I spent today surfing the top deck of an insert car manhandling a Hydrascope as we swerved around the city. Here are some thoughts:
Agree on a speed. Our driver is a professional. He won't go rogue and get us into a hairy situation. If you're going to go faster, the First AD should consult with you. It's your ass hanging out in the breeze.
Harness yourself. A crane arm will drag you right off a car, especially around a corner. It's a lot of mass and you can't fight physics. I know the old argument about wanting to be able to get off the car if it goes out of control. If you have to let go of the arm to save yourself, it's like letting a wild animal loose. It won't stop until it hits something. Strap yourself in.
Watch your height. Especially if you are shooting off the back of the car. You can't see what's coming. The top height on the bucket or camera shouldn't be more than about 12 feet in the city Code is 13'6". Give yourself some leeway. Watch for overhanging branches and low lines. I did all my work on the top deck today from my knees (no jokes) to keep from being decapitated.
Have a spotter. Someone should watch what's coming up behind you. If nothing else it gives you peace of mind.
Let the arm do the work. Don't try to hold it perfectly steady. Let the head take the bounces. That's what it's for. You'll needlessly wear yourself out fighting the arm.
Watch the turns. This is where the mass of the arm can drag you. Your driver should take them slow.
Don't let yourself get talked into anything stupid. A movie ain't worth dying for.