U. S. Department of Transportation
Federal Highway Administration
TECHNICAL ADVISORY
T 5040. 28
October 17, 1988
Par.
This subsection clarifies that federally funded nonfreeway RRR projects shall be constructed to preserve and extend the service life of existing highways and enhance highway safety.
(1) develop and adopt geometric design criteria specifically for nonfreeway RRR projects,
(2) adopt and apply current geometric design criteria for new construction (referenced in 23 CFR 625. 4(a) (1) ) to nonfreeway RRR projects, and/or
(3) continue to use previously approved geometric design criteria for nonfreeway RRR projects which have been in existing Certification Acceptance or Secondary Road Plan agreements, provided such criteria are consistent with 23 U. S. C. 109(o) .
(1) The criteria adopted by a highway agency, and approved by the FHWA, become the benchmark for evaluation of the design of a RRR project.
(2) The RRR design criteria should address, by modification or incorporation, all controlling elements, and may address additional items selected by a highway agency.
(a) As indicated in paragraph 3d, 13 geometric elements were established as the controlling criteria for geometric design. The controlling criteria are design speed, lane and shoulder widths, bridge widths, structural capacity, horizontal and vertical alignment, stopping sight distance, grades, cross-slopes, superelevation, and horizontal and vertical clearances.
(b) New construction standards apply for those controlling elements not addressed by special RRR criteria.
(3) Adoption of RRR criteria for the geometric elements for nonfreeways does not relieve agencies from meeting new construction policies, standards or standard specifications for all nongeometric elements. Deviations substandard to these policies and standards require approval on a project-by- project basis as discussed in paragraph 5a(4) .
(4) Deviations substandard to the adopted RRR criteria require justification on a project-by-project basis. The documentation justifying the lesser criterion might include, as appropriate and depending on the scope of the project, a discussion of the proposal including alternatives to the proposed action, compatibility of the exception with adjacent sections of roadway and future improvements on the route; a complete description or a sketch showing the design feature and its relation to other roadway elements; a cost analysis; an accident analysis; proposed mitigation measures, if any; the expected safety consequences; and other considerations to support the recommendation to use a design exception.
(1) The purpose of RRR is to preserve and extend the service life of existing highways and enhance highway safety (23 U. S. C. 109(o)) .
(a) The most current source of data, procedures and recommendations regarding geometric design and its relationship to safety for RRR projects is contained in TRB Special Report 214.
(b) The information in Report 214, together with current program guidance and other technical material can be used to develop or modify criteria, processes and practices to achieve the twin objectives of RRR type projects -preservation and safety enhancement.
(2) By their purpose and definition, RRR projects reflect and emphasize the management of the highway system by extending the service life and deriving the maximum benefit from existing highways. Economic considerations are a major factor in determining the priority and scope of RRR work.
(a) Special geometric design criteria developed for RRR projects should acknowledge this factor and emphasize implementation of cost-effective improvements where practical.
(b) Special Report 214 contains economic evaluation procedures for several of the elements included in its recommendations. These evaluation procedures may be used to consider the economic consequences of a proposed improvement.
(3) The topics addressed in paragraph 7, "Design Practices for Key Highway Features," include 10 of the 13 controlling criteria as they relate to RRR.
(a) The 10 controlling criteria discussed under appropriate headings are design speed, horizontal and vertical alignment, lane and shoulder widths, bridge widths, cross-slope, superelevation, stopping sight distance, and horizontal clearance.
(b) The three not addressed here or in the TRB Special Report 214 are vertical clearance, structural capacity, and grades. No new data was available or developed on which to base specific recommendations. However, if these elements are modified by special RRR criteria, special consideration should be given to the size and weight of trucks legally allowed to operate on the affected route.
(1) Correction of safety deficiencies and inclusion of appropriate enhancements must be integrated into the design process in the early stages of project identification as well as during each phase of project development.
(2) The RRR work often provides an opportunity to incorporate safety improvements into a project in conjunction with the pavement and geometric work. Consideration of the roadway, the roadside andoperational features is required to integrate the safety improvements.
(a) Safety improvements can include intersection and access point adjustments that increase sight distance and reduce vehicle conflicts, replacement or rehabilitation of obsolete bridge rails and guardrails, removal of roadside obstacles and unnecessary guardrails, slope flattening, ditch relocation and/or regrading, upgrading roadside appurtenances, new or improved signing, pavement markings and other traffic control devices.
(b) Special Report 214 provides information to develop programs and procedures that insure the consideration for safety is included in the initial scope and estimate for a project.
(1) the determination of existing geometric, safety and operational features throughout the project. The designers of RRR projects can draw on a substantial amount of information in the preparation of a design.
(a) The information available includes lane and shoulder widths; degree, length and superelevation of horizontal curves; length of vertical curves; stopping sight distances; grades; sideslopes; clear recovery areas; available right-of-way; potentially hazardous obstacles; location and design of intersections; type and location of highway signs; pavement markings; delineation and traffic signals.
(b) Line diagrams, strip maps, as-built plans, photologs, etc. are useful sources of information.
(2) A procedure to gather and analyze accident, speed and volume data. The analysis of this information can be used to identify specific safety or operational problems and develop appropriate countermeasures.
(3) A method to obtain speed data, using generally accepted study procedures, at various locations where they are to be used for design within the project limits for speed dependent design elements. The use of various speed measures is discussed in paragraphs 7a, 7c(1) , 7c(2) , and 7d(1) .
(4) A thorough field review by personnel knowledgeable about and trained in design, safety, traffic operations and maintenance to identify potentially hazardous locations and features, and recommend appropriate safety enhancements. Field reviews are also beneficial to verify existing conditions and identify recent changes.
(5) Consideration and incorporation, as appropriate, of high hazard locations, intersection, roadside and traffic control improvements that may result in enhanced safety. There are many relatively low-cost improvements that can be highly cost effective when incorporated into certain RRR projects. Paragraph 7h discusses alternate safety improvements.
(6) A procedure for routine review of projects during development by traffic and safety specialists. This should include periodic consultation with these specialists before final approval of the project plans.
(1) This report can serve as documentation of the design process undertaken to develop the RRR project, assist in design decisions and provide the background information needed to obtain any necessary design approvals.
(a) The components which should be incorporated in the report include the existing and proposed geometric and roadside features, current and estimated future traffic volumes, speeds, accident history, applicable design standards and design options.
(b) Specific safety problems or concerns should be identified and addressed along with options, costs and recommendations to alleviate the problem.
(c) Any identified design exceptions (geometric and nongeometric) and appropriate mitigations should also be included in this report.
(2) While neither a format nor a length for this report is specified, it should be as detailed as the size, scope, and complexity of the project requires. A simple form summarizing the information may be sufficient for many projects, while a detailed report may be necessary for more complex projects or in situations where accident and traffic histories warrant consideration.
(1) These problems are evident in those locales with significant adjacent development or where existing right-of-way for the highway is narrow. These factors are frequently the cause for delay in advancing the project to construction.
(2) These potential conflicts should be taken into account early in the RRR process. A process to screen candidate projects to identify locationswhere improvements are desirable and require right-of-way should be instituted.
(a) With these locations identified, the design at these sites can be expedited to determine the actual right-of-way requirements. Using timesaving techniques, the acquisition of the necessary real estate for the project can be expedited to insure its availability in time for construction.
(b) A process can be instituted to work in advance with affected parties to identify environmental and community impacts in order to develop an acceptable balance between community concerns and project needs.
(1) Highway agencies are encouraged to perform a periodic assessment of the potential for systemwide, route, or route section safety upgrading design in connection with or in addition to programs to identify and correct specific hazardous locations.
(2) These periodic assessments of improvements on the basis of one of the preceding classifications can increase the positive impact of RRR projects on safety in several ways.
(a) The results could be used to help tailor design practices and standards to the circumstances of a particular highway agency.
(b) The results could detect opportunities for effective safety improvements that warrant project programming earlier than previously anticipated.
(c) The assessment could be linked to other safety programs to gauge overall progress toward improving highway safety.
(d) Along with the results of other analyses, the assessments could serve as input for establishing future highway programs and funding requirements.
(1) There are two methods that can be used to select the design speed for a RRR project. These procedures may be used alone or in combination. In either case, the objective is to coordinate the various geometric elements to produce a safe highway.
(a) One method is to select an overall project design speed. This is defined as the speed that equals or exceeds the posted or regulatory speed on the section of highway being improved. All the various geometric elements on the project are correlated by this one design speed.
(b) A second method involves a series of design speeds. This method requires the determination of the speeds that affect four of the individual elements.
1 The average running speed throughout the project length may be used as the design speed in determining lane and shoulder widths. The average running speed is the average speed of a vehicle over a specified section of highway.
2 The 85th percentile speed may be used for horizontal and vertical curves. The 85th percentile speed is the speed below which 85 percent of the vehicles are operating.
(c) The specific applications of these speeds are discussed in paragraphs 7c(1) , 7c(2) , and 7d(1) .
(2) When a speed less than the posted or regulatory speed is used, speed studies using generally accepted study procedures are required to establish the speed at each location where the average running or 85th percentile speed is to be applied. The results of these studies are to be used as the basis for determining the design speed for the element whether the posted or regulatory speed is exceeded or not.
(1) Design decisions for particular highway features should be based on conditions that reflect the anticipated service life of the feature even though the expected performance period of the pavement rehabilitation work may be much less than the performance period for geometric improvements.
(a) For RRR, the need for a formal forecast of future traffic is the greatest when the current traffic is approaching the capacity of the highway, and decisions must be made regarding the timing of major improvements such as additional lanes.
(b) Studies to determine future traffic are not normally necessary on very low-volume roads where even high-percentage increases in traffic do not significantly impact design decisions.
(2) Preferably, the design traffic volume for a given feature should match the average traffic anticipated over the service life of the affected feature such as alignment and widths.
(1) Horizontal Curves
(a) An existing horizontal curve may be retained as is without further evaluation if:
1 the existing curve design, assuming correct superelevation is provided, corresponds to a speed that is within 15miles per hour (mph) of the 85th percentile speed of the approaching vehicles; or within 15 mph of the overall project design speed, and
2 the design volume is less than 750 vehicles per day.
(b) Reconstruction to either new construction standards or to approved RRR standards is to be considered and evaluated when the above speed and/or volume criteria are exceeded.
(c) If the curve reconstruction is not justified, or if it is reconstructed to less than new construction standards, appropriate safety and other mitigation measures should be applied. Safety measures that are less costly than reconstruction include, but are not limited to, those enumerated in paragraph 7h(2) . These measures may be applied either separately or in combination.
(d) The 85th percentile speed, defined in paragraph 7a(1) (b) 2 , is to be measured at a point ahead of each end of the curve where vehicle operators have not begun adjusting their speed. Project design speed is as defined in paragraph 7a(1) (a) .
(2) Vertical Curves
(a) An existing vertical curve may be retained as is, without further evaluation if:
1 the existing curve design speed, based on the stopping sight distance provided, corresponds to a speed that is within 20 mph of the 85th percentile speed of vehicles on the crest, or within 20 mph of the overall project design speed; and
2 the design volume is less than 1,500 vehicles per day.
(b) Reconstruction of crest vertical curves to either new construction standards or to approved RRR standards is to be evaluated when the above speed and/or volume criteria are exceeded, and the vertical curve hides major hazards from view.
(c) Whether or not an evaluation is required, designers should routinely examine the nature of potential hazards such as intersections, sharp horizontal curves, or narrow bridges hidden by a vertical curve, their location in relation to the portion of the highway where sight distance falls below new construction standards, and other options to reconstruction such as relocating or correcting the hazard or providing warning signs.
(d) If curve reconstruction is not justified, or the curve is reconstructed to less than new construction standards, appropriate safety and other mitigation measures should be applied. Safety measures that are less costly than reconstruction include, but are not limited to, those identified in paragraph 7h(2) . These measures may be applied separately or in combination.
(e) The 85th percentile speed, defined in paragraph 7a(1) (b) 2 , is to be measured on the crest of individual vertical curves for vehicles traveling in both directions. Project design speed is as defined in paragraph 7a(1) (a) .
(f) While the preceding discussion focused on crest vertical curves, sag verticals should not be ignored. Substandard sag vertical curvesshould be investigated to insure that potential hazards do not exist, especially ones that become apparent when weather conditions or nighttime reduces visibility.
(3) Curves in Series . Frequently the alignment of a segment of a roadway consists of a series of reverse curves or curves connected by short tangents. A succession of curves may be analyzed as a unit rather than as individual curves, applying the criteria in paragraphs 7c(1) and 7c(2) as appropriate.
(a) The first substandard curve in a series should receive special attention because this change in alignment prepares the driver for the remaining curves in the series.
(b) Any intermediate curve in a series of substandard curves that is significantly worse than the others in the series should also be analyzed individually.
(c) These controlling curves can be used to determine the safety and/or other mitigation measures to apply throughout the series.
(d) When improvements are considered to any curve in a series, the effect on the series of curves as a whole should be evaluated.
(1) Lane and Shoulder Widths . Wide lanes and shoulders provide motorists increased lateral separation between overtaking and meeting vehicles and an opportunity for safe recovery when their vehicles run off the road. Additional safety benefits include reduced interruption of the traffic flow as the result of emergency stopping and road maintenance activities, less pavement and shoulder damage at the lane edge, improved sight distance at critical horizontal curves, and improved roadway surface drainage.
(a) Suggested minimum lane widths and combined lane and shoulder widths are provided in Table 1 of Attachment 1. The suggested minimums explicitly consider vehicle speed and the amount of truck traffic, which influence the safety benefits derived from wider lanes and shoulders.
(b) Either of the two methods may be used as the speed parameter for determination of appropriate lane and shoulder widths.
1 Average running speed throughout the project length is one method. This speed is defined in paragraph 7a(1) (b) 1.
2 The overall project design speed is the second method that may be used. Design speed is defined in paragraph 7a(1) (a).
(2) Bridge Widths . Hazards associated with bridge widths can be significant. Roadway constriction at narrow bridges reduces the opportunity for safe recovery of out-of-control vehicles and can result in end-of-bridge collisions. Furthermore, bridge approaches are often on a downgrade, a factor responsible for increases in speed, and particularly in the case of older spans, are often sharply curved. When coupled with other factors such as premature icing in winter and substandard bridge rail, the special hazards associated with bridges are readily understood.
(a) An existing bridge may be retained when the suggested bridge widths in Table 2 of Attachment 1 exist.
(b) A bridge should be evaluated for replacement or widening on a case-by-case basis when the criteria suggested in Table 2 are not met.
(c) Safety at narrow bridges can also be improved by transition guardrails at bridge approaches, new or rehabilitated bridge rails and warning devices.
1 If an existing bridge is to be retained, substandard bridge rail should be upgraded to current standards and "safety" curbs which can cause vehicles to vault the rail should be eliminated. Exceptions may be considered on a case-by-case basis where safety can be adequately enhanced but cost effective considerations prevent full widening or full upgrading of the bridge rail.
2 On all projects involving bridges, the approach guardrail should be evaluated and upgraded to current standards. Approach guardrail must be properly anchored to the bridge.
3 The transition between the approach guardrail and the bridge rail should be smooth and of sufficient strength (i. e. , reduced post spacing) to prevent snags and vehicle pocketing.
4 Only approved crash-tested bridge rails, guardrail, and transitions should be used.
5 A partial list of alternate safety measures is identified in paragraph 7h(2) .
(3) Cross-Slope and Superelevation
(a) On RRR projects that include resurfacing, pavement cross-slopes should be restored to new construction standards.
(b) Superelevation rates on horizontal curves should be increased, if necessary, to the appropriate rate for new construction for the design speed being used at the location.
(4) Roadside Features . Accident data firmly establish that roadside characteristics are important in determining the overall level of safety provided by a highway. Accident rates are lower and accidents are less severe on highways with few obstacles near the travelway.
(a) Consistent procedures should be developed for evaluating and improving roadside features with the following objectives:
1 Remove, relocate, shield, or reconstruct to a breakaway design isolated roadside obstacles.
2 Flatten sideslopes that are 3:1 or steeper at locations where run-off-road accidents are likely to occur (e. g. , on the outside of sharp horizontal curves) .
3 Retain current slope ratios (i. e. , do not steepen sideslopes) when widening lanes and shoulders unless warranted by special circumstances.
(b) Clear zone policies can be tailored to particular types of obstacles commonly encountered by a highway agency to reflect differences in the cost of removal, relocation, or shielding.
(1) The existing pavement condition and the scope of needed pavement improvements dictate to a large extent those improvements which are feasible, prudent, or practical. More significant geometric upgrading might be appropriate if the pavement improvements are substantial, but may not be appropriate or economical if needed pavement work is relatively minor. Conversely, the geometric deficiencies may be so severe that the overall highway improvements must be more substantial than those which may be appropriate with only minor pavement improvements.
(a) Geometric design criteria should indicate how existing pavement condition and the scope of pavement improvements will interrelate with the scope of geometric improvements and the values used for design.
(b) Pavement rehabilitation is to be developed in accordance with current FHWA pavement policy.
(2) A skid resistant surface is an essential part of any pavement surface improvement, regardless of the scope of geometric problems or upgrading. Current policy requires that each Federal-aid project, including RRR projects, involving pavement construction shall provide a skid resistant surface.
(3) Pavement edge drops are undesirable, no matter how they develop, because of the safety implicationsassociated with the vehicle recovery maneuver. Pavement edge drops, defined as vertical discontinuities at the edge of the paved surface, often develop between the pavement surface and the adjacent unpaved shoulder or roadside. They can result from adding a layer of surfacing without regrading the existing shoulder; wear or erosion of gravel, turf, or earth shoulder materials.
(a) Properly designed and constructed RRR projects can reduce edge drop related accidents. Existing policy requires that edge drops be eliminated on Federal-aid projects. Any RRR criteria developed should include procedures and practices to eliminate designs and construction operations which lead to creation of edge drops, and that reduce their occurrence along existing highways.
(b) There are several practices which can reduce the occurrence or mitigate the impact of edge drops. These practices include:
1 paving the full top width between shoulder breaks;
2 selectively paving shoulders at points where vehicle encroachments are likely to create pavement edge drops, such as on the inside of horizontal curves; or
3 constructing a beveled or tapered pavement edge so that any edge drop that develops has a reduced impact on the recovery maneuver.
(c) Any paving of the shoulder area should incorporate a pavement structure capable of supporting anticipated loadings.
(1) Although specific guidelines for intersection improvements are not appropriate because of the wide variety of physical and operational features affecting safety, it is recommended that consistent procedures and checklists be developed for evaluating intersection improvements on RRR projects.
(2) Intersection improvements should be tailored to each individual situation with due recognition being given to traffic volumes on each of the intersecting roadways, prior accident pattern and physical characteristics of the site.
(a) The improvements at intersections generally focus on reducing conflicts and improving driver guidance. Reducing approach speed and improving skid resistance can be important also.
(b) There are several useful analysis procedures available to assist in selecting safety improvements, including collision diagrams, condition diagrams, and a field review of the intersection.
(1) Signs and markings in conformance with the Manual on Uniform Traffic Control Devices (MUTCD) are required on all federally funded highway projects, including RRR.
(2) While traffic control devices cannot fully mitigate all problems associated with substandard geometric features, they are a relatively low-cost measure that can compensate for certain operational deficiencies.
(a) Where roadway geometry or other roadway or roadside features are less than standard, do not meet the driver's expectancy, and reconstruction is not appropriate, additional signs, markings, delineation, and other devices beyond normal requirements of the MUTCD should be considered.
(b) Judicious use of special traffic regulations, positive guidance techniques and traffic operational improvements can often forestall expensive reconstruction by minimizing or eliminating adverse safety and operational features on or along existing highways.
(1) Highway design practice provides a broad range of alternative measures that can be used alone or in combination with others to mitigate the effects of geometric deficiencies and provide for safer operations on existing highways.
(2) A partial list of alternatives to reconstruction for several geometric deficiencies is provided in the following table.
GEOMETRIC DEFICIENCY ALTERNATE SAFETY MEASURE Narrow lanes and shoulders Pavement edge lines Raised pavement markers Post delineators Steep sideslopes; Roadside hazard markings roadside obstacles Slope flattening Round ditches Obstacle removal Breakaway safety hardware Guardrail Narrow bridge Traffic control devices Approach guardrail Hazard markers Pavement markings Poor sight distance at hill Traffic control devices crest Fixed-hazard removal Shoulder widening Driveway relocation Sharp horizontal curve Traffic control devices Shoulder widening Appropriate superelevation Slope flattening Pavement antiskid treatment Obstacle removal Obstacle shielding Hazardous intersections Traffic control devices Traffic signalization Fixed lighting Pavement antiskid treatment Speed controls
Thomas O. Willett, Director
Office of Engineering
10
Percent or More Trucks b |
Less
Than 10 Percent Trucks b |
||||
Design
Year Volume (ADT) |
Running Speeda (mph) |
Lane Widthc (ft) |
Combined
Lane and Shoulder Widthd (ft) |
Lane Widthc (ft) |
Combined
Lane and Shoulder Width d (ft) |
1-750 | Under 50 | 10 | 12 | 9 | 11 |
50 and over | 10 | 12 | 10 | 12 | |
751-2000 | Under 50 | 11 | 13 | 10 | 12 |
50 and over | 12 | 15 | 11 | 14 | |
Over 2000 | All | 12 | 18 | 11 | 17 |
a Highway segments should be classified as "under 50" only if most vehicles have an average speed of less than 50 mph over the length of the segment.
b For this comparison, trucks are defined as heavy vehicles with six or more tires.
c If the highway is included on the National Network or is an access road for the network, a 12-foot lane width should be used.
d One foot less for highways on mountainous terrain.
Design Year Volume (ADT) | Usable Bridge Width(ft) |
0 - 750 | Width of approach lanes |
751 - 2000 | Width of approach lanes plus 2 ft. |
2001 - 4000 | Width of approach lanes plus 4 ft. |
Over 4000 | Width of approach lanes plus 6 ft. |
If lane widening is planned as part of the RRR project, the usable bridge width should be compared with the planned width of the approaches after they are widened.
1 From Special Report 214, "Designing Safer Roads, Practices for Resurfacing, Restoration, and Rehabilitation," TRB 1987.
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