Etiquette of letting someone in

[Patrick del Poker Grande]

Alright, so you're in a bumper-to-bumper, merging lanes, traffic jam type situation. Maybe you're on the highway and there's an accident or road construction or you live in Southern California, maybe you're leaving a sporting event or concert. Someone lets you into their lane and you're now in the lane that's moving. You come to a point where another guy wants in and you could let him in. What's the play, assuming you're motivated by more than selfishness and aren't going to just not let the guy in because you want to get through asap?

Do you:
a) Let the guy in? Someone was nice enough to let you in and it's your turn to return the favor for someone else who's in your same position.

or

b) Don't let the guy in. The guy behind you just let you in and now you're going to go on letting everyone in ahead of you? He did you a favor and now you're [censored] him over?