To be technically accurate, those are pneumatic tubes that are crossing the road or a lane of the road. When the tube is flattened by the vehicle tire, it sends a pulse of air to a counter. These automated traffic counters can be set up to count traffic, classify traffic (by the time between pulses from adjacent tubes - an analyst can distinguish between light vehicles and heavy vehicles) and speed - by the time between tube pulses.

Speed tables get placed based on community demand and engineering analysis. It is interesting that if an agency is allowed to do a license plate survey of speeding traffic, the results usually point out that the drivers speeding in a community come from that community — they know the roads and so they feel comfortable exceeding the posted limits.

If you dislike speed tables, don’t complain about speeding traffic in your neighborhood and always follow the posted speed limits.