Does Free-Flowing Car Traffic Reduce Fuel Consumption and Air Pollution?

Cities and Automobile Dependence (1989)

by Jeffrey Kenworthy and Peter Newman

Kenworthy and Newman, in this landmark book, argue from their worldwide survey of cities that the goal of “free-flowing” traffic (through such strategies as road widenings) actually results in MORE fuel consumption and air pollution.

“Does free flowing traffic save fuel (and lower emissions) in a city? This question is often asked in response to our contention that traffic [calming] will ease automobile dependence and gasoline use; it is generally asserted that building up congestion will in fact make cars use more fuel and we will be worse off than before…There is a longstanding observation that automobiles get high miles per gallon in smooth, free-flowing traffic and poor miles per gallon in stop-start, congested traffic…”

One form of research bias is that of looking only at the emissions from individual cars, and not taking into account the inevitable changes in travel behavior that result when cars move faster and more freely. When conditions change to allow easier use of the car, people will inevitably drive more often and further, and are more likely to use a car instead of walking, bicycling or using the bus. This is because it becomes more unsafe and unpleasant to walk, bicycle, or ride the bus when travel by car is easier and more frequent. In other words, car travel tends to be a “zero sum game” — that is, when we design our streets to improve conditions for cars, we almost inevitably worsen things for other forms of travel.

“…research treats the road traffic system as an independent factor, capable of being modified in isolation from feedback effects within the rest of the urban system, for example changes in land use patterns…

…[However], it is cities with the highest average traffic speeds that have the highest per capita gasoline consumption…congestion…increases progressively as gasoline use declines…

…while congestion diminishes significantly from central to outer areas (e.g., speeds improve 54 percent) and vehicle fuel consumption improves, actual per capita fuel use by residents in these areas increases significantly. Vehicles in central areas have 19 percent lower fuel efficiency than the Perth [Australia] average due to congestion but the central area residents use 22 percent less actual fuel, and conversely, congestion-free outer suburban driving is 12 percent more fuel efficient than average but residents use 29 percent more actual fuel…

…the feedback parameters such as land use factors and [forms] of travel exert an influence on gasoline use far in excess of the fuel efficiency of vehicles as determined by traffic conditions. In other words, in the congested but denser and more compact central and inner areas travel distances are shorter for all [forms of travel] and there is greater use of [transit], walking and cycling.

…while vehicles driven in the Perth central area at [21 mph] average speed have 17 to 27 percent more hydrocarbon and carbon monoxide emissions respectively per [mile] of travel relative to the average for Perth, residents of the area actually generate 19 to 21 percent less total carbon monoxide and hydrocarbons respectively due to their smaller use of the automobile and greater use of other modes…residents of the outer suburbs actually generate some 24 to 27 percent more carbon monoxide and hydrocarbons respectively than the average…higher average vehicle speed appears to spread the city, creating lower density land use, a greater need for cars, longer travel distances and reduced use of other less polluting or pollution-free modes. The benefits gained in terms of less polluting traffic streams appear to be overwhelmed by the sheer amount of extra travel and resulting bulk of emissions…Barry et al., showed that it is the lower density, more automobile oriented US cities that have the worst total environmental pollution…

…the central focus needs to be on automobile dependence rather than any tinkering with how well a vehicle is performing in the traffic stream. Land use patterns which give rise to automobile dependence…are usually promoted by most attempts at easing congestion and are thus counterproductive in the longer term…extra speed is not used to save time but to travel further in response to more far flung land uses, and this extra travel involves time expenditure in excess of that saved by travelling faster.”

(pg 142-159)