Its about your attitude too. If they respect your car, your driving, and your attitude, then you and them are gonna be all smiles. If they're doing a riceboy flybye, then you're never going to get a thumbs up- because they EXPECT to win, and if they do, then they dont care about you, and if they don't, they'll pissed.
On purpose or not, I always got the best reactions when you win and lose. IE, win the first time around, then he wins, etc. Everyone wins, you're all happy, thumbs up all around. If you constantly spank him tho, he's not in the mood (and will probably pull a left or right turn out of their ass). Also, if YOU constantly lose to a car that you dont think is worthy of losing to, YOU'RE gonna be an ass about it- I am!
Unfortunetly, the city I live in, Mission Viejo, is too cool to be rice. Street races happen when you line up next to a car, and you both just "happen" to go at the same time with your pedal to the ground. Eye contact is STRICTLY avoided. Bumping your sound system so you can't hear them even if they wanted to be friendly, is ENCOURAGED. lol