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Adding in-city highway capacity cannot reduce congestion. In fact, by attracting vehicles to the expanded roadway it will induce highway traffic growth without reducing congestion.
Surprisingly, most in-city highway travel is by residents making local trips, using the highway as what they hope will be a quick route to another destination within the city. Residents will choose highway routes over surface street routes until growing volumes on the expanded highway route makes the travel times equal across routes. There never will be enough highway capacity to absorb all the traffic currently on parallel surface streets. Therefore, in-city highways will continue to congest in peak travel periods—namely rush hour or major evacuation events—no matter how much capacity is provided.
So why do these in-city highways get built and expanded in the first place? The benefits of in-city expansion are grossly overstated in alternatives analyses. There are dozens of examples across the U.S. where major expansion projects have failed to reduce travel times as promised. The false promises result from reliance on poor computer models. The conventional models—many developed decades ago—used in these analyses overestimate traffic and underestimate travel time.
With advances in computer hardware and software, there is a practical alternative today called Dynamic Traffic Assignment (DTA) that properly models real-world traffic behavior. DTA should be used for all modeling of urban freeway alternatives.