They don’t. They merely display information that’s sent to them. The information comes from.a central location where current traffic conditions are analyzed in real time.

Same way it’s done in your phone and other GPS systems I would guess.

equation = distance = rate * time

The central computer knows the distance – say 12 miles.

The traffic management system knows the rate (speed) because of sensors in the road. say, 30 mph.

So it divides distance by rate and comes up with the amount of time.

That makes sense. I wonder if the road sensors can be seen.

@LostInParadise Many of the sensors are imbedded in the pavement, although some are overhead. If you live in an area that has toll lanes that collect via an overhead fastrack type toll reader, you are also seeing traffic monitoring instruments.

The sensors can tell if one or more lanes come to a halt while others are moving. They are typically placed 1,500 to 3,000 feet apart.

How do sensors measure speed? One crude way would be to measure the time between successive hits and then calculate speed by assuming the length of the car.

Can the sensors instead measure speed directly? It doesn’t seem as if lasers could do the job when the sensors are under the car. Lasers detect speed by looking at reflections from .distant vehicles Maybe the sensors work like speedometers, using magnets to generate a current from the movement of the car.

WAZE Mobile feeds speed and traffic delays to it users, Maybe signs are connected to WAZE mobile servers

The sensors are magnetometers, and sense how long it takes for a vehicle to pass (knowing the average length of a vehicle). They also kno how long until the next vehicle passes. Calculations using both pieces of data can tell yoou both the avg speed and the level of traffic.

Near me, there are closed-circuit TV cameras every mile or so – I am sure they know distances from top of screen to bottom of screen, and calculate from that.