Cars are space-inefficient

Cars are extraordinarily space-inefficient. An urban street has a hard physical limit: every added car lane competes with buildings, trees, sidewalks, and other ways of moving. If nearly every trip requires a car, congestion is inevitable (unless the city is destroyed into a multi-lane stroads and parking lots). It is the predictable result of using the least space-efficient mode as the primary mode.

The geometry does not scale#

NACTO’s peak-capacity estimates show how many people different modes can move through the same width of urban street under normal peak conditions:

Note that this is about capacity; a disconnected bike lane will not attract thousands of riders, and a bus stuck in car traffic will not reach transitway capacity. But the order-of-magnitude difference is the point: private cars require way more street space per person. Whenever we turn a car lane into a connected transit, bike lane, or sidewalk that can be used well, we double, triple, quadruple the capacity of the street without widening it.

More lanes create more driving#

Road expansion can reduce delay temporarily, but the easier trip changes people’s behavior. They change routes, departure times, destinations, modes, and how often they travel. Moreover, wide, multi-lane roads make them drastically more hostile to pedestrians and cyclists, further suppressing alternative and way more space efficient modes. Over time, households and businesses also change locations. This is Induced demand.

Gilles Duranton and Matthew Turner found that vehicle-kilometers traveled rose roughly in proportion to interstate lane-kilometers across US cities (2011). The added traffic came from more driving by current residents, more commercial traffic, and migration. The authors also found no evidence that merely providing public transit reduced total driving. Alternatives must be actually useful, and cities must stop continually making driving cheaper and easier at the same time.

Fewer car trips, not worse access#

The scalable goal is not to prevent people from reaching jobs, schools, shops, or friends. It is to provide the same access with fewer car trips. Walking, biking, and transit must be safe, connected, reliable, and competitive enough that people can choose them without accepting a major penalty.

Congestion pricing, parking reform, compact land use, telework, and better non-driving infrastructure use different mechanisms, but they share the same basic logic: fewer cars compete for limited road space at the busiest times. If no practical alternative exists, reducing car capacity merely makes travel harder. If good alternatives exist, some trips shift modes, times, destinations, or disappear from the road network altogether.

Not everyone needs to stop driving. Every person who can make a trip without a car frees a disproportionate amount of space for people who still need to drive. Good walking, biking, and transit can therefore make driving better too.

Traffic can disappear#

Road-space reallocation does not mean that every displaced car moves to the next street. A review of more than seventy cases across eleven countries found that traffic problems were usually less severe than predicted. The median share of traffic formerly using the treated road or area that could not be found in the surrounding network was 10.6%; the mean was 21.9%. People adapted in many ways rather than all sitting in the same queue somewhere else. This is Traffic evaporation.

Traffic demand is not a fixed quantity waiting to be accommodated. As Traffic projection explains, street design helps create the traffic that models later treat as inevitable.

Bottom line#

You cannot solve urban traffic by fitting more cars into a city. The durable solution is to make fewer trips require one by making other modes genuinely viable.