Allen Foster wrote:

> I am currently designing a system to measure the distance between two
> cars when they are moving at speed.  Would ultrasonic transducers be
> suitable for the job?

  No, too far, too much ambient noise and attenuation.

> I'm a bit worried about what effect the wind will have on them and if
> they will operate over a long enough range.

  Well, you bolt them down firmly for starters ;-)

  Seriously, the wind *will* have a major effect in distorting transit
time and literally "blowing your signal away".

> Also, does anyone know what mercedes use to measure the distance in
> their cruise control system which measures the distance between cars?

  No, I don't know, but I'd say it'll be radar or lidar.  I know
ultrasonics are used for parking sensors, but the speed, range and
accuracy involved in that is quite different.
--
  Cheers,
        Paul B.