Most of the 80% suggest that it is the sheer volume of traffic that is the main cause of this congestion, though poor road design and road works also play their part. Road accidents do not help the situation and are felt to cause problems on motorways in particular. Road congestion doesn’t just cost more directly in terms of fuel and wage bills it has a profound effect on forward planning to make best use of expensive staff and machines. There is a knock on effect for an articulated truck that is delayed by, say, 40 minutes during one journey; it means the next job starts late or not  at all, or a connection is missed and so on.

Source: Waiting in Line, p.8.