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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies for Design Exceptions - July 2007

List of Figures


Comparison of nominal and substantive concepts of safety

NCHRP Report 500 series

Steps in the design exception process

Advanced technology is making the collection of in-service data more effective and more efficient

Because it is a design control, design speed affects the curvature, stopping sight distance, superelevation, and other features of this horizontal curve

Accident Modification Factors for lane width on rural two-lane highways

Shoulders on this urban freeway provide enough width for crash avoidance, storage of disabled vehicles, maintenance activities, and enforcement

Partially-paved shoulders on this rural arterial improve bicycle accommodation and reduce risky passing maneuvers

Pavement edge drop-off

Comparison of how shoulder width affects stopping sight distance past concrete bridge rail along horizontal curves

Accident Modification Factors for shoulder width on rural two-lane highways

Bridge width

Vehicle shying towards the centerline on a narrow bridge

Horizontal alignment


Horizontal curve at the base of a steep grade

Vertical stopping sight distance at a crest vertical curve

Headlight sight distance at a sag vertical curve

Sight distance at an undercrossing

Horizontal stopping sight distance

Not all locations with limited stopping sight distance are the same in terms of safety risk

Stopping sight distance profile

Cross-slope break on the high side of a superelevated curve

Interstate closure after an impact with a bridge

Lateral offset to obstruction is an operational offset and is not the same as clear zone

Signs can be used to warn drivers in advance of a change in lane width

Wide pavement markings

Recessed pavement markings

Raised pavement markings

Post-mounted delineators


Shoulder rumble strips

Centerline rumble strips

Painted edgeline rumble strips

Partially paved shoulders

Safety edge (top) and after the shoulder has been graded over the edge (bottom)

Fixed object removal. Separate box culverts were extended, connected, and covered at this interchange

Breakaway light poles

Shielding fixed objects with barrier

Pull-off area on the inside of a horizontal curve

Signs can be used to warn drivers in advance of a narrow bridge

Reflector tabs on guardrail

Object markers and post-mounted delineators at a narrow bridge

White concrete bridge rail

Anti-icing system on a bridge

Bridge rail and guardrail transition in compliance with NCHRP Report 350

Turn warning sign with flashing beacon

Curve warning sign.

Dynamic curve warning system

Delineation with large chevrons

Delineation with post-mounted delineators

Delineations with reflectors on barrier

Advance warning of a steep grade

Climbing lane

Truck escape ramp

Continuous drainage system

Sign for crest vertical curve with inadequate stopping sight distance

Intersection warning sign with flashers activated by vehicles entering from the side road

A STOP sign with a flashing beacon improves visibility of the sign at this intersection with limited vertical sight distance

Intersection lighting


Longitudinal texture applied to fresh pavement to improve surface friction

Transverse grooving to improve surface drainage and friction

An example of transitioning the cross slope of the shoulder to mitigate a cross-slope break greater than 8%

Vertical clearance signing

Vertical clearance signing

Warning sign with hanging chimes installed at the same height as the vertical clearance of the structure

Reflective sheeting on utility poles

Reflective sheeting on utility poles

The project runs through Des Moines, Iowa

Approximate project limits of design exception

Interstate 235 before reconstruction

Interstate 235 after reconstruction

Cross section within the unconstrained locations

The unconstrained cross section

Cross section within the area of restricted width

Narrower lane widths would place the lane lines in a different transverse position than the underlying longitudinal joints

Inside lane and shoulder widths within the constrained areas were narrowed

Lighting was placed on the outside of the freeway through the design exception area

Lighting was placed down the center of the median where more cross-sectional width was available

The project is located in north-central Wyoming

Approximate project limits

The steep cut on the north side of the highway and deep canyon on the south illustrate the difficult terrain faced by designers

The Tensleep-Buffalo Highway leads into the Big Horn National Forest in the Rocky Mountains

A massive retaining wall illustrates the difficult site constraints that were encountered

Signing for the steep grades is provided throughout the project

Electronic signing provides enhanced warning for the non-standard horizontal curves

Advance signing for the truck escape ramp

Advance signing for the brake-check turnout

A pull-off area provides tourist information and spectacular views

Runaway truck ramp with arrestor system

Runaway truck ramp

Electronic signing provides advance warning when the truck ramp is closed

Climbing lane

Guardrail on the outside of a horizontal curve

State Route 99 is an urban arterial in Seattle, Washington

State Route 99 parallels Interstate 5 through the Seattle metro area

SR 99 before and after reconstruction in Des Moines

SR 99 before and after reconstruction in SeaTac (top) and Shoreline (bottom)

Drawing of proposed improvements to SR 99 in Shoreline

Left-turn lane and U-turn areas after reconstruction in Federal Way

New transit stop in Des Moines

SR 99 before and after reconstruction in Des Moines

Median tree plantings along SR 99 in SeaTac

Impact with tree in median (SeaTac)

Information from City maintenance personnel provided data on tree impacts that were not reported

When the in-service evaluation showed that many tree hits were occurring at the narrow-median locations adjacent to turn lanes, trees were no longer
planted in these areas in subsequent phases of the project (SeaTac)

A low-profile barrier was used along the median of SR 99 in Des Moines.

Sloped-down end section adjacent to turn lane

Two low-profile barriers have passed NCHRP Report 350 test-level 2 (45 mph) crash testing

Los Angeles, California

State Route 110. The Arroyo Seco Parkway

SR 110 shortly after construction in 1940

Historic structures along the Arroyo Seco Parkway

The San Gabriel Mountains on the horizon illustrate the Parkways scenic context

The Arroyo Seco Parkway has a narrow cross section, a curvilinear alignment, and non-standard interchange geometry

The Arroyo Seco Channel runs adjacent to the Parkway, constraining its width on one side

Single-slope concrete median barrier at a horizontal curve

Pull-off areas provided periodically along the outside lanes

Enhanced delineation with raised pavement markers and pavement markings with high retroreflectivity

Warning signs for curvature, slowing traffic, and vertical clearance

Geometry at the State Street Exit Ramp


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