Whenever you are doing someone a favour and giving them a ride in your vehicle there are 3 options for payment for this favour:

1. Cash
2. Grass (i.e marijuana)
3. Ass (e.g. sex)
Joe "Hey dude, can I get a lift after hockey?"
Mark "No problem, cash, grass, or ass though - sorry bro, rules of the road"

Mary "Can I get a ride home after work?"
Bill "Sure you can; cash, grass, or ass?"
by F.Castro March 18, 2009
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Referring to road head, this is usually used in a sentence when someone is talking about a car ride with a girl. The rule of the road is when and if you give a girl a ride somewhere in your car and she gives you no money or gift, she must follow the rule of the road and give you road head.
guy 1: i gave cindy a ride home last night.
guy 2: did she follow the rule of the road?
guy 1: naw, she gave me $10

bill: i gave sarah a ride to wal mart earlier.
jimmy: you use the rule of the road?
bill: yeah, she tried to stop by an ATM to give me cash but i refused and made her sucky sucky
by John Chargualaf December 2, 2006
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Don't be an asshole.

Especially if you're normally an asshole (you know who you are), try your hardest not to be one. Being a dick to anyone will RUIN an entire trip.
Guy 1: Dude, Guy 3 has been such an douchebag to Guy 4.
Guy 2: I'd kick him out on account of violating Rule 1 of Road Trips, but we're stuck with his sorry ass for another two weeks...
by VanguardVince June 9, 2012
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