If you're willing to spend DOD type bucks you can get some insanely good (like the old boring legacy tech will keep track of a city's worth of vehicles and dismounts without getting them all confused with each other) radar. The secret sauce is in the algorithms though and lord knows how many millions of dollars and man hours have been poured into that.
Limited aperture on a car means limited angular resolution even with millimeter wave. This can mean the information needed to tell whether something stationary is in your path just isn't there-- especially in the vertical direction where aperture is smaller.
A big flat surface that's not normal to you like the box truck may also not be a very large return.
