Trimet bus

A TriMet 3100 series bus shown at the agency's Central Garage on July 22, 2013.

(The Oregonian/File)

Some TriMet buses have red, blinking lights on the front readerboard. What are they?

Those lights are a secret handshake between buses and traffic signals. They help late buses make up time by cruising through intersections rather than falling further behind at a red light.

TriMet started installing transit signal priority equipment in the early 2000s. Every bus is fitted with a transmitter, and more than 250 intersections in the Portland area are outfitted with receivers.

The system is similar to one used to clear the way for emergency vehicles. Here's how it works:

When a bus is behind schedule -- and only then -- its transit signal priority system switches on, beaming infrared signals ahead to those specially equipped traffic signals.

Those signals then either lengthen the green light to let the bus squeeze through or shorten the red light to let it get through faster. If the bus gets back on schedule, the system switches off again.

"These can help buses improve on-time performance, reliability and fuel savings," TriMet spokesman Andrew Longeteig said in an email. "It also can free up dollars for added service as opposed to spending money to add time to schedules in response to congestion and increased activity -- such as what we've seen on Division Street, Williams Avenue and Mississippi Avenue."

The infrared signals themselves are invisible. Newer buses, however, have a visible LED indicator.

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Have a commuting question? Contact Elliot Njus at enjus@oregonian.com or on Twitter @enjus