Electrical – Beacon testing Microchip RN4871

Barring externally designed limitations or strange relativistic cars, the main concern will be if the vehicles are close enough for long enough to establish a connection and communicate the necessary information.

Suppose car A is moving at speed \$\Delta v\$ relative to car B, your wireless device has an effective range of \$d\$, and the two cars are initially out of communication range. Then the time you have available to connect and communicate is:

\begin{gather} t = \frac{2 d}{\Delta v} \end{gather}

Typical passing speed might be ~10mph, and assuming your bluetooth device has the class 2 radio range of 10m, the time available is about 4.5 seconds. If we assume instead that \$\Delta v = 80mph\$, you have ~0.56 seconds. Whether this is enough time depends on your actual devices and the amount of data you need to send.

When you have clear line of site (no obstructions), you will get 1/4 the power (6 dB less) every time you double the distance.

So in this case you would get -59 dBm - 6 dB * log2(range / 1 meter) or -59 dBm - 19 dB = -68.5 dBm

For much more info on this, check out: https://en.wikipedia.org/wiki/Friis_transmission_equation