being judgmental: types of car people drive, and the potential risk they present: Nissan Micras tend to be driven by little old ladies, who a) may be as blind as a bat, b) quite possibly won't react as quickly as you if anything happens (and indeed, they may not see the risks until late on).
Corsa's / Saxo's (esp with dustbin exhausts, or have mobile discos installed) - usually driven by clueless little kids: loud music / car full of mates mean that they are highly unlikely to be paying attention to their driving. May well attempt to race you from traffic lights. Frequently un-insured.
Subaru Imprezas / Mitsubishi Evo's / older BMW 3-series - driven by slightly older but none-the-wiser kids, and almost certainly will attempt to race you - wet roads and wet roundabouts being favourites.
Number 1: alertness - think of the time of day: 8am - 9am / 5pm - 6pm may be the peak times of day, but think a bit more: 2pm - 3pm / 10pm - 11pm should be taken into account: shift work frequently changes on 6am/2pm/10pm cycles (give or take an hour) - are the drivers around you as alert after a day at work? Similarly, early evening / night / early morning (esp weekends): If someone is driving erratically, have they maybe had a drink?
HGV's: these have significant blind spots, and they don't stop easily - keep out of their way. Bear in mind also the point of alertness above - he could have had several hours on the road already (thus possibly tired) when you've only just gone out on the road.