I generally try to say something positive first. Or... when you think they know they've done something dodgy, when you get to the lights and catch them up, politely ask them to wind the window down - them expecting you to start having a go at them, but NO! You completely don't! Just ask for directions, or let them know that their nearside rear tyre could be a bit flat, or something driverish like that. Works a treat! makes you have contact with them... real, human, friendly contact.
Total Derren Brown stuff right there!
I likes it