please help me about my ultrasonic project!please

Ultrasound will not work well for this.   Typical vehicle
separation distances are too large, and it won't work
well in rain, snow, or high wind conditions.  You should
use radar like everyone else.

Google for "adaptive cruise control".

Ultrasonic is not appropriate for this application.
You should look at the RADAR options.

You may start with the following URLs;
http://en.wikipedia.org/wiki/Radar
http://www.williamson-labs.com/autordar.htm
http://www.ieeconferences.org/Events/Hoare.pdf
http://www.hamhud.net/darts/rdr_comp.htm
http://www.shfmicro.com/gunn.htm
http://www.upv.es/menuconf/CD%20MENU%20CONFERENCE/Poster/jose_m_zamanillo_2.pdf
http://www.autocruise.net
http://www.autocruise.net/MMIC-GUNN%20 (English).pdf
http://www.epsilonlambda.com/#CW



Cheers,

Miem

    I have an Eaton VORAD radar sitting on my desk, pointed out
the window at an intersection.  You get range, range rate, and azimuth for
car-sized targets.  It was at one time fun to watch the cars go by.
We used one of these on the Overbot in the DARPA Grand
Challenge, as a backup to the more sophisticated sensors, and
this was an extra unit used for debugging.

   This is an older device; it's more of an emergency warning device
than something useful for speed control.  There's been progress since
the VORAD, which is mid-90s technology.

                John Nagle