this is the truth, like it or not... i've mellowed out in my "old" age..
but if 'yer a punk, and don't give a $h!t, i'll share my techniques. i've already ruined my rep on this site for being a poor ambassador to motorists.
1) learned this from an old hell's angel- carry a couple ball bearings in your pocket- industrial size. i got some 3/4" and 1"-ers from a compressor pump. a simple, underhanded toss into the windshield of a moving car will go through. if the car has already passed, you can throw those dense little suckers with amazing range and accuracy. the spark plug thing is an urban myth.
2) use clipless shoes that accept screw-in track spikes. usually two 'fangs' up front. works as a knee-jerk reaction to closely passing cars. woks on paint like a key.

this is provided for informational purposes only, for the sake of discussion and must not be taken out of context. the author is not liable for illicit use of this information. (keep the prudes at bay!)