November 1997

To design for the continuous opportunities for free-flowing vehicles (as is the case with 10 feet wide and greater travel lanes) is to create situations where most of the time passenger cars—far and away the pre-dominant vehicle—will travel at speeds greater than are desirable for nearby pedestrians. This becomes a self-worsening situation of degradation of the pedestrian environment: faster vehicles are noisier and more dangerous to pedestrians; faster vehicles generally mean fewer pedestrians; and fewer pedestrians generally mean even faster vehicles.

Institute for Transportation Engineers, 1997

Because a vehicle's kinetic energy, sound, and the difficulty of seeing the driver all increase dramatically

with vehicular speed, speeds at or below 20 mph are also the speeds that are generally the most aesthetically pleasing for pedestrians and bicyclists. Pedestrian perceptions are also very important; it has been noted that it is actually the "feeling of being unsafe, the experience of a certain threat emanating from traffic" that ends up dedicating street to primarily vehicular travel and discouraging pedestrian traffic.

Many planning texts contain references to appropriate vehicular speeds so that nearby non-motorists are not too adversely impacted by the moving vehicles. Some recently published guides and adopted standards also reflect the desirability of lower design and travel speeds for similar reasons. It is noteworthy that the speed cited as preferred is quite often given to be approximately 20 mph.

There are many more planning and design references that are not specific but relate such qualitative terms as "safety" and "comfort". When actual examples are shown or referenced, the actual travel speeds of moving vehicles on a particularly favored street is at or below 20 mph. In similar fashion traffic calming texts also try to establish various levels of vehicular speeds, and one of the commonly important thresholds of speed cited is, again, the 20 mph threshold.

Donald Appleyard, 1981

Peter Calthorpe, 1993

City of Toronto Traffic Calming Policy, 1994

Devon County Council Traffic Calming Guidelines, 1991

Fernandez, John. 1994

Institute for Transportation Engineers, 1997

Oregon DOT, 1995

Organization for Economic Development and Co-Operation, 1979

Eric Uhlfelder, 1997

Richard Untermann, 1984

Although it is commonly believed that slowing down traffic will increase emissions, recent research has shown that the greater the speed of vehicles in built-up areas, the higher is the proportion of acceleration, deceleration and braking, all of which increase air pollution. By contrast, the research indicates that lower traffic speeds as a result of traffic calming measures, can reduce idle times by 15%, gear changing by 12%, brake use by 14%, and gasoline use by 12%.

In fact, the Federal Highway Administration reports that German traffic calming experience shows that with a reduction of speed from 23 mph to 12 mph, traffic volume remained constant, while air pollution decreased between 10% and 50%.

The reasoning behind this is cars achieve near-maximum fuel-consumption efficiency at a very low speed. For instance, in most cars, fuel consumption reaches efficiency at speeds over 9 mph; heavier trucks achieve maximum efficiency at 22 mph. Hence, fuel efficiency is related to minimum, not maximum, speeds that are within pedestrian safety limits.

Reductions in Vehicle Emissions and Fuel Use from 30 mph to 18.6 mph:

Carbon Monoxide -13% to -17%

Hydrocarbons -10% to -22%

Nitrogen Oxides -32% to -48%

Fuel Consumption -7%

Kenworthy and Newman, 1992

Michael Replogle, 1993

Scottish Department of Transport, 1992

Richard Untermann, 1991

By lowering traffic speeds, general reductions of the noise level can be achieved in the order of 1 to 3 dB, i.e. From the barely perceptible to the just perceptible.

Kenworthy and Newman, 1992

The variables involved in the planning design and operations of signalized arterial streets include: traffic speed; signal cycle length; signal spacing, and; efficiency of progression. As such, it is difficult to determine the exact impact of reduced traffic speeds on a roadway's carrying capacity. For example, it is generally accepted that traffic flow is maximized at 35 to 40 mph. However, studies have pegged the flow- maximizing speed at anywhere from 9 to 45 mph, depending in part on the type of the roadway.

European traffic calming experience has shown that traffic volumes can be maintained despite a reduction in traffic speeds. In fact, there are numerous cases where mid-block, travel lanes where reclaimed as bike lanes or parkway without resulting in a reduction in traffic volumes.

V.G. Stover, 1991

Reid Ewing, 1996

...it is often extremely difficult to make adequate provisions for pedestrians. Yet this must be done, because pedestrians are the lifeblood of our urban areas, especially in the downtown and other retail areas. In general, the most successful shopping sections are those that provide the most comfort and pleasure for pedestrians.

American Association of State Highway and Transportation Officials (AASHTO), 1990, pp. 98-99

Planners will not be able to effect much change in creating new places or rejuvenating existing ones unless they alter the long-standing priority given to the automobile. Any attempt to fix up streets will be handicapped until municipal authorities and private developers stop thinking of streets only as means of getting somewhere else and begin re-embracing the concept of the street as a place.

Eric Uhlfelder, 1997

Great streets form the core of our cities' most elegant districts, commanding both premium rents and real estate taxes, proving the strong link between aesthetics and financial assets. In other words, the strategy to attain vibrant urban commercial districts is to concentrate land use and traffic, slowing it down in the heart of these districts to help. An essential part of motivating people to get out of their cars and walk around is to slow them down and encourage them to take a look.

The faster cars and trucks move, the more road space they require due to increases in following distances required to maintain safety margins.

Wolfgang Zuckermann, 1991

Street pavement is a major source of stormwater runoff. Pollutants from autos, as well as fertilizer, pesticides and other contaminants, are collected in stormwater, which flows into storm sewers. Eventually, this dirty water reaches area streams and rivers. Reducing pavement reduces stormwater runoff and allows more water to soak directly into the ground.

Michael Hough, 1989

City of Portland, 1997

Asphalt and concrete are major contributors to the creation of urban heat islands, a characteristic of urban climates which results in higher air conditioning bills, greater discomfort, and in a prolonged heat wave, more strokes and death. These materials absorb heat quicker and store it in greater quantities than plants, soil, and water. Pavement radiates as much as 50% more heat than grass does. While plants and water absorb solar radiation, much of that energy is expended in evaporation and transpiration—resulting in heat loss rather than gain.

Slowing traffic through traffic calming addresses the impact of urban heat islands by replacing asphalt and concrete for less heat absorbing materials such as plants and soil. For example, by narrowing the width of a street, traffic is forced to travel at slower speeds, while the reclaimed space can be converted to landscaped areas.

Michael Hough, 1989

Anne Whiston Spirn, 1984

The specific suggested perceptual characteristics that should distinguish pedestrian spaces from those designed for motor traffic can be derived from the variable speed of travel and the different ways of perceiving the environment: free and flexible for pedestrians, constrained and "tunnel" for motorists. Speed influences how noticeable differences occur, how long they are seen, and, hence, whether they are observed. Subtle cues need a slow pace, yet driving is not only fast, but it also demands concentration, leaving little time or capacity to appreciate the environment.

If people who move quickly are to be able to perceive objects and people, representations must be enlarged greatly. Therefore the automobile city and the pedestrian city have different sizes and dimensions. In the automobile city, signs and billboards must be very big and bold to be seen. Buildings are comparably large and poor in detail.

Jan Gehl, 1987

Amos Rapoport, 1991

There is a disproportionate relationship between speed and the severity of pedestrian injuries. Research shows that when pedestrians are struck by a moving car traveling at 20 mph, 5% are killed, most injuries are slight, and 30% suffer no injury at all; at 30 mph, 45% are killed and many are seriously injured; and at 40 mph, 85% are killed.

In other words, research shows that pedestrians are usually not seriously injured when hit by a car moving at a speed of less than 20 mph at the time of impact, noting that "if impact speeds are between 20 and 35 mph, injuries are usually serious, while at or above 35 mph they usually endanger life or are fatal".

The total required stopping distance increases by a factor of somewhat more than three when vehicular speed doubles from 20 to 40 mph. Due to driver reaction time, a vehicle traveling 20 mph will travel approximately 73 feet before it even begins to slow down from the effects of a braking action. At 40 mph, this reaction-distance doubles to 147 feet of vehicle travel before the vehicle begins to slow from braking.

The actual distance traveled by the vehicle as it slides to a stop after the brakes have been applied is five times more at 40 mph than at 20 mph (increasing from 33 to 167 feet); this is also a function of physics not related to driver skill or awareness. Even if an increase of only 10 mph is evaluated (from 20 to 30 mph), 2.5 times the braking distance alone is required to stop (33 to 86 feet).

The total kinetic energy associated with a moving vehicle is related to the square of the velocity of that vehicle. A vehicle's kinetic energy can also be subjectively perceived by most pedestrians by the noise associated with the moving vehicle. Not surprisingly, the level of pedestrian injury that is likely if a pedestrian is struck by a moving vehicle also increases with the square of an impacting vehicle's velocity.

In the event of a pedestrian being struck by a motor vehicle there is also a secondary impact that occurs when the pedestrian strikes the ground. With some further analysis, it can readily be determined that the risk of very serious injury to a pedestrian increases dramatically as the speed of the impacting vehicle exceeds 20 mph. This increase in risk is due to the effects of both the initial and the secondary impacts.

The probability of fatal injury becomes likely from initial impact alone as vehicular speeds reach and exceed approximately 35 mph. Rudolph Limpert, in his text Motor Vehicle Accident Reconstruction and Cause Analysis, states that "analysis of car/pedestrian accident statistics have shown that the probability of (the pedestrian) receiving fatal injuries is 3.5% at 15 mph, 37% at 31 mph and 83% at 44 mph".

County Surveyor's Society, 1994

Federal Highway Administration, 1997

Institute for Transportation Engineers, 1997

Scottish DOT, 1992

American Association of State Highway and Transportation Officials. 1990. AASHTO Policy on Geometric Design of Highways and Streets. AASHTO: Washington, D.C.

Appleyard, Donald. 1981. Livable Streets. University of California Press: Berkeley.

Calthorpe, Peter. 1993. The Next American Metropolis: Ecology, Community, and the American Dream. Princeton Architectural Press: New York.

City of Eugene. August 1996. Eugene Local Street Plan. City of Eugene Planning and Development Department: Eugene, Oregon.

City of Portland. 1997. Skinny Streets in Residential Neighborhoods. City of Portland Office of Transportation: Portland, Oregon.

City of Toronto. June 1994. Traffic Calming Policy Report. City of Toronto Department of Public Works and the Environment: Toronto.

County Surveyor's Society. 1994. Traffic Calming in Practice. Landor Publishing: London, England.

Devon County Council. 1991. Traffic Calming Guidelines. Devon County Council Engineering and Planning Department: Great Britain.

Ewing, Reid. 1996. Best Development Practices. American Planning Association. Planners Press: Chicago.

Federal Highway Administration. 1997. Flexibility in Highway Design. U.S. Department of Transportation: Washington, D.C.

Fernandez, John. Boulder Brings Back the Neighborhood Street. In: Planning. Spring 1994. American Planning Association: Chicago.

Gehl, Jan. 1987. Life Between Buildings. Van Nostrand Reinhold: New York

Hough, Michael. 1984. City Form and Natural Process. Routledge: New York.

Institute for Transportation Engineers. 1997. Traditional Neighborhood Development: Street Design Guidelines. ITE Transportation Planning Council Committee 5P-8: Washington, D.C.

Kent County Council. 1994. Traffic Calming: A Code of Practice. Kent County Council: United Kingdom.

Kenworthy, Jeff and Peter Newman. 1992. Winning Back the Cities. Pluto Press: Australia.

Oregon Department of Transportation. 1995. Oregon Bicycle and Pedestrian Plan. Oregon Department of Transportation Bicycle and Pedestrian Program: Salem, Oregon.

Organization for Economic Co-Operation and Development. 1992. Policies to Influence Urban Travel Demand. OECD Project Group on Urban Travel and Sustainable Development: Paris, France.

Rapoport, Amos. Pedestrian Street Use: Culture and Perception. In: Moudon, Anne Vernez (ed). 1991. Public Streets for Public Use. Columbia University Press: New York. pp. 80-94.

Replogle, Michael. 1993. Transportation Conformity and Demand Management. Environmental Defense Fund: Washington, D.C.

Scottish Department of Transportation. 1992. Killing Speed and Saving Lives. Scottish Office: Scotland.

Sprin, Anne Whiston. 1984. The Granite Garden: Urban Nature and Human Design. BasicBooks (HarperCollins).

Stover, V.G. Signalized Intersection Spacing: An Element of Access Management. In: Institute for Transportation Engineers 1991 Compendium of Technical Papers. ITE: Washington, D.C. pp. 176-181.

Uhlfelder, Eric. January 1997. The Downtown Street as a Place. In: Urban Land. Urban Land Institute: Washington, D.C. pp. 30-31.

Untermann, Richard. Changing Design Standards for Streets and Roads. In: Moudon, Anne Vernez (ed). 1991. Public Streets for Public Use. Columbia University Press: New York. pp. 255-60.

Zuckermann, Wolfgang. 1991. End of the Road. Chelsea Green Pub.: Vermont.

Back to the Walkable Streets home page.