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Traffic impact analysis: when a development needs one

A traffic impact analysis is an engineer's study of the trips a new development adds to nearby roads and the improvements those trips require. NCDOT asks for one when a project reaches its trip threshold or sits in a sensitive location. Many towns and counties set their own triggers.

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Quick summary: This guide explains what a traffic impact analysis is, when NCDOT or a local government requires one, what the study measures, and what it can change on a tract. It is for builders, tract buyers, and landowners who plan a subdivision or a larger project. The key takeaway: the study can add turn lanes, a signal, or access limits to a project, and those costs belong in the deal before you close.

What a traffic impact analysis is

A traffic impact analysis is a study of the vehicle trips a proposed development adds to the roads around it. It compares conditions with and without the project, then names the road improvements needed to carry the new traffic. NCDOT calls it a traffic impact study and describes it as a review of safety, capacity, and access on the state highway system. See NCDOT access policy.

The two names refer to one kind of study. Local ordinances often use “traffic impact analysis” or the short form TIA. NCDOT’s policy uses “traffic impact study” or TIS.

The study sits inside a larger approval. It supports an NCDOT driveway permit, a rezoning, a site plan, or a preliminary plat. See NCDOT driveway permit: how access to a state road works.

When NCDOT requires one

NCDOT may require a study when a development is expected to generate 3,000 or more vehicle trips on an average weekday. The NCDOT access policy counts trips with the Institute of Transportation Engineers trip generation method. It allows no reductions for pass-by trips or internal capture when it tests the threshold.

NCDOT gives examples of projects that usually reach that threshold. Its driveway permit FAQ lists 300 single family homes, 55,000 square feet of retail, 250,000 square feet of office, and a 350 room hotel.

Location can trigger a study at a smaller size. The policy names these cases:

  • Interchanges. A proposed access within 1,000 feet of an interchange.
  • Crash history. An access near a high accident location.
  • Major roads. An access on a major arterial road.
  • Medians. A project that involves an existing or proposed median crossover.
  • Planned road work. A site near a project in the Transportation Improvement Program or an active construction project.
  • Engineer’s call. Any other case at the discretion of the NCDOT District Engineer.

The Division Engineer may also waive the study. The District Engineer sets the scope, and the parties can change it at a pre-submittal meeting.

When a town or county requires one

A town or county can require its own traffic study as part of its development rules. State law lets a subdivision regulation provide for the distribution of traffic “in a manner that will avoid congestion.” See N.C. Gen. Stat. § 160D-804.

Local triggers differ from NCDOT’s, and from each other. Two adopted ordinances show the range:

Ordinance What triggers the study
Durham UDO Sec. 3.3 Rezonings with a development plan, site plans, and preliminary plats expected to generate at least 150 vehicle trips at the peak hour
Raleigh UDO Sec. 8.2.2 A tiered test: a trip generation report, then a traffic assessment, then a full traffic impact analysis when queues and delays are unacceptable

Durham also adds separate projects together when they share access or infrastructure. A TIA there stays valid for no more than eight years, if the project does not change in a significant way.

A project on a state road can face both reviews. The local study supports the zoning or plat decision. The NCDOT study supports the driveway permit.

The short version: NCDOT tests for 3,000 daily trips or a sensitive location. Your town may test for peak-hour trips at a lower number. Ask both offices before you set a lot count.

What the study measures

The study measures traffic before and after the project, at each intersection the project affects. The NCDOT access policy sets out the parts.

  1. Existing conditions. Turning movement counts at each intersection, usually for the morning and evening peak hours. Counts taken within twelve months of submittal may be used.
  2. Background growth. Traffic growth to the build-out year, plus trips from other approved developments nearby.
  3. Site traffic. Trips the project generates, from the Institute of Transportation Engineers trip generation method.
  4. Capacity analysis. Level of service and queue length for each approach, by Highway Capacity Manual methods.
  5. Safety. Recent crash history near the proposed access points, when NCDOT asks for it.

What the study can change

The study can require road improvements, and the developer usually builds or funds them. NCDOT’s policy requires the applicant to identify mitigation when one of these results appears:

  • Total average delay at an intersection or approach rises by 25 percent or more at the same level of service.
  • Level of service drops by at least one level.
  • Level of service is F.
  • The 95th percentile queue in a turn lane exceeds the lane’s storage.

Mitigation can include a turn lane, a longer turn lane, a traffic signal, or a change to signal timing. It can also move or limit the access points on the site. Improvements away from the site frontage can be required when the study shows a significant impact. A new signal still needs its own NCDOT approval, and it must meet the warrant criteria in the Manual on Uniform Traffic Control Devices. See the NCDOT access policy.

Local rules can set their own response. Raleigh’s street adequacy rule applies when a project degrades an intersection below level of service E. The site plan can then be approved only within set density or floor area limits, and the applicant submits a traffic mitigation plan. Approval waits until the city’s transportation director accepts that plan.

Who prepares and reviews it

A licensed North Carolina professional engineer with traffic engineering experience prepares and seals the study. NCDOT’s policy requires the seal, and so does Durham’s ordinance.

The NCDOT District Engineer reviews the study for a state road and decides the required mitigation. A city’s transportation staff review the local study. For a rezoning, the governing board can weigh the study in its decision. See Special use permit, conditional zoning, or rezoning.

Take action: Before you price a tract by its lot count, ask the planning office and the NCDOT district office whether the plan triggers a traffic study. Start with How many lots is this tract, really?

What it means for a tract

A traffic study adds time, cost, and possible road work to a project. It can also reduce what fits on the land. A required turn lane can take frontage. An access limit can move the entrance or remove a second one. A long queue at a nearby signal can make a planned access point unworkable.

The public record shows part of the picture before a study exists. Road ownership, recent NCDOT traffic counts, and planned projects in the Transportation Improvement Program are public. A prior TIA for a nearby project can be on file with the planning office. None of those records replaces a sealed study. See A road beside the land is not a way in.

A study fits into the order of the approval work. See The land development process: from raw tract to recorded lots and How to subdivide land in North Carolina.

Key recap

  • A traffic impact analysis measures the trips a development adds and the road improvements those trips require.
  • NCDOT may require one at 3,000 or more daily trips, or near an interchange, a high-crash location, or planned road work.
  • Towns and counties set their own triggers, such as Durham’s 150 peak-hour trips.
  • A licensed professional engineer prepares and seals the study. NCDOT and local staff decide the mitigation.
  • Turn lanes, signals, and access limits can change cost, schedule, and lot count.

Questions

Does a small subdivision need a traffic impact analysis?

Often it does not, but the answer depends on the trip count, the road, and the local ordinance. NCDOT gives 300 single family homes as a project that usually reaches its daily threshold. A local peak-hour trigger or a sensitive location can require a study at a smaller size, so ask planning staff and the NCDOT district office.

Who pays for a traffic impact analysis?

The developer or applicant pays an engineer to prepare it. The developer also usually builds or funds the improvements it identifies, and the reviewing agency decides which ones are required.

Is a traffic impact analysis the same as a driveway permit?

No. The study is technical evidence, and the driveway permit is NCDOT’s approval of the connection. NCDOT uses the study to decide the conditions of the permit, and a local government uses it to decide a rezoning, site plan, or plat.

References

Primary sources cited on this page, in APA style.

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