You need to be stationary for this game. Perhaps the car is parked in a lay-by while you eat lunch. Perhaps the driver has gone to fetch some shopping. Perhaps (horror of horrors!) the car has broken down and you are waiting for the mechanic to arrive.
The players divide into two teams. One team watches the traffic travelling from right to left, and the other watches traffic going from left to right. Each team scores points for the vehicles they see. Of course, most of these will be ordinary cars, so a saloon car scores only one point. Less common vehicles score more highly:
Estate car or hatchback 3
Four-wheeled lorry 4
Articulated lorry 6
Coach or bus 6
Car and caravan 7
The winning team is the first to reach fifty points.