of
useless.
It's not a useless idea, theoretically it could be put to fairly practical use provided the obvious problems of using sound to communicate on the freeway. Provided that you could transmit/receive a signal approx 500' I could envision a system whereby:
1) the car in front could know fairly accurately how far the car behind it was 2) it could also know how fast the car is going (i.e. how fast it is encroaching)If the front transponder was appropriately calibrated to a specific frequency, the rear transponder of the leading car could use the frequency of a received signal (and it's own speed) to calculate the speed of the vehicle in behind it.
To calculate the distance, the onboard GPS's (all new vehicles will eventually have one) 1 second pulsed output can be used to determine when pings should be sent from the front transponder. By analyzing the time shift of the start of the received signal, the rear transponder of the leading vehicle can determine the distance to following vehicle up to about 500'. A single bit of data could be embedded into the signal to indicate that it was transmitted on odd vs. even numbered second to extend range to about 1000'.
To distinguish transmitted return signals from reflections, a different frequency is used. Standard compensation techniques for atmospheric variations would be applied of course.