate on low speed facilities, and in work zones with higher speeds where lane widths are constrained and the potential for high angle hits is limited. Potential problems with these non-redirecting attenuators include vaulting over the nose of the attenuator into the work area, and inadequate clear run out areas behind the devices. Side redirect impacts into these devices are known to push acceptable safety limits and as such, their use in the field requires scrutiny to ensure adequate safety for both vehicle occupants as well as workers in downstream construction areas. Typical examples of non-redirecting/gating attenuators installed in a manner that increases the likelihood of compromised impact performance include: 1) not providing adequate clear run-out areas behind these units and 2) positioning them at sites where the probability of high-speed, severe-angle (i.e. - high energy) impacts is high.
All users of these devices should be made aware of the factors that contribute to proper performance as outlined in the crash test report. Examples of non-redirecting, gating crash cushions include all sand barrel arrays, the Triton CET (Concrete End Treatment), ACZ350, NEAT, and the ABSORB 350.
Please note that adequate clear run out areas are required for both non-redirective and redirective devices. The run out areas are longer for non-redirective devices. The additional length is typically the length of the non-redirective device.
It should also be noted that non-redirective crash cushions such as sand barrel arrays can pose a hazard if impacted in the reverse direction on the heavy barrels adjacent to the rigid hazard. Impact in the reverse direction at this point in the array is untested and the large mass of the final barrels could cause rapid and violent deceleration of the impacting vehicle that would exceed our occupant risk limits.
What type of crash cushion should I use?
A. When more than one crash cushion / impact attenuator system is approved for use in your State, carefully evaluate the structural and safety characteristics of each candidate system for the site in question. These include such factors as impact decelerations (as indicated by the Test Level to which the device was tested), redirection capabilities, anchorage and back-up structure requirements, and debris produced by impact. All of the systems described in the Roadside Design Guide as meeting MASH or NCHRP Report 350, TL-3 evaluation criteria have the capability to stop compact cars and pickup trucks impacting head-on at 100 km/h [62 mph] within tolerable deceleration levels, and to redirect or contain those vehicles impacting on the sides of the units within the system's length-of-need. However, the costs of initial installation and maintenance, the ease of repair, and the system's durability (to environmental conditions and impacts) vary greatly. In addition, the contract plans need to include enough details showing the type of system desired, the hazard needing protection, and/or how the specifying agency intends to have the device attached to the hazard.
Attenuators that are categorized as self-restoring or low maintenance are premium systems that are designed for high traffic areas where impacts can be expected to be frequent. The high initial cost can be offset by the long-term savings in maintenance and repair costs.
Traffic speeds, geometric constraints, weaving maneuvers, congestion, worker exposure, space available to repair, potential for secondary impacts, and sight distance are just a few of the factors that should be considered at potential locations for redirective crash cushions. They have demonstrated the ability to redirect vehicles away from the corner of the hazard, as well as safely decelerate vehicles hitting the nose of the attenuator head-on.
Sand barrel arrays are most appropriate for hazards that are located well off of the traveled way where impacts are expected to be infrequent, yet very serious when they do occur. The relatively low initial cost of these sacrificial crash cushions is a good investment to prevent serious injury when a crash does occur.
What if my state challenges all or part of the Roadside Design Guide?
A. The Roadside Design Guide is neither a standard nor a design policy. It is intended to be used as a resource document offering guidance from which individual highway agencies can develop standards and policies. Although much of the material in the Guide can be considered universal in its application, several recommendations are subjective in nature and may need modification to fit local conditions.
Is it appropriate to use re-galvanized or salvaged guardrail posts and rail in longitudinal barriers for new construction and/or maintenance repair projects?
A. No, only new posts and rails that are accompanied by a material certification should be allowed on Federal-Aid projects or on NHS routes. FHWA also recommends that State DOTs should not use salvaged or reconditioned guardrail material on State projects off the NHS because w-beam guardrails are at performance limits when all the materials used conform to specifications. Non-documented components should not be used.
New steel guardrail posts should conform to AASHTO M270 / ASTM A-36 steel and AASHTO M111 / ASTM A-123 for the galvanizing. New W-beam rails should conform to AASHTO M-180 specifications. When delivered to construction sites, these components are typically accompanied by mill certifications. Salvaged material is often an assortment of varying ages, bolt-hole locations, steel grades, etc. State highway agency should be able to track and verify the source of these materials to ensure the barrier will perform as designed because it is difficult to establish that salvaged guardrail material meets proper specifications. The use of new material is recommended.
An exception exists where "remove and reset" conditions apply. If the highway agency approves the condition of the in-situ barrier components, then it may be adjusted to current specifications within the limits of the project.
Concerns about the environmental aspects of old guardrails may be reduced because most salvaged posts and rails are recycled and used to produce new steel. FHWA's general guidance on salvage credit is located at: http://www.fhwa.dot.gov/programadmin/contracts/core02.cfm#s2C06
Is it appropriate to use re-straightened guardrail w-beam panel?
A. No. W-beam rail is placed under significant tensile loading when the barrier is impacted. A minimum 27 ¾ inch high w-beam rail is at its performance limit when tested to the AASHTO Manual for Assessing Safety Hardware (MASH) Test 3-31 using the quad cab pickup truck at 25 degrees and 100 km/hr. Any potential alteration of the strength of the rail by deformation during an impact or by restraightening could compromise its performance.
Useful FHWA links:
W-Beam Guardrail Repair Guide
http://safety.fhwa.dot.gov/local_rural/training/fhwasa08002
Criteria for Restoration of Longitudinal Barriers
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_656.pdf
Task Force 13 Guide to Standardized Highway Barrier Rail Hardware:
http://www.aashtotf13.org/Barrier-Hardware.php
How do you handle guardrail posts when using in expanded polystyrene (EPS) fill, the geofoam, lightweight fill since it won't create the support needed for proper resistance of the guardrail when hit? Can you simply extend the posts?
A. There are a few ways to do this: 1) bury the EPS deep enough and cover with conventional soil that develops the length needed, 2) construct as a moment slab and barrier (similar to Mechanically Stabilized Earth walls), or 3) use a load distribution slab (a reinforced concrete slab overlying the EPS) an anchor the guardrail in it.
Would drilling a new hole in the Midwest Guardrail System (MGS) guardrail weaken the system?
A. FHWA does not recommend altering a conventional w-beam rail by drilling new holes to accommodate the MGS. If the rail does not come with slots pre-punched at the 3' 1 &frac1/2;" mark, attempting to drill a new hole may compromise the performance of the rail or constrain its lateral movement. The cross section of all w-beam rail is already reduced at the splices, and there is a hole at the mid-span location. Providing additional factory-punched holes or slots at the 3' 1&frac1/2;" marks does not reduce the effective cross-section.
Are there other products (w-beam guardrail systems and terminal sections) currently being tested?
A. Yes, research sponsored by the NCHRP and pooled fund studies at the Midwest Roadside Safety Facility (Lincoln, NE) and the Texas Transportation Institute (College Station, TX.) is underway. Placement next to slope break points, transitions, terminals, etc., will be tested and/or evaluated. Proprietary terminals are also being developed and tested under MASH criteria.
Have terminal sections for 31" w-beam guardrail been found eligible without blockouts or do all systems have either 8" or 12" blockouts? Can they be used with systems without blockouts?
A. Currently eligible terminals for 31" guardrail were listed in Appendix C of our May 17, 2010 memo: http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/policy_memo/memo051710. Appropriate terminals are also listed in Chapter 8 of the RDG. Alternatively, a 27 or 27 ¾ inch terminal may be installed and transitioned to 29 inches or 31 inches when you reach 25 feet beyond the downstream end of the terminal. The terminal manufacturer should be consulted for current design details.
May existing eligible NCHRP350 w-beam terminals that were tested at minimum height of 27¾ inches be installed as per existing FHWA Memorandum 'ACTION: Roadside Design: Steel Strong Post W-beam Guardrail' dated May 17, 2010?
A.Yes. To clarify the existing guidance in the Memorandum dated May 17, 2010, it recommends strong post w-beam guardrail be installed at 29" +/- 1". This also applies to w-beam terminals that have been found crashworthy for strong-post w-beam guardrail at 27 to 27 ¾ inches. This has been FHWA recommendation for guardrail and the associated terminals for over 5 years. In addition, installing at a ragged-edge minimum of 27 ¾ inches provides no utility for the installer regarding installation tolerances.
Prior acceptance / eligibility for strong post w-beam terminals does NOT apply to 31" tall w-beam systems like the Midwest Guardrail System (MGS). Terminals should be crash tested at 31" when used with these taller barriers which differ from strong post W-beam in other ways.
Q. Are "lead anchors" acceptable when connecting the guardrail end shoe to the concrete parapet or end block?
A. No. Lead anchors can work loose over time due to vibration from traffic. The only sure method of attachment is to continue the bolt through the concrete and place the nut on the outside of the structure. A good quality epoxy anchor is acceptable if properly installed according to the manufacturer's instructions.
Q. Do bridge railings on reconstructed bridges off the NHS need to meet NCHRP Report 350 criteria?
In general, FHWA standards apply to projects on the NHS. State transportation agencies may establish different standards for non-NHS projects if desired and may elect to use roadside hardware that has not been successfully tested to NCHRP Report 350 guidelines. Nonetheless, the FHWA strongly recommends the use of crashworthy devices on all public facilities where run-off-the-road crashes may occur.
Regarding the design of new railing standards for both "on and off" NHS routes, Load and Resistance Factor Design (LRFD) Section 13 should apply to all new bridges and rehabilitated bridge projects where railing replacement is required. However, repair or retrofit to an existing railing system that has been found acceptable under the previous crash testing and eligibility criteria (such as NCHRP Report 230, the 1989 AASHTO Guide Specifications for Bridge Railings, or equivalent) does not require further testing to the NCHRP 350 requirements and are at the owner's discretion. Further support for this position can be referenced to the FHWA memorandum dated May 30, 1997. Please also be reminded that a new railing detail solely designed to the LRFD geometric and resistance requirements does not necessarily warrant "passing" of a full scale NCHRP 350 crash test at the specified performance level.
Information on crashworthy bridge railings may be found on the Task Force 13 web site www.aashtotf13.org.
FHWA is considering developing FAQs for the following topics:
