two things to consider: as speed increase, braking distance increase squared

as distance increase, the angular size of an object decreases inversely.

a radar requires an increase in power both to see at a longer distance and to see a smaller object, because the signature reduces with 2*distance^4

( this is an oversimplification to help introduce the topic, you can see the details here http://www.radartutorial.eu/01.basics/The%20Radar%20Range%20... )

a static object is the worst case scenario: you can't use its doppler signature because it's not moving against the background and the radar becomes less efficient the moment you need it the most (at high speed)

as an aside, given the problem statement it should be possible to find a maximum speed that allows for timely detection and stopping.

chances are it's still slow.

I wonder why the radar/lidar isn't helped by an ir coincidence rangefinder.