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Cities must make investments that consider the life of major infrastructure investment and account for anticipated future growth and development. Yet, traditional traffic forecasting substantially overestimates traffic growth. Even as trends show otherwise, many transportation models still assume an upward trend in traffic demand, accepting more vehicle kilometers traveled as inevitable. Instead, cities must link design capacity for each mode to the desired mode split and activity on a street. Capacity should be measured based on total person capacity rather than vehicle level of service, using vehicle capacity to understand operational decisions.
Street design should be goal oriented and policy driven, with design year decisions supporting these goals. City transportation policies often prioritize walking, cycling, and transit. Many set explicit mode share targets to reduce dependence on single-occupancy vehicle use. Meeting these aggressive goals and targets will require a shift in both infrastructure investment and travel behavior.
Design year is applied to a project as the future conditions it should accommodate. If increased traffic is assumed, a self-fulfilling projection of increased traffic will be established. Conventional scenarios may also be at odds with community goals and preferences. For example, a 2% annual growth rate in vehicle volume represents a doubling of vehicles within 35 years, or less than two generations.1 Most cities and neighborhoods cannot afford such growth.
Traffic Evaporation and Induced Demand
Induced Demand. Travel in a given mode increases when it gains advantages in comfort, cost, travel time, or perceived convenience. Increased vehicle capacity on a street or network can result in increased vehicle travel, displacing other activity and slowing transit.
Traffic Evaporation. In urban areas, private vehicle volume decreases when road capacity is shifted to transit, cycles, and walking. This is known as traffic evaporation. Research shows that when road capacity is shifted to other modes, some vehicle traffic is absorbed by parallel routes, but drivers also shift to other modes, make trips at other times, or shift destinations. It has been shown that traffic disappears at a rate of 11%.2
Reducing VKT. To reduce Vehicle Kilometers Traveled (VKT), construct streets that include dedicated transit facilities, comfortable sidewalks, cycle facilities, and compact development. Dedicated transit facilities help shiftmovement from private vehicles and taxis to higher-efficiency collective transport, increasing the capacity of the street to move people while decreasing VKT.
Modal Capacity and Mode Splits
Appropriate modal capacity helps achieve a desired mode split. Build new streets in large developments assuming that most short trips will be made on foot or by cycle, including access to transit.
On existing streets with new development, implement operational measures that change vehicle capacity, such as signal timing or lane assignments, only after a new development’s traffic impacts are known rather than relying on self-fulfilling predictions.
Cycle facilities often experience substantial growth in use when highcomfort facilities are provided. Account for growth where expected.
Capacity and Development Review
Development review is a critical juncture for street design, when longterm decisions are made. Use existing transit capacity and opportunities for increased capacity to determine where new development is desirable. Estimate the person and freight capacity needed. Setting a desired and realistic mode split based on transit connectivity and proximity to destinations helps to determine the facilities needed to achieve an equitable modal mix.
1. National Association of City Transportation Officials, Urban Street Design Guide (Washington, DC: Island Press, 2013)
2. Stephen Atkins et al. “Disappearing Traffic? The story so far,” Municipal Engineer 151, No.1 (2002): 13-22.
Adapted by Global Street Design Guide published by Island Press.
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