Letting Traffic Lights Talk to Each Other

Employing a wireless technology developed by the military to hide radio signals, New York State's Department of Transportation is bringing traffic light synchronization to several upstate cities-- particularly to streets where the infrastructure for traditional traffic-data transmission is unavailable. Along Troy's Hoosick Street, for instance, cables that would ordinarily be used to connect signals along the road with central computers are inaccessible to engineers.

To bring an element of intelligent traffic systems to that roadway, DOT engineers will use spread-spectrum radio. It works by rapidly switching among a range of radio frequencies that do not require an FCC license. This "frequency hopping" allows the signal to avoid interference.

"It's much easier to install and eliminates many problems," says David Mitchell, a signals specialist in the Department of Transportation in Texas, where the system is in widespread use. The Texas DOT is, in fact, systematically replacing hardwired communications on its roads with spread-spectrum radio.

Wireless communication infrastructure is politically popular since it can be installed quickly and without major roadwork. With no cables for utility workers to accidentally sever, there are fewer disruptions of service. Radio transmitters use little energy and can often be powered by solar panels. And at $800 to $1,800 for each unit, spread- spectrum radios are a bargain compared with installing cables or leasing existing lines from telecom firms.

"It's much more economical than to dig a trench, put in a pipe and feed a wire through," says Paul Mayer, a New York State DOT engineer who is working on the Hoosick Street spread-spectrum installation.