I was heading to the Interstate at the time and could have been severely hurt. The dealer just wants to replace the bolt that broke and send me on my way. Wow, that is a bad one, you are lucky you were not going at speed. Contact the National Highway Traffic Safety Administration and file a complaint, get the old parts and send them to their investigation division as well. Take some Polaroids as well.
Bolts just dont break, unless they are over tight or too loose and flex causing breakage over time.
It is a serious problem that should be brought to the attention of authorities. Do not count on the dealer to do anything more than replace the broken part, have them make sure all the other hardware is snug and nothing is damaged. I had three lug nuts break off. The metal was weak. I weigh only 135 lbs.