Wait until it's raining (shouldn't take long this time of year) , then sneakily climb on to the roof of their car while they are stopped in traffic.
Squat over their windscreen - not so far forward so they can see your arse from their driving position - then quickly drop one out so it'll land on their windscreen and the wipers will smear it all over. Then quickly hop off and ride on.
Don't shit on the bonnet.
Wait until it's raining (shouldn't take long this time of year) , then sneakily climb on to the roof of their car while they are stopped in traffic.
Squat over their windscreen - not so far forward so they can see your arse from their driving position - then quickly drop one out so it'll land on their windscreen and the wipers will smear it all over. Then quickly hop off and ride on.