ABSTRACT: For the twelve years that traffic sheds have been a part of the Williamson County zoning ordinance, the county has experienced a very high rate of growth, while generally maintaining its rural character outside the fringes of its urban areas. Williamson County's traffic shed procedures have generally kept new development in balance with the capacities of its county roadways, which generally do not meet published design standards.

With the enactment in 1998 of urban growth boundary legislation statewide in Tennessee, Williamson County's traffic shed procedures can be modified to be much more restrictive in areas outside urban growth boundaries and planned growth areas. Modified traffic shed procedures can be utilized to encourage the concentration of new growth within those areas where it can best be served.

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WILLIAMSON COUNTY OVERVIEW

Williamson County is a high growth high income county located on the southern boundary of Nashville and Davidson County in Tennessee. The City of Brentwood is located in Williamson County just south of the Davidson County line, and the City of Franklin, the county seat, is nearly in the middle of Williamson County. The county population has more than doubled since 1980, from just over 58,000 to just over 126,000. Brentwood's population is 23,445 and the population of Franklin is 41,842.

Although the two communities are both growing rapidly, Brentwood is more of an edge city with several large office parks, whereas Franklin has a more historical looking Main Street and town square. Within the widely extended city limits of both cities, there are many low density upscale residential developments. Along both sides of the boundary between Brentwood and Franklin, there is a large concentration of retail, light industrial, and office development known as Cool Springs.

In spite of the general pattern of low density residential development around Franklin and Brentwood and all along the Davidson/Williamson County line, the county as a whole has managed to maintain a very attractive rural character. Its attractiveness is greatly due to its topography of low but steeply rolling hills, but credit should also go to the county's commitment to growth management planning.

ORIGIN OF TRAFFIC SHEDS IN WILLIAMSON COUNTY

In the late Eighties, Williamson County's leaders were confronted with what at that time was known as "Saturn Fever," a wave of land speculation that was triggered by the announcement in 1985 that GM would locate a new auto assembly plant just south of Williamson County in Spring Hill TN. Up until then Spring Hill's main claim to fame was that Peter Jenkins had chosen to live there soon after the publication of his best selling book entitled "Walk Across America."

Once "Saturn Fever" took off, it developed a life of its own, leading to strong pressures both for and against new development. To deal with these issues, Williamson County retained a consulting team to prepare a growth management plan and a performance zoning ordinance. Williamson County's traffic shed procedures were developed as a part of that ordinance, which was initially adopted in 1988.

The consulting team was headed by Lane Kendig Inc., and also included Tocknell & Associates. Tocknell & Associates has administered Williamson County's traffic shed procedures on a continuing basis since late in 1989.

HOW TRAFFIC SHEDS WORK IN WILLIAMSON COUNTY

Figure 1 is a flow chart that depicts Williamson County's traffic shed procedures.

Figure 1: Highway Capacity Adjustment Flowchart
Williamson County Zoning Ordinance
Section 5230

In Williamson County the traffic sheds function as a comprehensive set of overlay zones, so that the maximum permitted density for proposed new developments is the either the base zoning density or the density as determined by the traffic shed procedures, whichever is more restrictive.

In turn there are actually two sets of traffic shed procedures in Williamson County: one for arterial sheds and one for collector sheds. Depending upon which is the more restrictive, the controlling traffic shed density is either the arterial shed density or the collector shed density. Arterial sheds are groupings of individual collector sheds.

Figure 2 shows a portion of Williamson County's traffic shed map, including both numbered arterial sheds with solid line boundaries, and letter-designated collector sheds with dashed line boundaries.

Figure 2: Part of Traffic Shed Map
Williamson County TN

The maximum permitted density for collector sheds is relatively straightforward - the area of the shed divided by the capacity of the principal collector roadway designated for that shed. For arterial sheds the maximum permitted density depends upon the existing level of service on the main arterial that traverses each arterial shed - the lower the level of service, the lower the permitted density.

For arterials and for designated collectors, the capacities were determined using the chapter on two lane highways from the 1985 Highway Capacity Manual. The same procedures for determining rural highway capacities were carried forward through to the 1997 Highway Capacity Manual.

TRAFFIC SHEDS AND TENNESSEE'S URBAN GROWTH BOUNDARY LEGISLATION

In Williamson County both the arterial and collector traffic shed capacities were factored up so that the base zoning district densities and traffic shed densities would each prevail in roughly the same number of instances. If this adjustment factor had not been applied, then the traffic shed densities would have been nearly always more restrictive than the base zoning district densities.

One of the reasons that Lane developed traffic sheds for Williamson County was to implement a system for growth management on a countywide basis. A system of growth boundaries was not considered to be politically feasible in the late Eighties, when the traffic shed system was developed for Williamson County.

But in 1998 urban growth boundary legislation was enacted on a statewide basis in Tennessee. Tennessee Public Chapter 1101 also allows the designation of "planned growth areas" in counties."Planned growth areas" are different from urban growth areas in that "planned growth areas" do not have to include municipalities.

With Public Chapter 1101 now in place across Tennessee, this may be the time for Williamson County to revisit its traffic shed procedures, in order to implement a more restrictive set of standards to control development outside urban growth boundaries and planned growth areas.

A CASE STUDY

I'd like to wrap up this presentation with a 1994 case study that illustrates a real world application of traffic shed procedures as a tool for controlling development triggered by a sewer line extension into Williamson County from the City of Franklin.

The landowner requested a change in the base zoning district to reflect the availability of sewerage services to his property. Williamson County responded by requiring a traffic shed analysis of the entire area that would potentially be served by the sewer line extension. Figure 3 shows the study area for this rezoning study: about 3,500 acres.

In this analysis, committed and proposed roadway improvements were added to the county roadway network, and traffic shed procedures were used to determine if the roadway improvements would add enough capacity to support the change in base zoning.

Figure 4 shows State Route 96, the arterial roadway that serves this area, Figure 5 shows a typical rural collector roadway, and Figure 6 shows a new street being built to serve a new residential development in this area.

Procedures were followed to determine how the proposed widening of the arterial roadway would have increased the traffic shed capacity of this area. But what I'm going to show today is how the proposed construction of new collector roadways would have increased the number of collector sheds, so that each collector would serve a smaller area, and the collector shed capacity, expressed in trips per acre, would be increased.

Figure 7 shows a revision to the collector shed map based on the construction of both a new collector and a new arterial roadway. The right side of the figure shows additional two new collector sheds that would be fed by a new arterial that has been proposed to be built within the City of Franklin. With this new arterial in place, traffic from Collector Sheds 50A and 50B would be expected to flow directly north and out of the county's planning jurisdiction.

Table 1 shows that without 50A and 50B, Traffic Shed 49D is 484 acres and has a capacity of 2.89 PM peak hour trips per acre. But when Traffic Shed 50A is created out of 49D, then the number of acres in 49D goes down from 484 acres to 261 acres, and its capacity in trips per acre goes from 2.89 PM peak hour trips per acre up to 5.36.

In this case the proposed improvements to the arterial roadway were not considered to be definite enough to warrant a change in the arterial shed capacity, so the requested rezoning was denied. But in another case, in 1999 in a different area of the county, traffic shed procedures were followed in order to determine the costs of collector roadway improvements needed to support higher density zoning. These costs were the basis for the enactment of a fee schedule that functions more or less the same as an impact fee for that particular area. Developers have supported the payment of this fee, in order to secure development approvals and also in order to fund improvements to what would otherwise have been a bunch of substandard rural roadways.

Figure 3
Study Area - Pratt Rezoning Study
Williamson County TN

 

Figure 4
Typical Arterial
Williamson County TN

Figure 5
Typical Collector
Williamson County TN

Figure 6
Typical New Street
Williamson County TN

Figure 7
Effect of New Traffic Shed

Table 1
Effect of New Shed

CONCLUSION

For the twelve years that traffic sheds have been a part of the Williamson County zoning ordinance, the county has experienced a very high rate of growth, while generally maintaining its rural character outside the fringes of its urban areas. Williamson County's traffic shed procedures have generally kept new development in balance with the capacities of its county roadways, which generally do not meet published roadway design standards.

With the enactment in 1998 of urban growth boundary legislation statewide in Tennessee under Public Chapter 1101, Williamson County's traffic shed procedures can be modified to be much more restrictive in the areas outside urban growth boundaries and planned growth areas. Modified traffic shed procedures can be utilized to encourage the concentration of new growth within those areas where it can best be served.

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Author and Copyright Information

Copyright 2002 by author

Stephen Tocknell, AICP, was educated at the University of Pennsylvania (BA, 1971) and at Northwestern University (MS in Transportation, 1974). He earned his AICP certificate in 1994, and is currently serving as president of the Tennessee Chapter of the American Planning Association. Since 1982 Mr. Tocknell has been the principal of Tocknell & Associates, which prepares transportation studies and provides transportation planning technical assistance for growing communities located mainly within Middle Tennessee.

Stephen Tocknell grew up on Long Island in Freeport NY. He is married to Margaret Dick, a native of Ireland who was once a planner for the State of Delaware. They have two daughters: Erin (23) and Cathleen (19).

CONTACT INFORMATION
Stephen Tocknell, AICP
Principal
Tocknell & Associates
1330 Otter Creek Road
Nashville TN 37215
stevetock@aol.com
615 269 5435 phone
615 269 0831 fax