Roadside sensors are a natural for a mesh network. Each sensor can use something akin to WiFi to talk to the next one in each direction along the road. Data can pass from each sensor to it's neighbor in two directions, providing a measure of fault tolerance and detection.
However, cars are gradually becoming connected - it might be easier for them just to talk to each other and back to the net using the sensors and radio gear that they already have. That way you don't have to monitor sections of road where nobody's driving.
To iterate is human, to recurse, divine. -- Robert Heller