INTRODUCTION

Horizontal curves account for more than 25 percent of fatal crashes and the average crash rate for curves 

is approximately three times higher than tangent segments.1

 Transportation agencies have a number of 

horizontal curve safety countermeasures at their disposal to improve the safety of collision-prone curves. 

This flyer introduces a family of curve solutions that are underutilized but still highly effective, and can be 

considered if other treatments do not address site- or crash- specific needs. The following sections will 

demonstrate that these countermeasures may be within the agencies’ scope or ability to deploy.

Curve Safety Solutions

Roadway 

Departures

List of Treatments

SIGNS OR DEVICES

DYNAMIC SPEED FEEDBACK SIGN (OR SPEED ACTIVATED SIGNS)

Dynamic speed feedback signs are typically radar-activated roadside signs 

that display a vehicle’s current speed in relation to the posted speed limit. 

As a vehicle approaches at a speed higher than the advisory speed, the 

dynamic portion of the sign alerts drivers. This creates an opportunity to raise 

drivers’ awareness. These devices can be portable or permanent depending 

on the agency’s requirement for a particular location. 

Application Conditions Limitations

 Typically installed in

transition zones to

slow traffic upstream

in advance of curves or

lower speed areas.

 Requires increased maintenance

and a power source.

 Drivers may become immune if

overused and with no perception

of further enforcement.

 Single directional results unless

posted on both travel directions.

Safety Benefits

 CMF = 0.93 (For 2-lane undivided roads)

o Applicable to all crash types and severities.

o Studies have shown that dynamic feedback reduces average

speeds by 5 mph and 85th percentile speeds by 7 mph,

which can help drivers navigate curves more safely.

Source: Dwernertl at the English 

Wikipedia, CC BY-SA 3.0

Figure 1. Dynamic 

speed feedback sign.

Cost: This treatment has a 

high initial cost (compared 

to standard signing).

1 FHWA, Horizontal Curve Safety, https://safety.fhwa.dot.gov/roadway_dept/countermeasures/horicurves/. http://safety.fhwa.dot.gov
DYNAMIC CURVE WARNING SYSTEMS

Dynamic curve warning systems can take a variety of forms. 

One example combines a speed measuring device (such as 

loop detectors or radar) with flashing beacon and a warning 

sign (see Figure 2). It works by measuring the speeds of 

approaching vehicles and providing either static or dynamic 

messaging to drivers traveling too fast. 

Another related treatment is known as the Sequential 

Dynamic Curve Warning System (SDCWS) as shown in Figure 

3. This system includes chevron signs with solar powered

flashing lights embedded in the sign. The lights can be 

programmed to flash in different patterns in order to grab 

the driver’s attention.

Application Conditions

 Locations with poor performance especially those

involving fatalities and injuries.

 SDCWS have been implemented as a countermeasure on

two-lane rural and multilane urban highway curves as a

means to reduce vehicle speeds and improve curve

delineation.

Limitations

 Even the least expensive dynamic curve warning system

is significantly more costly than static signs.

o As such, agencies may limit their application to

locations where other less expensive devices have

failed to solve the issue.

Safety Benefits

 CMF = 0.56 (For SDCWS)

o Applicable to all crash types and severities.

o Studies have shown speed reductions as high as

8.8 mph.

Source: FHWA.

Source: FHWA.

Figure 2. Flashing beacon 

with warning sign.

Figure 3. Example of sequential 

dynamic curve warning system.

Cost: 

o Cost of these systems varies

depending upon the specific 

design and the availability of 

a power source. 

o Overall, this countermeasure

is moderate cost.

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MARKINGS

OPTICAL SPEED BARS (OR SPEED REDUCTION MARKINGS) 

Optical speed bars are transverse stripes on the roadway spaced 

at gradually decreasing distances. They are used to increase 

drivers’ perception of speed, causing them to slow down. As 

spacing between bars slowly shortens, drivers feel they are 

going faster and will reduce speed accordingly. 

Application Conditions

 Reserved for unexpected horizontal or vertical curves or

other feature where drivers need to decelerate in advance of

the feature.

 Supplements, and does not substitute for, appropriate

warning signs and other traffic control devices.

Limitations

 Only allowed where longitudinal lines are present on both

sides of the lane.

 Requires maintenance due to extensive exposure to traffic

over time.

 Not intended for use on long roadway sections.

 Overuse could jeopardize the visual effect of the

countermeasure.

Safety Benefits

 No CMF associated with this countermeasure.

 The 2016 Low-Cost Treatments for Horizontal Curve Safety

found speed reduction for this safety countermeasure and

the details can be found in the report

https://highways.dot.gov/sites/fhwa.dot.gov/

files/2022-06/fhwasa15084rev011720_508_FINAL.pdf

Source: FHWA.

Source: @Getty Images, Inc.

Figure 4. Optical speed bars.

Cost: This is a low cost treatment.

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SPEED ADVISORY MARKINGS IN LANE

This treatment involves in-lane pavement markings 

that supplement curve warning signs, by providing the 

same information in the driver’s direct line of sight, 

re-emphasizing the message to the driver. 

Application Conditions

 Higher speed roads where curve advisory speed is

significantly lower than the posted speed.

 Curves where crash reports indicate speed-related issues.

 Corridors where speed studies indicate excessive

speeding.

 Rolling or mountainous terrain where the horizontal

curve may be hidden by a vertical crest.

Cost: This is a low cost treatment.

Source: FHWA.

Figure 5. Design guidelines for 

pavement markings.

Limitations

 Requires maintenance due to extensive exposure to traffic over time.

Safety Benefits

 CMF = 0.652 (For 2-lane undivided roads)

o Applicable to all crash types and severities.

 Studies have shown that speed advisory markings significantly reduce vehicle speeds at the entrance to

the curve.

o Refer to NCHRP 600 Human Factors Guidelines for Road Systems for further details.

FOR MORE INFORMATION

 Low-Cost Treatments for Horizontal Curve Safety 2016. FHWA-SA-15-084.

https://safety.fhwa.dot.gov/roadway_dept/countermeasures/horicurves/fhwasa15084/

 CMF Clearinghouse. http://www.cmfclearinghouse.org/index.cfm

 NCHRP Report 600: Human Factors Guidelines for Road Systems: Second Edition.

http://www.trb.org/Main/Blurbs/167909.aspx

 FHWA, Roadway Departure, Horizontal Curve Safety.

https://safety.fhwa.dot.gov/roadway_dept/countermeasures/horicurves/

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CONTACT

Joseph Cheung

Joseph.Cheung@dot.gov

http://safety.fhwa.dot.gov FHWA-SA-21-092