Promoting cycling as an alternative to short car trips has several advantages: it contributes to health, it reduces pollution through noise and exhaust emissions, and it reduces congestion problems. A drawback of promotion of cycling might be an increase in crash rates.
In general, the expected number of crashes is the product of exposure and crash rate. Therefore, one would expect that an increase in the number of cyclists - as a result of the promotion of cycling - would increase the number of crashes. However, it is increasingly recognised that the crash rate is also related to the amount of cycling per inhabitant; it has been shown that the fatality rate for cyclists varies in inverse proportion to the amount of cycling per cyclist. In countries where people cycle a lot, cyclists have a lower fatality rate. A similar inverse relationship exists for the number of pedestrians or cyclists crossing at intersections. Summersgill et al.  have shown that for pedestrians crossing at intersections, increasing pedestrian flows result in lower crash rates per crossing pedestrian .
Several factors may account for the tendency of crash rates to decline as the amount of exposure increases. In the first place, as each cyclist accumulates more kilometres, he or she becomes more experienced and more aware of the hazards of traffic. In the second place, when cyclists become more numerous in traffic, drivers of motor vehicles become more aware of the presence of cyclists and may behave more considerately towards them. In the third place, countries where cycling is common, like Denmark or the Netherlands, are likely to provide better facilities for cyclists than countries where cycling is less common . Similarly, increased numbers of cyclists in other countries will result in more and better cyclist facilities.
Effects on health
The beneficial effects of cycling on health have been assessed in terms of prevention of cardiovascular risk. In a study of 9,400 men in sedentary occupations (executive grade civil servants), 70% cycled at least an hour a week to work or at least 25 miles of other cycling a week. They were found to have an incidence of coronary heart disease of 2.5 per 1000 man years. This compares with 5.6 for non-cycling civil servants. Those cycling less kilometres had a rate of 4.5 . This health aspect is 5 to 10 times more important than the safety aspect. ECF  cites Hillman (1993), who calculated that years of life gained by cycling outweigh years of life lost in crashes by 20 to 1 .
Motorised forms of transport cause pollution through noise and exhaust emissions. Cycling and walking do not produce such emissions. The table below gives some estimated effects of replacing car kilometres with cycle kilometres.
Estimated effects of a one-third reduction in the number of car trips from 44% to 30% of all trips in a city:
http://ec.europa.eu/transport/road_safety/images/bullet.gif); background-position: 0px 7px; background-repeat: no-repeat no-repeat;">30% less traffic jams,
http://ec.europa.eu/transport/road_safety/images/bullet.gif); background-position: 0px 7px; background-repeat: no-repeat no-repeat;">25% reduction in pollution from motor vehicles (all types),
http://ec.europa.eu/transport/road_safety/images/bullet.gif); background-position: 0px 7px; background-repeat: no-repeat no-repeat;">25% reduction in petrol consumption (cars only),
http://ec.europa.eu/transport/road_safety/images/bullet.gif); background-position: 0px 7px; background-repeat: no-repeat no-repeat;">9% reduction in the number of people suffering from noise pollution,
http://ec.europa.eu/transport/road_safety/images/bullet.gif); background-position: 0px 7px; background-repeat: no-repeat no-repeat;">42% reduction of the barrier effect of major highways.
Source: The above figures are estimations in the 1980s of the effects of a pro-bicycle
policy in Graz, Austria (252,000 inhabitants; cited by EC DGXI, 1999).
A Cyclists' Public Affairs Group study  has demonstrated that modest increases in cycling could readily reduce transport sector emissions by 6% of the total in Great Britain, while at Dutch levels there would be a 20% reduction.
Car traffic is moreover the major source of noise in towns. In France, since 1 January 1998 any renovation or construction of urban thoroughfares must include provision for cyclists. In addition, all conglomerations in France with more than 100,000 inhabitants had to adopt an urban mobility plan. The purpose of this is to reduce pollution-producing town traffic .
Energy savings would also be an important benefit of increased level of cycling. The space consumption of a cyclist was calculated to be only 8% of the space consumption of a car UPI report Heidelberg 1989, cited by EC DGXI .
Cost-benefit analysis of mode switching
Cycling does not impose the same external costs on society as car driving does. The major external costs of car driving include:
The major external costs of cycling are the costs of injuries. However, contrary to car driving, cycling may also generate benefits for society. These may include, for example, savings in public health care as a result of improved physical fitness.
In the PROMISING project , a cost-benefit analysis was carried out of switching from driving a private car to cycling. External costs that were included in the calculation were air pollution, traffic noise, 40% of the costs of crashes, and savings from reduced absence from work. The researchers concluded that despite the fact that crash costs of cycling are higher than those of car driving, the total social costs of cycling are lower than those of driving a car.