Measuring car speed between two points: What sensor to use and how to cover the distance?

Question

PROBLEM

Cars regularly exceed the speed limit in our street

SOLUTION

Measure each car's speed and clearly display the speed to the driver

EXECUTION

1. Measure the time the car takes to cover a set distance (e.g. 25 meters) to calculate the speed
2. For this I'd set up two sensors and calculate the difference between the timestamps recorded between the car passing the first and second sensors -> then convert into km/h
3. Display the result on an LED screen

CHALLENGES & QUESTIONS

A. What sensors should I use here? Photoresistor with laser diode, digital or analog IR sensor, ...

B. How can I best cover the distance between the sensors and connect them to the Arduino? Long wire, wireless, connect two Arduinos, ...

Thanks!

Answer 1

Why 25meters? If you place the two sensors 1 meter apart, you only need 1ms accuracy to get km/h (assuming a 50km/h speed limit). The Arduino can handle 1ms resolution with ease.

I'd probably go with 3-5 meters, so you don't need your setup to be that precise (i.e. distance and alignment of the two sensors). Lowering the distance also makes it less likely for two cars, from both directions, to simultaneously be inside your sensor area.

For the sensors, the biggest problem is the legal one. Having objects on/near the road could cause some liability issues. The laser solution is probably the least likely to cause this issue. Though I'm unsure about having a display next to the road.

You could go with laser and photo-transistor on one side of the road. And a reflector/mirror on the other side. I'd probably go with an IR laser, so it's invisible. You'd have to shield the photo-transistor as much as possible from ambient light, so a sudden cloud won't trigger it. You probably still need to do some filtering in software.

There are radar modules available, but I think they are pretty expensive.

Another option is a Laser range sensor, like this one from sparkfun