CHARLESTON, S.C. - Tourists and residents fleeing the coast before a hurricane don't notice, but emergency officials can monitor how quickly they leave thanks to pavement sensors that count vehicles and calculate their speed.

There are now more than 120 pavement sensors on South Carolina highways, including on all the main hurricane evacuation routes.

In good weather, the sensors are used to count cars to help with highway planning. During storms, they can tell how well an evacuation is going.

"The Department of Transportation knows the roadway capacity and we can measure whether roads are at capacity or under capacity," said Highway Patrol Lt. Col. Harry Stubblefield, the state's traffic czar. "And we can monitor reductions in speed."

The sensors are similar to those that cause intersection traffic signals to change when cars approach.

This week, they have helped officials gauge traffic flowing into South Carolina along Interstate 95 from Florida and Georgia as residents and tourists flee from Hurricane Frances.

At midday Thursday, for example, the sensors told emergency officials that traffic on the interstate flowing into South Carolina was twice as heavy as normal.

The sensors are solar-powered and connected by phone line to Columbia where the information can be fed to the state Emergency Management Division, said Dick Jenkins, safety and systems engineer with the Transportation Department.

While highways in urban areas are monitored with cameras, the pavement sensors also are placed along rural stretches of highway, letting officials know how evacuations affect roads farther inland.

The state had some sensors before Hurricane Floyd in 1999. But after the traffic jams in the Floyd evacuation, sensors were installed on all evacuation routes, Jenkins said.

"We can determine how folks are responding to an evacuation order," he said.

For example, if the governor asked for a voluntary evacuation, the traffic counts would tell officials whether people were heeding the order or staying along the coast. That information could be factored into whether to order a mandatory evacuation.

The pavement sensors are used in other hurricane-prone states "although I'm not sure they have as many as we do," Jenkins said.

Gov. Mark Sanford ordered a mandatory evacuation of areas east of U.S. Highway 17 Business in Georgetown and Horry counties last month as Hurricane Charley bore down on the coast.

That evacuation affected an estimated 180,000 people and the eastbound lanes of U.S. Highway 501 into Myrtle Beach were reversed to let traffic out.

Besides pavement traffic counters, fixed and portable video cameras fed information to Columbia on how well the evacuation was going.

"We think it went very well," Stubblefied said. "Like anything we're going to go back and review the events that took place."

Traffic for the most part was heavy, but not bumper-to-bumper and moved steadily during the lane reversal.

"We think it was a very successful operation," Stubblefield said.