Q: There were recent questions and answers about traffic-light sensors embedded in the road. What do sensors actually detect? Magnetics? Weight? Vibration?

I once got behind a horse-drawn wagon at such an intersection. The horses and wagon did not trigger the lights. A wagon rider had to go out into the intersection and stop traffic, as if he were a traffic cop, and allow the wagon to proceed.

A: Traffic signal sensors are made of wires buried in the asphalt in a rectangular shape, typically 6 feet wide by 40 feet long. At most intersections the perimeter of the sensor is easy to see.

Electricity is run through the wires and this generates an electromagnetic field. When a vehicle interrupts this field, the light’s control system detects this and understands there is a vehicle waiting for the green light.

These magnetic fields emanate from the wire in concentric circles, and would look like a big Slinky coiled around a wire if electromagnetic fields were visible. As such, the field strength drops off as you move away from it.

A metal car or motorcycle is needed to interrupt the electromagnetic field, which in turn activates the traffic signal. The lack of metal in the horse drawn wagon is probably the reason the signal was not triggered.

Rest area lights

Q: There was a single vehicle crash into the electrical panel that ran the lights for the rest area on Route 146 South in North Smithfield. The panel has been replaced, but the rest area lights are still dark. When will these lights be working again?

A: RIDOT is aware that the lights along Route 146 are out, but they are currently being fixed. Work is about half done right now and should be completed within the next two weeks.

Dana Alexander Nolfe, chief public affairs officer for the state Department of Transportation, has agreed to answer questions of general interest posed by Journal readers about state roads and other state transportation matters.

The DOT is responsible for the state’s transportation infrastructure, which includes highways, bridges, traffic signals and bikeways.

To ask a question that would also be of interest to other readers, send a letter to Ask RIDOT, Features Department, The Providence Journal, 75 Fountain St., Providence, RI 02902. You can also e-mail your question to projocars@projo.com. Please put “Question for the DOT” in the subject field.

Questions or complaints of a specific nature should be posed to the DOT directly and will not be answered in this column.