This calculation was done by DTU Transport, at the Technical University of Denmark. Taken into account were the costs associated with the use of the chosen vehicle, the time required for a trip, as well as costs associated with unsafe traffic conditions, environmental impact, and possible traffic jams.
The factors Health (a plus for the bike) and Time (a plus for the car, a minus for the bike) were particularly influential in the calculation. The costs of traffic accidents influenced the bicycle factor, but this was generously compensated by the health gains.
When everything is taken into account, the conclusion is that every kilometre travelled by bike costs 8 cents, while a kilometre travelled by car costs 50 cents. The Danish researchers admit that these figures must be taken with a grain of salt, for they are based on a number of assumptions - such as the average occupancy of a car, its average speed, worldwide economic developments, etc., but the outcome is applicable to the overall estimation of investments in infrastructure.