I have a cam on the windscreen and its been very useful.
have a look at smartwitness.com
i recorded an unsafe load of steel last week, i was following it and
flashed the driver to pull over and he carried on regardless. the steel had sliced
a strap and it was all getting loose on the ■■■ end.
the cam records continuous and when ya break hard it saves the previous 30 seconds,
or if ya see something interestin you can record it manually.