FHWA-SA-21-029

Roadside Design 

Improvements at Curves

Horizontal curves account for 27 percent of all fatal crashes and 80 percent of 

all fatal crashes at curves are roadway departure crashes.1

 Roadside design 

improvements at curves is a strategy encompassing several treatments that 

target the high-risk roadside environment along the outside of horizontal curves. 

These treatments can reduce roadway departure fatalities and serious injuries 

by giving vehicles the opportunity to recover safely and by reducing crash 

severity.

Roadside design improvements can be implemented alone or in combination, 

and are particularly recommended at horizontal curves—where data indicates 

a higher risk for roadway departure fatalities and serious injuries.

Roadside Design Improvements to 

Provide for a Safe Recovery

In cases where a vehicle leaves 

the roadway, having strategic 

roadside design elements, including 

an added or widened shoulder, 

flattened sideslopes, or a widened 

clear zone can provide drivers with 

an opportunity to regain control and 

re-enter the roadway in their lane or 

come to a safe stop before rolling 

over or encountering a fixed object.

• A clear zone is an unobstructed,

traversable roadside area that

allows a driver to stop safely or

regain control of a vehicle that has

left the roadway. Agencies should

avoid adding new fixed objects

such as trees and utility cabinets or

poles in the clear zone. AASHTO’s

Roadside Design Guide details the

clear zone width adjustment factors

to be applied at horizontal curves.

• Slope flattening reduces the

steepness of the sideslope to

increase drivers’ ability to keep the

vehicle stable, regain control of the

vehicle, and avoid obstacles. Slopes

of 1V:4H or flatter are considered

recoverable (i.e., drivers can retain

control of a vehicle by slowing or

stopping). Slopes between 1V:3H

and 1V:4H are generally considered

traversable, but non-recoverable

(i.e., errant vehicle will continue to

the bottom of the slope).

• Adding or widening shoulders

gives drivers more recovery area

to regain control in the event of a

roadway departure.

Roadside Design Improvements to 

Reduce Crash Severity

Since not all roadside hazards can be 

removed, relocated, or redesigned 

at curves, installing roadside barriers 

to shield unmovable objects or steep 

embankments may be an appropriate 

treatment. Three common types of 

roadside barriers are:

• Cable barrier is a flexible barrier

made from steel cables mounted

on weak steel posts. Flexible barriers

are more forgiving and have the

most deflection.

• Metal-beam guardrail is a semirigid barrier where a W-beam or

box-beam is mounted on steel

or timber posts. These deflect less

than cable barriers, so they can

be located closer to objects where

space is limited.

• Concrete barrier is a rigid barrier

that has little to no deflection.

Clear zone provided on the outside of 

the curve. Source: FHWA.

8%

reduction for 

single-vehicle crashes.2

Flatten sideslope from 

1V:3H to 1V:4H: 

12%

reduction for 

single-vehicle crashes.2

Flatten sideslope from 

1V:4H to 1V:6H: 

Safety Benefits:

1 Fatality Analysis Reporting System.

2 (CMF ID: 4627,4632) NCHRP Report 617: Accident Modification Factors for 

Traffic Engineering and ITS Improvements, (2008).

3 (CMF ID: 35,36) Elvik, R., and Vaa, T. Handbook of Road Safety Measures, (2004). 

22%

reduction for all crashes.3

Increase the distance to 

roadside features from 

3.3 ft to 16.7 ft:

44%

reduction for all crashes.3

Increase the distance to 

roadside features from 

16.7 ft to 30 ft:

OFFICE OF SAFETY

Proven Safety 

Countermeasures

For more information on this 

and other FHWA Proven Safety 

Countermeasures, please visit 

https://highways.dot.gov/

safety/proven-safety-counter 

measures and https:// 

highways.dot.gov/safety/

rwd/provide-safe-recovery/

clear-zones/clear-zones.