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Stumptown Stumper

by: L.E. BASKOW, Electric-field sensors in the ground, not the weight of a car, tell a left-turn signal that a car is waiting.

Every Friday in Stumptown Stumper, the Portland Tribune offers a trivia question and answer to help you boost your Rose City IQ.

Q: What makes the city's left-turn lane signal lights recognize that a car or bike is waiting to turn?

A: Contrary to what you might expect, the signals aren't triggered by the weight of a vehicle, but a cleverly designed system that involves electric fields.

Bill Kloos, the city's manager of signals and street lights, explains:

'Most of the left-turn signals are activated by inductive loop detectors buried in the pavement in the left-turn bay. Our contractor cuts a 6-foot diameter circle into the pavement about 3 inches deep, then imbeds wire into the slot. We call this a 'loop.' A wire is run from the loop to our traffic controller cabinet at the intersection. We use an electronic piece of equipment called a loop amplifier to establish an electric field around the loop in the pavement.'

He continues: 'When a vehicle (or any object that conducts electricity) moves through this electric field in the lane, the loop amplifier detects a change in inductance and tells the traffic controller that a vehicle is present. Simple and reliable.'

Kloos said there is a trick for bicyclists, who may find it difficult to activate the signal sometimes. The issue isn't whether the bike has enough conductive material - most do, he said.

The issue is making sure the bike waits on the spot over the loop that provides the most sensitivity - the 'sweet spot,' so to speak.

At many locations, Kloos said, the city has placed bike markings on the appropriate waiting spot.

In places without markings, cyclists should try to position their wheels in the center of the lane, and if that doesn't work, try moving the bike a few feet to the side and leaning it toward the center of the lane.

Next week's Stumper: A Portland father wants to know: What's the etiquette on spitting in the gutter of a public pool?

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