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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 DistanceRevised braking distance portion of equation

Now based on deceleration ( a ) rather than friction factor ( f )

Adopted a - 3.4 m / sec^{2}[ 11.2 ft / sec^{2}]

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

BicyclesAdded emphasis throughout

AASHTO- Guide for the Development of Bicycle Facilities 1999