Variable Speed 

Limits

Selecting appropriate speed limits on roadways is important in maintaining 

a safe and efficient transportation network. Speed limits are established with 

an engineering study based on inputs like traffic volumes, operating speeds, 

roadway characteristics, and crash history. However, conditions on the roadway 

are susceptible to change in a short amount of time (e.g., congestion, crashes, 

weather). Drivers typically determine their operating speeds under normal 

weather conditions on a straight roadway section with good pavement quality 

and adequate sight distances. If ideal conditions do not exist and the roadway 

does not meet the driver’s expectations, there is a greater chance that a driver 

error could result in a crash. Providing variable speeds limits (VSLs) capable of 

adapting to changing circumstances could reduce crash frequency and severity.

Speed management strategies, including VSLs, are integral to the Safe Speeds 

element of the Safe System Approach. Because humans are unlikely to survive 

high-speed crashes, VSLs reduce speeds so that human injury tolerances are 

accommodated in three ways: improving visibility, providing additional time for 

drivers to stop, and reducing impact forces. 

Applications

VSLs use prevailing information on the 

roadway, like traffic speed, volumes, 

weather, and road surface conditions, 

to determine appropriate speeds 

and display them to drivers. This 

strategy improves safety performance 

and traffic flow by reducing speed 

variance (i.e., improving speed 

harmonization). VSLs may also improve 

driver expectation by providing 

information in advance of slowdowns 

and potential lane closures, which 

could reduce the probability for 

secondary crashes. VSLs can mitigate 

adverse weather conditions or to slow 

faster-moving traffic as it approaches 

a queue or bottleneck.

Agencies can implement VSLs for 

the following applications:

CONGESTION INCIDENTS

WORK ZONES INCLEMENT WEATHER

Considerations 

• Particularly effective on urban and

rural freeways and high-speed arterials

with posted speed limits greater than

40 mph.

• Often implemented as part of Active

Traffic Management (ATM) plans

or incorporated into existing Road

Weather Information Systems.

• When used with ATM, VSLs can

mitigate rear-end, sideswipe,

and other crashes on high-speed

roadways.

• May be implemented as a regulatory

and/or an advisory system.

• Can be applied to an entire

roadway segment or individual lanes.

Source: WSDOT

1 (CMF ID: 11002,11005,11003) Avelar et al. Developing Crash Modification Factors 

FHWA-SA-21-054 for Variable Speed Limit. FHWA, (2020).

OFFICE OF SAFETY

Proven Safety 

Countermeasures

Safety Benefits:

VSLs can reduce crashes 

on freeways up to:

34%

for total crashes.1

65%

for rear-end crashes.1

51%

for fatal and injury crashes.1

Benefit/Cost Ratios 

range between1

9:1- 40:1

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/

speed-management/reference-materials.