A minute saved on a journey cannot normally be used to earn extra money. Therefore in mode choice models the wage rate should be replaced by the expenditure rate (purchasing power per unit of time available to spend it), based on per capita family income.

Firms using the Humber bridge regularly for goods transport were able to benefit in a number of ways, and in this case it is appropriate to value savings in drivers’ time by the wage rate. But this conclusion may not be applicable to the more usual case of smaller incremental road improvements.

Modal split may vary from day to day, and uncertainty is an important element in the commuter’s choice.