Okay, so the end result I am going for is a guy running across a traffic-stricken street. It's a composite shot from two static shots from the same angle: the first is of the guy running across the street with no cars; the second is just of the traffic.
My problem is when the guy gets about halfway through, a bus drives right in front of him, and the guy's edges still catch the light as though the bus wasn't there.
How can I mask out this edge light without losing pieces of the guy?
Unfortunately, the light is on the guy, so you can't really mask the light and keep the guy.
One possibility is to duplicate the man layer, mask around the bits that need to be darker, then reduce the lightness using your preferred colour correction tool. Animate the effect so that it comes on and off in time with the bus.
A better option would be to scrub through the traffic footage and use a bit with no bus, but I suppose you'd have already thought of that if it was an option.