– When nasty weather or heavy traffic congestion pops up ahead, drivers in a growing number of states may see electronic signs showing speed limits that are lower than usual.

The idea behind these variable speed limits, which can change depending on road, weather or traffic conditions, is to make traffic flow more smoothly and give drivers more time to react to changing conditions. The intent is to help prevent crashes such as rear-end or lane-changing collisions.

Some states say the high-tech systems have helped curb congestion and make the roads safer. But some critics question whether the costly technology is worth the investment, while others view the signs as speed traps. At least one state — Missouri — dumped variable speed limits after drivers and law enforcement gave them a thumbs down.

More than a decade ago, only a small number of states used variable speed limits. Today, at least 15 states, from Maine to Washington, deploy them on certain highways when there is congestion, inclement weather or both.

Here’s how they work: Road sensors gather information about traffic speed, volume, weather and road conditions. The data is transmitted to a traffic operations center and analyzed using an algorithm or a review by staffers, who make decisions about what speed limits should be posted.

In some states, transportation officials have the authority to decide whether to adopt these changeable speed limits on their own. In others, they need legislative approval.

Some states say variable speed limits have improved road safety.

In Wyoming, which started using them in 2009 during winter storms or vehicle emergencies, the changeable signs made a “dramatic difference” within a year, said Vince Garcia, a state Department of Transportation ­program manager.

“We have a lot of severe weather conditions, and we were consistently having a number of large multi-vehicle crashes in certain trouble-spot areas,” said Garcia, who pointed to a 2010 University of Wyoming study that found road closures had dropped and crashes were the lowest in 10 years in the year after the signs were installed.

The program was so successful that the state expanded it to several other highways over the next seven years, Garcia said. And the public hasn’t raised a stink about the signs, which can gradually reduce the speed on some interstates from 75 mph to 45 mph in a bad storm, he said. “When people can see a reason for a speed reduction, they’re more ­tolerant.”