Strobe-enhanced traffic signals have been developed to aid in the preemption of road intersections for emergency vehicles. The strobe-enhanced traffic signals can be incorporated into both new and pre-existing traffic-control systems in which the traffic-signal heads are of a relatively new type based on arrays of light-emitting diodes (LEDs). The strobe-enhanced traffic signals offer a less expensive, less complex alternative to a recently developed system of LED-based warning signs placed next to traffic signals. Because of its visual complexity, the combination of traffic signals and warning signs is potentially confusing to motorists. The strobe-enhanced traffic signals present less visual clutter. In a given traffic-signal head, the strobe-enhanced traffic signal is embedded in the red LED array of the "stop " signal. Two strobe LED strips — one horizontal and one vertical — are made capable of operating separately from the rest of the red LED matrix. When no emergency vehicle is approaching, the red LED array functions as a normal "stop "signal:all the red LEDs are turned on and off together. When the intersection is to be preempted for an approaching emergency vehicle, only the LEDs in one of the strobe strips are lit, and are turned on in a sequence that indicates the direction of approach. For example (see figure), if an emergency vehicle approaches from the right, the strobe LEDs are lit in a sequence moving from right to left.

The LEDs in the Horizontal Strobe Strip are lit in sequence from right to left to indicate that an emergency vehicle is approaching from the right.

Important to the success of strobe-enhanced traffic signals is conformance to city ordinances and close relation to preexisting traffic standards. For instance, one key restriction is that new icons must not include arrows, so that motorists will not confuse new icons with conventional arrows that indicate allowed directions of movement. It is also critical that new displays like strobe-enhanced traffic signals be similar to displays used in traffic-control systems in large cities.For example, Charleston, South Carolina uses horizontal strobes on red traffic lights to alert motorists and thereby help motorists not to miss red lights.The one significant potential disadvantage of strobe-enhanced traffic lights is initial unfamiliarity on the part of motorists.

This work was done by 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:

Intellectual Assets Office
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Refer to NPO-30716, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Strobe Traffic Lights Warn of Approaching Emergency Vehicles

(reference NPO-30716) is currently available for download from the TSP library.

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