I read with interest the recent Salt Lake Tribune article on widening I-15. You were correct to note that adding more highway lanes does not reduce traffic in the long run because people respond by driving more. This is the induced traffic effect.
You would also have been correct to note that adding highway lane miles changes land use patterns, often encouraging urban land development at more peripheral locations. This is the induced land development effect.
These two effects combine in a self-reinforcing cycle of cause and effect shaping most North American metropolitan areas. It is a vicious cycle where slowly road building encourages -> low density urban land development that -> generates longer trips and more traffic -> creating congestion that -> requires building more highway lane miles that -> induces more urban land development at even lower densities that -> generate more traffic -> requiring more roads.
This cycle unfolds slowly, almost imperceptibly, intractably over a generation or more. It results in slowly enveloping urban sprawl and traffic congestion. The characteristic behavior of this system is to require an increasingly greater effort in road-building in pursuit of the ever-receding goal of alleviating traffic congestion.
The only escape is to combine (1) anything that lowers auto-dependency with (2) anything that maintains or increases urban land development densities. Together and only together will these two break the cycle that kills American cities.
Philip Emmi, Salt Lake City
