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# THE 2001 GREEN BOOK

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Chapter 3
Elements of Design

Sight Distances
Transition Designs
Urban Street Design
Offtracking
Horizontal Alignment
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.

• 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
• 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

• Equation provided to determine minimum length
• Lengths listed in Exhibit 3 - 29

Spiral Curve Transitions

• Corrected metric equation for minimum length
• 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

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 ft
10.5 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 ]

• Speed - Distance Curves:
• Developed for 120 Kg/kW [ 200 lb/hp ]

• 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
• Combined traffic control devices under one heading

Chapter 4
Cross Section Elements

Surface 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
• 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