> There is little dispute among transportation researchers that expanding highway capacity increases vehicle use. This phenomenon is commonly known as induced demand, and it demonstrates a fundamental economic principle: individuals tend to consume more of a good as the price of the good falls. In other words, wider highways increase traffic speeds and reduce the time cost of driving, thereby inducing additional vehicle travel. In the short run, when residential and employment locations are fixed, faster peak period highway speeds attract drivers from alternate routes, modes, and times of day. Then, in the long-run, faster speeds encourage additional social and economic behavior in areas made more accessible by the new highway capacity, which further increases traffic volumes.
> Research studies since the 1960s have suggested that, because of induced demand, the hoped-for benefits from highway expansion tend to be short-lived and do not provide lasting relief to traffic congestion. Early studies by Downs (1962), Smeed (1968), and Thomson (1977) go so far as to argue that, over time and without any other offsetting deterrent, rush-hour traffic speeds tend to revert to their pre-expansion levels. The finding has even been dubbed the Fundamental Law of Road Congestion (Downs, 1962), which asserts that the elasticity of vehicle miles traveled with respect to lane mileage is equal to one, implying that driving increases in exact proportion to highway capacity additions.
If you don't like them, try any of many other sources. Quoting "If you build it, they will drive: Measuring induced demand for vehicle travel in urban areas" (2018) at https://www.sciencedirect.com/science/article/abs/pii/S09670... :
> There is little dispute among transportation researchers that expanding highway capacity increases vehicle use. This phenomenon is commonly known as induced demand, and it demonstrates a fundamental economic principle: individuals tend to consume more of a good as the price of the good falls. In other words, wider highways increase traffic speeds and reduce the time cost of driving, thereby inducing additional vehicle travel. In the short run, when residential and employment locations are fixed, faster peak period highway speeds attract drivers from alternate routes, modes, and times of day. Then, in the long-run, faster speeds encourage additional social and economic behavior in areas made more accessible by the new highway capacity, which further increases traffic volumes.
> Research studies since the 1960s have suggested that, because of induced demand, the hoped-for benefits from highway expansion tend to be short-lived and do not provide lasting relief to traffic congestion. Early studies by Downs (1962), Smeed (1968), and Thomson (1977) go so far as to argue that, over time and without any other offsetting deterrent, rush-hour traffic speeds tend to revert to their pre-expansion levels. The finding has even been dubbed the Fundamental Law of Road Congestion (Downs, 1962), which asserts that the elasticity of vehicle miles traveled with respect to lane mileage is equal to one, implying that driving increases in exact proportion to highway capacity additions.