Filling the Gaps

Motorists approaching an entrance ramp along Massachusetts' well-traveled Route 128 recently were pleasantly surprised at the ease with which they could enter the expressway. Instead of the usual pile-up along the ramp, car after car moved effortlessly into the mainstream of traffic. Drivers barely glanced back to look for an opening. They did not have to. Like the flashlight of a theater usher, moving green lights at the edge of the ramp led them surely and safely up and out onto the highway.

The guidance system that performed this small miracle was developed by Raytheon and is being tested by the U.S. Department of Transportation. It consists of a computer, guidance lights, three traffic lights and magnetic sensing loops embedded in the outside lane of the freeway and in the entrance ramp. As cars move over the sensors, the computer learns how many are on the ramp and whether there are gaps available for them on the highway. When a space shows up, the computer begins lighting the string of green lights in sequence at the proper speed, producing a pacing light that moves up the ramp. The motorist need only follow the pacer light by about a car length; theoretically he will then reach the freeway in time to swing into the space allotted him. If no gaps are available, cars are stopped up to 15 seconds at one of the three traffic lights while the computer "looks" for an opening. Should there still be no space, a merge-with-caution sign advises the motorist to proceed on his own.

Unbelievably Successful. The system still has its own gaps. For example, a motorist who refuses or is unable to follow the pacer lights can frustrate the computer, which tries frantically to backtrack and pick up his car again. A second phase of the testing will involve a less complex arrangement of moving bands of green and white light on an electronic railing along the ramp; a driver who cannot or will not keep abreast of a green band (programmed, like the pacer lights, to deliver him to a predetermined slot in highway traffic) can either fall back and pick up another, catch up with one ahead or ignore the bands completely and go it alone.

Although the completed guidance system will not be ready for widespread installation for at least five years, those who kept pace with the pacers deemed the experiment unbelievably successful. "It is incredible," marveled Woburn Supermarket Checker Jeannette Gillis. "When you pull out, there isn't a car there." Most motorists, in fact, like Billerica Store Manager Bob Gaughan, found the system almost suspiciously painless. "I still had a tendency," Gaughan remembers, "to turn around and look at the oncoming traffic. Just to make sure."

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