What range are blind spot monitors good at ?
I have seen several aftermarket sensors for cheap.
It seems that the alert signal could be used to signal the braking system as well with modification.
But what you are thinking of is a system that doesn't just apply brakes.
The adaptive cruise control modification seems like a good idea to start with.
This "long range" IR detector is only good up to 18 feet.
https://acroname.com/store/long-rang...16-gp2y0a710yk
Also, this could be modified :
https://youtu.be/qHYPG6x6Yqo