What Is a Traffic Impact Study? A Practical Guide for 2026

A traffic impact study — also called a traffic impact analysis (TIA) or transportation impact assessment — estimates how much traffic a new development will add to the surrounding roads, whether those roads can handle it, and what improvements are needed if they can’t. Cities, counties and state DOTs use it to decide whether to approve a project and what the developer must build or pay for.

This guide walks through when a study is needed, the steps engineers follow, what the report contains, and where most of the hours go.

When is a traffic impact study required?

Each agency sets its own thresholds, but most trigger a study when a project is expected to generate more than a set number of peak-hour trips — commonly somewhere between 50 and 100 trips in the AM or PM peak hour. Other common triggers:

A rezoning, site plan, or subdivision application above a size threshold
New access points on an arterial or a state highway
Known congestion or safety problems near the site
A change of use that significantly increases trips

Always start with the agency’s TIA guidelines. They define the study area, the analysis years, the peak periods, acceptable level of service, and the report format.

The steps of a traffic impact study

1. Scoping

The engineer agrees the study scope with the agency: which intersections and driveways are studied, which peak hours (usually weekday AM and PM, sometimes Saturday midday), which horizon years, and which nearby approved developments must be included as background traffic. A scoping memo avoids the most expensive mistake in a TIA — redoing the analysis because the study area was too small.

2. Existing conditions

Engineers collect or obtain turning-movement counts at each study intersection, document lane geometry and traffic control, and gather signal timing. The existing network is then coded into capacity analysis software, and existing level of service is calculated.

3. Background traffic

Existing volumes are grown to the opening year (and horizon year) using an annual growth rate, and trips from other approved or pending developments nearby — cumulative projects — are added.

4. Trip generation

The project’s land uses are converted into daily and peak-hour trips using published trip generation rates or equations, based on an independent variable such as dwelling units or building floor area. Reductions are then applied where appropriate:

Pass-by trips — vehicles already on the adjacent road that stop on the way (common for retail and gas stations)
Internal capture — trips that stay within a mixed-use site, such as residents walking to on-site retail
Mode share — transit, walking or cycling reductions where the agency allows them

5. Trip distribution and assignment

Distribution estimates where the new trips come from and go to — for example 40% to the north, 30% to the east. Assignment then routes those trips through each study intersection, movement by movement. This step is where engineering judgement matters most, and where reviewers ask the most questions, so the reasoning behind each percentage should be written down.

6. Capacity analysis

With existing, background and project volumes in place, the engineer runs capacity analysis for each scenario — typically using Highway Capacity Manual (HCM) methods — and reports delay and level of service (LOS) for each intersection and lane group.

Where the project causes a movement to drop below the agency’s acceptable LOS, or worsens one that already fails, the engineer proposes mitigation: a turn lane, signal retiming, a new signal, or a roundabout. Mitigation is re-tested to prove it works. Many agencies also require a fair-share calculation — the project’s percentage of the growth at that location — to set its financial contribution.

8. The report

Finally, everything is written up: introduction and project description, study area figure, existing conditions, trip generation table, distribution and assignment figures, volume figures for each scenario, LOS tables, mitigation, and conclusions — plus appendices with counts and analysis output.

What a reviewer looks for

Study area and analysis years that match the scoping agreement
Counts that are recent and taken on typical days
Trip generation inputs that match the site plan
Distribution percentages with a stated rationale
Volumes that balance between adjacent intersections
Capacity inputs (lanes, timing, peak hour factor) that match the field
Mitigation that is tested, feasible, and tied to project impacts

Where the time goes

On a typical study, the engineering decisions — scope, distribution assumptions, mitigation choices — take a small share of the hours. Most of the budget disappears into mechanical work: coding the network (often twice, once for analysis and once for figures), balancing volumes, re-keying signal timing, drawing volume figures, and formatting tables. Every revision repeats much of it.

That’s the work tools like LaneMarker AI Studio automate. The network is built from aerial imagery, counts and timing are read from the files you already have, trips are generated and distributed with the reasoning written out, HCM analysis runs on the same network, and figures, tables and the report update together when anything changes. The engineer still makes — and signs — every judgement call; they just stop spending days on formatting.

Key takeaways

A traffic impact study shows whether roads can absorb a development’s traffic and what must be improved if not.
Follow the agency’s guidelines from scoping onward; most rework comes from scope gaps.
Trip generation, distribution and capacity analysis are the technical core; mitigation and fair share are the outcome.
Most hours go to mechanical work that can now be automated, leaving engineers more time for judgement.