How do traffic impact studies shape development decisions?

Ashley Woods entrance
AUTO ANXIETY: Residents of Arden’s Ashley Woods subdivision are concerned that a proposed 244-unit apartment complex across the road will significantly worsen traffic in the area. Photo by Justin McGuire

​​In fast-growing Buncombe County, a familiar question emerges whenever a major new building project is proposed: How will it affect traffic?

Nearby neighbors wonder whether their commutes will get worse. Developers want to know if they will have to make road improvements to relieve congestion. And government officials need details to make the call on whether to allow a project.

That’s where traffic impact studies come in. Such studies, which are required by the state and the county for certain projects, are prepared by engineering firms to forecast additional traffic associated with a development and identify possible problems.

“When we receive a traffic study, basically it’s an added tool to help some of our development boards make informed decisions, particularly the Board of Adjustment,” says Buncombe County Planning Director Nathan Pennington. The county forwards reports to the N.C. Department of Transportation, which is responsible for approving the driveway permits that are needed for every new development that connects to state roads. (NCDOT maintains the majority of the roads in the county.)

In most cases, the developer is responsible for hiring a traffic engineer, paying for the study and making any changes required by the state. “If a new turn lane is needed, a stop sign needs to be installed or even a full traffic signal — all of that responsibility falls to the developer,” says NCDOT spokesman David Uchiyama.

But some neighborhood groups question the benefits of traffic studies paid for by the very people who stand to benefit from a proposed project. After all, they wonder, don’t developers have an incentive to downplay possible traffic problems?

Project projections

So, how do traffic impact studies work?

Traffic engineers start by literally counting cars at peak hours, usually on Tuesdays, Wednesdays and Thursdays. Peak hours are generally defined as the rush hours of 7-9 a.m. and 4-6 p.m., as well as 11 a.m.-1 p.m., when people may be traveling to lunch or running errands.

“Those are typically the three highest levels of traffic volume, which results in delays and congestion,” says Calvin Clifton, business development manager for the engineering firm Mattern & Craig. The Roanoke, Va.-based company has an Asheville office that conducts local traffic studies.

“You want to do it when you have normal, routine traffic. You wouldn’t want to do it on a Friday evening when there’s going to be a high school football game,” Clifton explains.

Counts are often done using devices attached to rubber tube strips that run across the road and record each car that drives over them. Studies also can be done manually — someone sitting and counting cars — or with a camera.

The count establishes a baseline for the area’s traffic flow using an A-through-F grading system. An intersection that motorists pass through with little or no delay would be classified as having a A level of service.

Engineers then project how much additional traffic the new development will generate by referring to the Trip Generation Manual produced by the international Institute of Transportation Engineers.”Say you have a hotel with a hundred rooms, then that hotel would generate X number of trips per room,” Clifton says. “And we would use that number to say that’s the additional traffic on this intersection or this corridor.”

The engineers then crunch the numbers and prepare a study that calculates what likely impact the additional traffic will have on traffic flow in the area. If the impact is significant enough, it could cause the area’s grade to go down.

A level of service of A, B or C is generally considered acceptable, Clifton says. But a D or lower usually means the developer will be responsible for mitigating additional traffic congestion through new turn lanes, traffic signals or other measures.

Down with the count?

Jim Pritchett, president of the Ashley Woods Property Owners Association, has some concerns about traffic studies.

In 2020, residents of the neighborhood off Brevard Road in Arden opposed a 244-unit apartment complex proposed for a wooded valley just across the road. The developer, SC Bodner Co. of Carmel, Ind., ultimately withdrew its application before the Buncombe County Board of Adjustment could make a decision.

At the time, a traffic impact study commissioned by the developer concluded the project would result in a left turn out of Ashley Woods Drive — the only entrance and exit to the neighborhood — onto Brevard Road being reduced from a D to an F in the morning and evening.

Now, says Prichett, the company is back with a virtually unchanged proposal. But a new traffic study, done by a different firm, rates the current level of service for the same intersection at a C. The study also projects that service levels after the development was built would remain a C in the morning and be reduced to a D in the evening without improvements.

“[The second study] shows that there would be very little difference in traffic from before the build-out until after the build-out, which in my mind defies common sense,” he says. “They want to put 244 apartments across the street, and there’s not going to be any impact on the traffic? Are you kidding me?”

Pritchett plans to talk about the conflicting traffic reports when the Board of Adjustment considers the proposal Wednesday, Aug. 10.

“If this were a civil case and we had discovery, we could ask them, did you contact any other traffic engineers?” says Pritchett, a former attorney, in reference to the legal process by which plaintiffs and defendants can demand information from each other. But because Board of Adjustment hearings are handled under quasi-judicial rules, he continues, that option isn’t available.

“If [SC Bodner Co.] contacted two or three [traffic engineers] before and just used this one, it’d be pretty clear they were shopping, but it’s hard to know in a case like this. I do know that it concerned them that we were harping on the F rating [in 2020], and I plan to introduce that and use it against them,” Pritchett says.

Pennington, the Buncombe County planning director, says he can’t discuss specific cases that have yet to go before the Board of Adjustment. But he dismisses the idea that developers shop around for favorable studies.

“Traffic studies have to be prepared by licensed engineers that are specifically trained in traffic management, and they’re putting their license on the line, just like any other professional,” he says. “When we receive a report that is signed and sealed by a licensed professional, we have to take those findings.”

Pennington adds that NCDOT has to review and sign off on the findings, approving the methodology used. And Clifton says a traffic engineer’s job is to address traffic volume and delays safely, not please developers.

“We’re basing everything on safety and congestion management,” Clifton says.


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About Justin McGuire
Justin McGuire is a UNC Chapel Hill graduate with more than 30 years of experience as a writer and editor. His work has appeared in The Sporting News, the (Rock Hill, SC) Herald and various other publications. Follow me @jmcguireMLB

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4 thoughts on “How do traffic impact studies shape development decisions?

  1. Robert McGee

    It would be interesting to study/analyze projects that have increased traffic beyond expectations the past 10 years.
    Then go back to the original TIA report and see what engineers and developers claimed, what community members said during public comment, which board members or elected officials listened and voiced concern.

    They say that ‘history repeats itself’ and that ‘we learn from history’, but the second half of that statement is rarely true.

  2. kw

    Developers totally ‘shop’ TIA reports and everyone knows it. Some of the same lame guys are hired time after time…

  3. Jay Reese

    Here’s a novel idea. How about we eliminate the need for the automobile thus negating the impact of our transportation systems on housing and commerce. We need housing and business but there are alternatives to driving solo in an automobile. As a resident of Portland Oregon I’m witness to the positive aspects active transit.

    • Robert

      That’s why we oppose so many of the current development proposals where developers jam apartment buildings into ill-suited areas lacking infrastructure, creating more and more car-dependent communities. We will never get out of this nightmare until people recognize that, yes, we need housing–but not stupidly conceived housing built ‘at any cost’.

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