It should have enough resolution to detect a vehicle directly ahead of you at 100 meters pretty easily. I have also noticed that it comes in pretty hot on stopped cars and my interpretation has been that autopilot wants to see radar confirmation if possible. The problem with stopped cars is that they are very hard to see on a doppler radar.
Just in case you haven't heard this stuff before: doppler radar sorts things by relative speed. Most of the stuff around you (called the background) has the same relative speed (trees, road, buildings) so there's a lot of clutter and noise in that category, which makes it hard get a usable signal for stuff that is stopped. This is probably why the autopilot manual warns users to be wary of stopped vehicles on the road - because the radar has a hard time with stopped objects.