Traffic Sheds

Generally

Traffic sheds are an analytical and regulatory technique to allocate development rights based on the capacity of infrastructure, particularly rural road systems. The concept can also be applied to any infrastructure for which the capacity that is available to a defined area can be determined, such as water and sewer systems.

Traffic Sheds as an Analytical Tool

Traffic sheds can be used to support comprehensive planning efforts and the development of zoning ordinances by providing useful information about the capacity of road systems and the residential densities and nonresidential intensities that identified areas can support without overwhelming the available infrastructure. The traffic shed technique offers a simplified alternative to expensive, sophisticated traffic models, particularly in areas where significant investments in new infrastructure are unlikely.

The system works using rules of thumb that are reliable at the planning level.

• First, traffic shed areas are identified based on road connectivity and the predominant direction of traffic during morning and evening peaks.
• Second, a desired level of service ("LOS") is chosen as a matter of policy (LOS is a way of measuring the amount of traffic compared to the capacity of the road to move traffic).
• Third, the capacity of roads is calculated using a combination of the desired LOS, the width of travel lanes, whether lanes are passing or no-passing, the distance to obstructions along the sides of the road (e.g., trees and ditches), the directional split (i.e., how many cars go in each direction during morning and evening peak hours), the paving type (e.g., gravel, chip and seal, asphalt, or concrete), and the anticipated use of the road by trucks or heavy vehicles.
• Fourth, existing traffic is estimated (or measured).
• Fifth, the remaining capacity of the road network within the traffic shed is allocated to the landowners served by the traffic shed's roads in a manner that is proportional to how much of the traffic shed they own.

This technique has been used in plans for Williamson County, Tennessee; Loudoun County, Virginia; Miami County, Kansas; METROPLAN, the Little Rock Arkansas regional planning and transportation agency; and a modified version is being used in Bucks County, Pennsylvania.

Traffic Sheds as a Regulatory and Growth Management Tool

Williamson County, Tennessee has used the traffic shed technique as a regulatory tool for the past 15 years. Used this way, the traffic shed is a highly effective zoning strategy. For Williamson County, it compares favorably to adequate public facilities ordinances because the community perceives it as more fair and it does not stop development.

In many cases, the traffic shed concept may also be preferable to impact fees. For example, in low growth areas or large jurisdictions, where money does not accrue quickly in large amounts, or where those who pay the fees are widely dispersed, impact fee revenues may not be sufficient to support the construction of the improvements for which they are intended -- and money may legally have to be returned to those who paid the impact fees.

Used as a regulatory tool, traffic sheds are "market performance zoning." That is, the regulatory structure relies heavily on the market to prevent poorly located development or development that would overwhelm public infrastructure. As discussed above, market performance zoning techniques can be readily applied to water supply sheds and sewer sheds, as well. All landowners have a fair share of available capacity, and there are many options that the market can use to increase the development allocation to any particular property, including privately funded improvements (the benefit of which, in terms of development rights, accrue to those who construct them) and the purchase of development rights from other owners in the traffic shed.

Traffic sheds are particularly useful for communities that face political challenges to "downzoning" as a growth management tool in areas that are currently "overzoned;" that is, where the exercise of all potential development rights would overwhelm the capacity of the available infrastructure. A simplified comparison of market performance zoning to other popular growth management techniques is presented in the table below.