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Chapter 3
Elements of DesignSight Distances
Transition Designs
Urban Street Design
Offtracking
Horizontal Alignment
Turning Roadway Design
Grades
Vertical Alignment
Stopping Sight Distance
Revised braking distance portion of equation
Now based on deceleration ( a ) rather than friction factor ( f )
Adopted a - 3.4 m / sec2 [ 11.2 ft / sec2 ]
A single value for each design speed replaces the range of values used previously.
Stopping Sight Distance on Grades
- Braking distance equation for grades modified to use deceleration rate (a)
Metric
- Reaction time must be added to braking distance from equation
- Exhibit 3 - 2 incorporates adjusted values
Other Sight Distances
- Decision Sight Distance
- Equations added to discussion
- Exhibit 3-3 modified - reaction times added
- Passing Sight Distance
- Exhibit 3-7 expanded to 130 km/h [ 80 mph ]
Criteria for Measuring
SSD Eye Height -- 1080 mm [ 3.5 ft ]
Object Height -- 600 mm [ 2.0 ft]PSD Eye Height -- 1080 mm [ 3.5 ft ]
Object Height -- 1080 mm [3.5 ft ]Trucks Eye Height- 2330 mm [ 7.6 ft ]
Superelevation
- Unchanged except for minor editing
- NCHRP 439 Superelevation Distribution Methods and Transition Designs - under consideration for next edition of Green Book
Transition Design Controls
- 3 transition methods discussed
- Tangent to curve
- Spiral to curve
- Compound curve
- Eliminated 2 - second rule
- Equation for minimum length of runoff provided
- Exhibit 3 - 27 Maximum Relative Gradients refined
Superelevation Runoff
- Exhibit 3 - 28 provides adjustments for runoff for number of lanes rotated
- Lengths vary proportionally to 3.6 m [ 12 ft ] lane width
- Exhibit 3 - 29 based on 3.6 m [ 12 ft ] width
- Discussion on the Location of Runoff revised
- Exhibit 3 - 30 added providing allocations
Tangent Runout
- Added section on runout
- Equation provided to determine minimum length
- Lengths listed in Exhibit 3 - 29
Spiral Curve Transitions
- Corrected metric equation for minimum length
- Added two exhibits
- Exhibit 3 - 33 Maximum radius for use of spiral
- Exhibit 3 - 34 Desirable length of spiral
- Added equations for minimum and maximum length of spiral
Transitions - Miscellaneous
- Exhibit 3 - 31 and discussion added on limitations to the use of superelevation rates
- Axis of Rotation with Medians replaces Runoff with Medians
- Section added on minimum transition grades
- Section added on Turning Roadway Design
Urban Street Design
- Upper limit for low-speed urban design now 70 km/h [ 45 mph ] rather than 60 km/h [ 40 mph ]
- Exhibits 3 - 40 and 3 - 41 expanded to include 20 km/h [ 15 mph ] and 70 km/h [ 45 mph]
Offtracking
- Widths of turning roadways and widening of traveled way combined into section on Offtracking
- Exhibit 3 - 51 based on WB - 15 [ WB - 50 ]
- Exhibit 3 - 52 added to provide adjustment factors for other design vehicles
- Tabular values based on dimensions in Chapter 2
Horizontal Alignment
- New height of eye and object incorporated into discussion.
- Exhibit 3 - 57 redone based on single stopping sight distance value.
- Passing sight distance discussion uses new height of eye and object
Horizontal Alignment
Speed M M 94 GB [90 GB] 2001 GB 80 km/h
[50 mph]7.9 - 12.0 m
30.3 - 42.5 ft10.5 m
[ 34.1 ft ]M = middle ordinate, centerline inside lane to sight obstruction
M shown is for horizontal curve with radius of 200 m [ 656 ft ]
Grades
- Speed - Distance Curves:
- Developed for 120 Kg/kW [ 200 lb/hp ]
- Critical length of grade adjusted to reflect design vehicle
Grades
- Climbing lanes - capacity discussion edited to eliminate duplicating HCM
- Passing Opportunities on 2 - lane Roads
- Passing lanes and 3 - lane sections combined
- Exhibit 3 - 68 Lengths of Turnouts revised
Vertical Alignment
- Length of vertical curve equations revised to incorporate new height of eye and height of object.
- Exhibits 3 - 75 and 3 - 76, K values
- Recalculated using new eye and object height
- Based on single value of stopping sight distance
2001 values are lower due to changes in height of eye ( 1070 mm to 1080 mm ) and height of object ( 150 mm to 600 mm ). Since L = K x A, length of vertical curves will be shorter.
Vertical Alignment
Speed 94 GB [90 GB] 2001 GB SSD K SSD K 80 km/h
[50 mph]112.8 - 139.4 m
[400 - 475 ft]32 - 49
[110 - 160]130 m
[425 ft]26
[84]K = rate of vertical curvature ( length ( m ) per % of A ) or [ length ( ft ) per % of A ]
K shown is for crest vertical curves
Since L = K x A, length of vertical curves will be shorter.
Vertical Alignment
- Equations for passing sight distance revised to incorporate new eye and object heights
- Exhibit 3 - 77 passing sight distance revised to reflect changes
- Exhibits 3 - 78 and 3 - 79 length of sag vertical curves recalculated using single value for stopping sight distance
Miscellaneous
- Emergency Escape Ramp bed depth increased to 1 m [3 ft]
- Sight distance at undercrossings added
- Discussion on fencing added
- Combined traffic control devices under one heading
Chapter 4
Cross Section ElementsSurface type
Curbs
Medians
Pedestrians
Bicycles
Curbs
- Section rewritten
- Visibility stressed
- Terminology
- Barrier replaced by vertical
- Mountable replaced by sloping
Miscellaneous
- Intermediate surface type deleted
- Discussion on medians updated
- Paragraph on grades of parking areas in Park - and - Ride Facilities revised
Pedestrians
- Separate topic on Sidewalks consolidated with Pedestrian Facilities
- Made compatible with proposed ADAAG standards and guidelines
- Sidewalk curb ramps
- Terminology changed
- Basic dimensions
Pedestrian References
AASHTO - Guide for the Planning, Design, and Operation of Pedestrian Facilities (forthcoming)
FHWA - Designing Sidewalks and Trails (2 parts)
PROWAAC - Building A True Community
Access Board - Accessible Rights-of-Way: A Design Guide
Bicycles
Added emphasis throughout
AASHTO - Guide for the Development of Bicycle Facilities 1999
