Short answer. A traffic impact study (TIS), also called a traffic impact assessment or TIA, is a technical analysis that estimates how a proposed development will change traffic on the surrounding road network, and identifies what has to be built or changed to accommodate it. It is normally required as a condition of planning or zoning approval, it is triggered by the number of new peak-hour trips the development will generate, and it rests entirely on observed existing-condition traffic counts.
What a traffic impact study is
A traffic impact study is a technical document that answers a single question for a reviewing agency: if this development is built, what happens to traffic around it, and what needs to change as a result. It is prepared by a transportation engineer, submitted with a planning or zoning application, and reviewed by the agency with jurisdiction over the affected roads — a city or county transportation department, a state DOT, or all three where the site touches a state highway.
The names vary and largely mean the same thing. In the US you will see traffic impact study and traffic impact analysis; in the UK, Ireland, Australia and much of the Middle East, traffic impact assessment, abbreviated TIA. A transport assessment or transport statement in UK practice is a broader document that includes the same traffic analysis inside a wider multimodal scope.
What matters is the structure, which is consistent everywhere: measure existing conditions, forecast conditions without the development, add the development's trips, compare, and mitigate the difference.
When a jurisdiction requires one
There is no national threshold in any country we work in. The requirement is local, and it is usually written as a trip-generation trigger. Commonly seen triggers include:
- A defined number of new peak-hour trips — thresholds between 50 and 100 new peak-hour trips are typical in US municipal codes
- A defined total daily trip figure, often in the range of 500 to 1,000 new daily trips
- A new or modified access point onto an arterial or state highway, regardless of trip count
- A rezoning or change of use, even where the floor area does not change
- A development within a designated congestion management or overlay area
- Discretionary request, where an agency or a neighbouring authority asks for one
Because the trigger is itself a traffic number, the honest first step is a scoping conversation with the agency, not an assumption. Most agencies will confirm in writing whether a study is required, which intersections must be included, which peak periods to count, and which horizon years to forecast. That letter defines the entire scope of the work and should exist before any data is collected.
Scoping is where cost is decided
Study cost scales almost linearly with the number of study intersections and the hours counted at each. An agency that asks for six intersections at 12 hours each is specifying roughly four times the data collection of one asking for three intersections at AM and PM peaks. Negotiating scope at the scoping stage — with a reasoned argument for why an intersection is outside the influence area — is far cheaper than collecting data you did not need.
What is inside the study
A conventional study runs through the following sections, in this order:
- Existing conditions — the road network, the intersections, the controls, and observed traffic volumes from current counts
- Trip generation — how many trips the development will produce, usually from ITE rates or observed counts at comparable sites
- Trip distribution and assignment — which directions the trips come from and which routes they take to the site
- Background growth — a forecast of traffic without the development, including other approved but unbuilt projects
- Future conditions with the development — background traffic plus the assigned site trips
- Capacity and level of service analysis — delay, level of service and queueing for each movement in every scenario
- Safety review — collision history at the study intersections and along the frontage
- Site access and circulation — driveway location, spacing, sight distance, internal circulation, queueing at the access
- Multimodal assessment — pedestrian, cycle and transit conditions and impacts
- Mitigation — what has to be built or changed for the development to be acceptable, and who pays
The data a study is built on
Every scenario in the study is a transformation of one dataset: the existing-condition counts. Get those wrong and the error propagates through trip distribution, capacity analysis, queueing and mitigation. Reviewers know this, which is why appendices get scrutinised.
The minimum dataset for a standard study is:
| Data item | Typical specification | What it drives |
|---|---|---|
| Turning movement counts | 15-minute bins, AM and PM peak periods, all study intersections | Capacity analysis, level of service, queueing |
| Peak hour factor | Calculated per approach from the 15-minute data | Capacity analysis — never assume 0.92 |
| Heavy vehicle percentage | By movement, from a classified count | Saturation flow and HCM adjustment factors |
| Pedestrian and cycle volumes | By crosswalk leg, same period as vehicle counts | Multimodal section, signal timing, crossing treatments |
| Driveway counts at comparables | Existing similar generators, full peak periods | Validating or challenging ITE trip rates |
| Observed queue lengths | Cycle by cycle at critical approaches | Model calibration, blocking-back checks |
| Collision history | 3 to 5 years from the agency | Safety review |
Counts must be current and representative
Most agencies reject counts older than one to two years, and counts taken during school holidays, major construction or atypical weather. If your last count was collected in an unrepresentative period, budget for a fresh one rather than arguing for the old one.
The process, start to finish
- Pre-application scoping with the agency; obtain the scoping letter defining intersections, periods and horizon years
- Collect existing-condition counts to that specification
- Build and calibrate the existing-conditions model against observed volumes, and observed queues where available
- Generate trips for the proposed land use and quantum
- Distribute and assign trips to the network
- Apply background growth and committed development trips to reach the no-build future case
- Analyse the build case and compare against no-build
- Identify deficiencies and test mitigation options
- Assess site access, circulation and multimodal conditions
- Document, submit, and respond to review comments
How long it takes
For a straightforward study — three to five intersections, one land use, one horizon year — four to eight weeks from scoping to submission is realistic. The critical path is rarely the analysis; it is the data collection window and the agency's review cycle.
| Stage | Typical duration | Note |
|---|---|---|
| Agency scoping | 1 to 3 weeks | Depends entirely on agency responsiveness |
| Data collection | 1 to 2 weeks | Must avoid holidays, closures and atypical conditions |
| Count processing | 24 to 48 hours | Where counts are processed from video |
| Analysis and modelling | 1 to 3 weeks | Scales with intersection count and scenarios |
| Documentation | 1 week | Longer where mitigation is contested |
| Agency review | 3 to 8 weeks | Frequently the longest single stage |
Where studies fail review
- Counts that are stale, or collected in an unrepresentative week — the single most common reason for a resubmission
- A default peak hour factor carried through instead of one calculated from the 15-minute data
- Committed-but-unbuilt developments omitted from the no-build case, which understates background traffic
- Trip generation taken from an ITE rate with no local validation where the land use is unusual
- A model that was never calibrated against observed queues, then used to predict future queues
- Site access and internal circulation treated as an afterthought, when it is often where the real operational problem sits
- Mitigation proposed on land or in a right of way the applicant does not control
Most of these are data problems, not analysis problems. That is the argument for treating existing-condition data collection as a distinct discipline with its own quality control, rather than as the cheapest line item in the fee proposal.
Frequently asked questions
What is the difference between a traffic impact study and a traffic impact assessment?
In practice, none — they are regional naming conventions for the same document. Traffic impact study and traffic impact analysis are the usual US terms; traffic impact assessment (TIA) is standard in the UK, Ireland, Australia and much of the Middle East.
Who pays for a traffic impact study?
The developer or applicant. The study is submitted in support of their application, and any mitigation the study identifies is normally a condition of approval that the applicant funds.
How long is a traffic count valid for a study?
Most agencies accept counts up to one or two years old, provided nothing material has changed on the network. Counts taken during school holidays, roadworks or unusual weather are commonly rejected regardless of age.
Can a traffic impact study stop a development?
Rarely outright. Far more often it identifies mitigation — a turn lane, a signal, a driveway relocation, a contribution — that becomes a condition of approval. The study's real function is to price and locate that mitigation.
Do you need a traffic impact study for a change of use?
Often yes, if the new use generates materially more peak-hour trips than the old one. Several jurisdictions trigger on the change in trips rather than on floor area, so a same-size building can still require one.
The service behind this guide
Traffic Impact Study Data
Existing-condition counts, peak hour factors and classification data, formatted for the trip generation and capacity analysis stages of a TIA.
See the service → Cost & procurementHow Much Does a Traffic Study Cost?
Almost nobody in this industry publishes numbers. Here are real ranges for counts, parking studies and full traffic impact studies, and the six variables that actually move the price.
9 min read → MethodsTurning Movement Counts Explained
Everything the count sheet is telling you: bins, peak hour, PHF, classification, and the specification mistakes that make a count unusable.
10 min read →Need this data collected?
We process traffic, parking and pedestrian surveys from your video and return them in your own template, typically within 48 hours.