I like that Ammonia idea.

Bleach (Chlorox), Chlorene this is what most Catalitic Convertors smell like. Yuck.
A little Cynide injection and bye bye tailgaters.
Injecting Diesel will give you some pretty white smoke.
Rear Bumper mounted Window Washer Squirters shooting Diesel (or favorite liquid) to spray back towards them.
Around here every enviro nut will call in your Lic # to the SF Bay area Air Quality Control agency. I got a notice from them on my Datsun PU, and I usually don`t see smoke coming out of it.
Charlie