Tech Briefs

System Would Predictively Preempt Traffic Lights for Emergency Vehicles

These systems could utilize traffic-light infrastructure already in place.

Two electronic communication-and- control systems have been proposed as means of modifying the switching of traffic lights to give priority to emergency vehicles. Both systems would utilize the inductive loops already installed in the streets of many municipalities to detect vehicles for timing the switching of traffic lights. The proposed systems could be used alone or to augment other automated emergency traffic-light preemption systems that are already present in some municipalities, including systems that recognize flashing lights or siren sounds or that utilize information on the positions of emergency vehicles derived from the Global Positioning System (GPS). Systems that detect flashing lights and siren sounds are limited in range, cannot “see” or “hear” well around corners, and are highly vulnerable to noise. GPS-based systems are effective in rural areas and small cities, but are often ineffective in large cities because of frequent occultation of GPS satellite signals by large structures. In contrast, the proposed traffic-loop forward prediction system would be relatively invulnerable to noise, would not be subject to significant range limitations, and would function well in large cities—even in such places as underneath bridges and in tunnels, where GPS-based systems do not work.

One proposed system has been characterized as “car-active” because each participating emergency vehicle would be equipped with a computer and a radio transceiver that would communicate with stationary transceivers at the traffic loops (see figure). Whenever a vehicle was detected passing over a traffic loop, the loop transceiver, possibly using the loop as an antenna, would transmit a signal identifying the location of the loop and the direction of travel. (Traffic-loop equipment that performs this function is already commercially available.) If the vehicle passing over the loop were a participating vehicle, its transceiver would receive the position signal. The computer in the vehicle would use the signal and the time of its receipt as a time-and-position fix in a continuous “dead reckoning” estimation of the current position of the vehicle as function of speed and compass heading. At time intervals of 1 second, the transceiver would broadcast the updated estimate of position to loop receivers at neighboring intersections. The stationary portion of the system would determine, on the basis of the updates, whether the vehicle was likely to pass through a given intersection within a suitable amount of time (typically of the order of 1 minute), in which case the system would preempt the switching of traffic lights at the intersection.

Traffic Lights Would Be Preempted for emergency vehicles by a “car-active” or a “car-passive” system.Both systems would utilize inductive traffic loops already installed in the streets of many municipalities.
The other proposed system has been characterized as “car-passive” because a passive radio transponder would be installed on the underside of a participating vehicle. When passing over a traffic loop, the transponder would be energized by a signal, at a frequency of 450 MHz, radiated via the loop. When so energized, the transponder would transmit a vehicle identificationnumber signal at a carrier frequency of 900 MHz. Unlike in the car-active system, there would be no continuous estimation of vehicle position. Instead, traffic lights would be preempted on the basis of simple proximity detection. For this purpose, upon detection of a participating vehicle at a loop at a given intersection, the detection would be signaled to neighboring intersections. Traffic lights at the next intersection or next few intersections through which the vehicle could be expected to pass could then be preempted until the vehicle passed through or until a specified time elapsed.

This work was done by Aaron Bachelder and Conrad Foster of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Intellectual Assets Office
JPL, Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240
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Refer to NPO-30573, volume and number of this NASA Tech Briefs issue, and the page number.

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