The market does not on itself account for externalities: it can be better for the individual to drive a car even if it's better for all the road's users collect

The market does not on itself account for externalities: it can be better for the individual to drive a car even if it's better for all the road's users collectively if as many people as possible took the bus or rode a bike (less congestion). Likewise your personal air pollution doesn't make a perceptible difference but if everyone is driving cleaner vehicles it benefits everyone collectively with cleaner air (this was very noticable when e.g. Delhi required auto-rickshaws to convert to natural gas).