She starts sucking you off.

You're barely halfway there when she lets out a pained moan.
"Sorry..." She breathes, "I can't take it..."
She reaches out with slender fingers and undoes your pants, slipping off your boxers and letting your cock spring free. In one fluid motion, her head is in your lap, bobbing up and down, tongue swirling around. You barely manage to keep your eyes on the road, but luckily the streets are quiet today.
She moans as she sucks you off, apparently getting a lot of pleasure from the act as well. You grit your teeth and try to focus on getting home. Surely you'll have a lot of fun when you get there...