Mystery in the fast lane

July 17, 2008 12:39 PM

Mostly on highways, but also on large surface streets, there are these configurations of tar lines. Usually two per lane. They're sort of octagonal -- basically squares with angled-off corners, making them 8-sided. Additional lines of tar lead from the roadway to the shoulder. What are they?
posted by mudpuppie to Travel & Transportation (12 answers total)

Best answer: Inductive speed sensors.

They're basically a metal detector. There are two of them a known distance apart and as your car goes over them it records the time difference between the two of them turning them on. Since they know the distance apart and the time they can calculate the speed of your car.

This is how your local news station can show you how fast different parts of the local highway system are traveling.
posted by Confess, Fletch at 12:47 PM on July 17, 2008

Response by poster: That was quick. Thanks!
posted by mudpuppie at 12:49 PM on July 17, 2008

Best answer: Yeah, they're vehicle sensors. When possible, they like to put them in the ground before the asphalt is laid, but they can retrofit lanes with sensors by saw-cutting and laying the wire in the kerf of the cut. In addition to speed sensing, they can force a traffic light to change if it has the logic to use the information.

The sawed off corners are so you don't have a sharp 90 degree bend in the wires.
posted by notsnot at 12:51 PM on July 17, 2008

Is that where Google Maps gets its traffic data too? I tried to figure out where that information came from and I failed.
posted by Dec One at 1:13 PM on July 17, 2008

Google maps gets its traffic data from "road sensors, as well as car and taxi fleets" but at least in the bay area I believe they pull from Fastrak as well.
posted by Phineas Rhyne at 1:27 PM on July 17, 2008

A bit off-topic, but... in the Bay Area there are also a multitude of what look like "a camera and a solar panel on a pole" installations along the highway. I believe the camera-looking thing is actually a radar speed sensor. I assume they're on the cellular network.
posted by lalas at 1:43 PM on July 17, 2008

Note that these are not only used for speed sensing, but also in some intersections to determine if a vehicle is waiting at a light; that way they can turn the light green for the lesser-traveled road only when a car is present, or less often if one is not present.
posted by davejay at 1:47 PM on July 17, 2008

Er, I should have read notsnot's answer before submitting mine. Ignore me.
posted by davejay at 1:48 PM on July 17, 2008

Most intersections that have signals that aren't strictly timed have these sensors. If you are the first to stop at a light, please make sure your car/bike/whatever is on the sensor so the light trips and we all don't spend five minutes at a red light.
posted by zsazsa at 2:08 PM on July 17, 2008

Response by poster: Note that these are not only used for speed sensing, but also in some intersections to determine if a vehicle is waiting at a light; that way they can turn the light green for the lesser-traveled road only when a car is present, or less often if one is not present.

I was familiar with the inductors at stoplights. Having once been a bike commuter, I know that it's impossible to get a green arrow unless there's a car in the turn lane with you. The sensors at stoplights I've seen are similar in that they have lines running to a central location, but they're not octagonal like the ones I see on the highway.

I assumed it was some sort of sensor, but just didn't know what it was sensing.

Thanks again, y'all.
posted by mudpuppie at 2:59 PM on July 17, 2008

Because motorcycles do not always trip the induction unit for the red light to change, Tennessee amended their laws to allow motorcycles to run the red light if the way is clear.
posted by bach at 8:49 PM on July 17, 2008

Some bikers affix magnets to the underside of their bike to get the road sensor to detect them. At least, that's what I read.
posted by Four Flavors at 3:48 PM on July 18, 2008