The key here is "assured clear distance". That is dependent on speed, traction, braking characteristics of your vehicle, etc. It's the responsibility of the driver to process all those factors and set his distance appropriately. If you CAN stop in time when the driver in front of you emergency stops then you DID have enough distance. If you CANNOT stop in time, then you DID NOT have enough distance. That's not measured in feet. From a purely legal/ACDA perspective, it's measured in "collision" or "no collision". Would the cop yell at the biker if the biker stopped for a kid running out into the street? Looks like the biker may have seen the cop getting close and figured he'd scare the cop using the merging car as cover. There was no reason to stop completely. REGARDLESS: The cop was too close. There was a collision, therefore he was too close. It's really that simple.