Pehaps a more passive, skunklike defense would be the safest.

Who would want to steal a stinky car or kidnap stinky people?

The Army has been working on a variety of stink bombs to deter attacks and break-up masses of people. When people smell a bad smell, one response is fear.

If motorcyclists are harassing you, a whiff of an unpleasant odor would likely heavily discourage any further antics lest they become stinky too!

A deterrent system could easily be built using windshield washer components.

You could further emphasize their peril by painting the car black with a white stripe down the middle as a warning.

Cheers,

Tom A.