ation for planning efficient routes or to improve future road designs, says Benjamin Coifman, assistant professor of electrical engi- neering and civil and environmental engineering at Ohio State University. Many drivers have probably noticed the buried detectors, called loop detec- tors, at intersections. The devices are marked by the square outline that road crews create when they insert a loop of wire into the roadbed. When a car stops over the loop, a signal travels to a control box at the side of the road, which tells the traffic light to change. Although loop detectors are not much more than metal detectors, they collect enough information to indicate the general speed of traffic. With Coifman's software, a small amount of roadside hardware, and a single PC, a city could use the detectors to significantly improve traffic monitoring without interfering with drivers. That's important, because good traffic man- agement can't be obtrusive. "If transportation engineers are doing their job well, you don't even realize they've improved travel conditions," Coifman says. Coifman's algorithms capture a vehicle's length as it passes over a detec- tor. Once the car or truck passes over the next loop, the computer matches the two signals and calculates the vehicle's travel time. Based on the travel times, the software can spot emerging traffic jams within 3.5 minutes. Because driver behavior isn't predictable, the algorithms take many human factors into account. Among other parameters, Coifman considered people changing lanes, entering and exiting from ramps, and "rubbernecking"- the delay to drive time caused by people who slow down to look at accidents or other events. "Traffic is a fluid like no other fluid," says Coifman. "You can think of cars as particles that act independently, and waves propagate through this fluid, moving with the flow or against it." THE UNBLINKING EYE-SECURITY AND SURVEILLANCE After an accident, it may take a long time for the telltale wave of slow- moving traffic to propagate through the det