Vehicles impose higher congestion costs in denser neighborhoods. To illustrate this point, the first graph below shows the population density of various neighborhoods and the street area per person in each neighborhood.
I show data for two neighborhoods in New York City, Midtown and Wall Street. For these two neighborhoods, I’ve included both residential density (blue circles; a better indicator of evening and weekend densities) and job density (red squares; a better indicator of daytime densities during the work week). For the neighborhoods in other global cities I have only the residential density.
Denser neighborhoods such as Null Bazar in Mumbai (with nearly 1,100 people per hectare) have less street area per person than lower density neighborhoods such as my own neighborhood in Glen Rock, New Jersey (with approximately 11 people per hectare).
The first graph also shows how much street area a car consumes under various circumstances. A parked car occupies 14 square meters of street, a car traveling at 15 kilometers per hour consumes 40 square meters of street, and a car traveling at 30 kilometers per hour consumes 65 square meters of street. Because denser neighborhood’s have less street area per person, the congestion costs posed by an additional car (parked or in motion) are considerably higher in densely populated areas.
The graph helps to illustrate two important points about land use and transportation:
- Transport is mostly a real estate problem. Vehicles consume very valuable land and, very often, drivers do not pay for the land their vehicles consume. Reducing this consumption would reduce transport problems. Policies that charge vehicles by the area of road they use would help. As would restricting parking to off street private facilities where drivers pay market prices to park their cars.
- Transport systems have to adapt to densities, not the other way around. Densities are determined by markets, there is no way for planners to increase densities where there is no demand. The streets area in dense neighborhoods like Midtown cannot be increased. It doesn’t make sense to use a motorcycle or a subway to move around Glen Rock, NJ but it is indispensable in Midtown Manhattan. The issue is not so much individual transport vs. mass transit. The issue is the large size of the current fleet of vehicles that freely use large areas of expensive real estate. In the future, personal motorized vehicles that are much smaller could be the complement to transit and bicycles in very dense urban areas like Manhattan, or downtown areas in Cairo or Mumbai.
The figure below from PlanNYC shows that streets cover 26.6% the land area for the entire city of New York. (In Midtown Manhattan, a place where land values are an order of magnitude larger than those in the rest of the city, roads cover 36% of the land.)
This suggests that the mis-pricing and mis-allocation of urban streets is no trivial matter. More than one-quarter of New York City’s land is used in a wildly inefficient manner. Of course, the issue is also one of equity as well — cities need to give serious consideration to whether parking for private cars is the best use of the publicly provided streets that tend to be the source of so much that is vibrant and interesting in densely settled neighborhoods.