If you want to play around with the numbers yourself, let R be the speed the redneck usually travels at, let C be the speed the cyclist slowed him down to, then

D = .25*R*C/(R-C)

is the distance he'd have to drive before he'd be 15 minutes late.

For example, if his normal commute speed is 30mph and he got slowed down to 15mph by the cyclist, then he'd be 15 minutes late if his commute was 7.5 miles long.

This ignores traffic lights and the fact that angry people like to exaggerate. I also don't think an angry redneck would keep driving behind a cyclist for that long.