An electronic communication and control system has been developed as a prototype of advanced means of automatically modifying the switching of traffic lights to give priority to emergency vehicles. This system could be used alternatively or in addition to other emergency traffic-light-preemption systems, including a variety of systems now in use as well as two proposed systems described in “Systems Would Preempt Traffic Lights for Emergency Vehicles” (NPO-30573), NASA Tech Briefs, Vol. 28, No. 10 (October 2004), page 36. Unlike those prior systems that depend on detection of sounds and/or lights emitted by emergency vehicles, this system is not subject to severe range limitations. This system can be retrofitted into any pre-existing traffic-light control system, without need to modify that system other than to make a minimal number of wire connections between the two systems.

This system comprises several subsystems, including a transponder and interface circuitry on each emergency vehicle, a monitoring and control unit at each intersection equipped with traffic lights, and a wide-area two-way radio communication network that connects the emergency vehicles and intersection units. Computers in the various intersections and vehicle units run special-purpose software that implements the traffic- light-preemption scheme. The operations of the intersection and vehicle units are synchronized by use of Global Positioning System (GPS) timing signals. The transponder in each vehicle estimates its own position and velocity by use of GPS signals, deductive (“dead”) reckoning, data from the onboard diagnostic (OBD) computer of the vehicle, and/or triangulation of beacon signals.

When the operator of an emergency vehicle turns on its flashing lights and sirens in response to a request for an emergency response, the transponder unit goes into action, reading the OBD data to determine speed and acceleration, and reading and gathering further navigational data as described above. The position, velocity, and acceleration data are combined with vehicle-identification data in a prescribed format, and the resulting set of data is transmitted to the intersections within communication range of the transponder. In each intersection unit that receives such a data signal, a processor estimates the time of arrival of the vehicle, compares it with the estimated times of arrival of other emergency vehicles approaching the intersection, and determines which vehicle will arrive first. The intersection unit notifies the transponders of all emergency vehicles of a potential conflict and states, as part of the notice, which vehicle has the right of way. At the same time, the processor collects information on the current operation of the traffic lights at the intersection and calculates when pedestrians should be alerted not to cross and when preemption of the traffic lights should start. When preemption starts, the traffic lights are augmented by textual displays of a message that emergency vehicles are approaching and graphical displays indicating the direction(s) of approach. Once the emergency vehicles have passed through the intersection, normal operation of the traffic lights is resumed.

This work was done by Conrad Foster and Aaron Bachelder of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Electronics/Computers category. 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:

Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena, CA 91109-8099 (818) 354-2240 E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it. Refer to NPO-30604, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).

Unfortunately the TSP An Alternative for Emergency Preemption of Traffic Lights (reference NPO-30604) appears to be missing from our system.

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NASA Tech Briefs Magazine

This article first appeared in the September, 2006 issue of NASA Tech Briefs Magazine.

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