Car speed ultrasonic alarm

i'd like to get some idea based on the topic.

my current idea is like 2 cars, 1 moving slow and another 1 is moving faster.
once it approached, the slower car will give alarm to the behind and driver as well.
(no driver in this story actually since this is just like modeling)

is there any idea that i can get to put some improvement to this system?

I think you want to use "proximity" as oppose to velocity.

I'v seen prototype cars that detect the car in front and "follow it" @a distance., problem is, is that fog and other elements causes issues .

thanks for your comment.
The car that i plan come with 2 sensor. at the back and front.
it means can detect both ways.
its not follow another car but just detect obstacle front and it back.

in case my supervisor ask me to do some improvement on it,what should i put in it.