Variable speed limit signs improve safety on highways, school zones, and more

3 min. read
November 18, 2020
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Whether you’re driving along a high-speed expressway in California or a quiet residential street in Maine, there’s one thing you’re bound to encounter: speed limit signs. Typically positioned at the roadside every few miles, these familiar black-and-white signs communicate the legal maximum speed that vehicles should travel under ideal conditions.

That last part is important. While 50 mph might be appropriate on a dry road with good visibility, when snow starts to fall or traffic starts to snarl, reductions to posted speed limits are often desirable, if not critical.

This is where a variable speed limit sign (VSL) comes in. By using VSLs in place of static signage, transportation officials can dynamically adjust posted speed limits to better reflect current conditions, be they related to poor weather, road work, congestion, or something else.

Here are some of the ways VSLs are being used around the world to increase safety on a wide variety of road types.

Mitigating congestion on high-volume corridors

Studies have shown that traffic flowing more slowly, smoothly, and consistently can help drivers get to their destinations more quickly and safely. To achieve this, the Washington State Department of Transportation (WSDOT) has added variable speed signs along a number of busy corridors that direct drivers to incrementally reduce their speed when volumes are building and congestion is likely. According to WSDOT, “[VSLs] decrease last second avoidance maneuvers and panic braking, which are both primary factors that contribute to collisions.”

Accounting for weather on high-speed highways

The stretch of I-80 that traverses Utah’s Parleys Canyon is notorious for its low visibility and ice during the winter months. Engineers at the Utah Department of Transportation (UDOT) knew the posted speed limit of 65 mph was much too high under these conditions, which is why they moved to replace 15 of their static signs with variable speed limit signs in 2014. Now, personnel can monitor conditions and adjust the speed accordingly— from 65 all the way down to 35 mph, according to UDOT.

Anticipating students and increasing safety in school zones

While the weather can be hard to predict, there are some conditions you can plan for, such as increased pedestrian activity in school zones at times when children are travelling to and from school. In New Zealand, it has become standard to use variable speed limit signs that substantially reduce speed limits during peak travel periods. Though static signs with restricted hours serve the same purpose, drivers often ignore or fail to see them. According to a report from the New Zealand Transport Agency (NZTA), “Variable signs, which are displayed only when relevant, offer a way in which this drawback can be minimized and may actually enhance driver acceptance of any restriction imposed.”

>>> VSLs can be paired with flashing beacons for increased awareness and safety in school zones.

Keeping crews safe in work zones

Resurfacing a highway or replacing a water main is gruelling work, and it’s certainly not helped by cars whizzing past—even if they are traveling within the legal speed limit. One way to improve the safety of workers in these zones is with VSLs, as demonstrated by the Georgia Department of Transportation (GDOT). Since most roadway construction in the state is done at night when traffic is lighter and vehicles are moving faster, traffic managers routinely dial down speed limits after dark, increasing safety for road crews, while preserving regular speed limits during the day.

Want to learn more about variable speed limit signs and how they can be used to support traffic calming and other safety initiatives in your region? Click the link below.


Learn more about SpeedCheck variable speed limit signs

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