A traffic study is a formal engineering analysis that measures how vehicles, pedestrians, and cyclists move through a specific area and predicts how a proposed change will affect that flow. Local governments typically require one before approving a new development expected to generate roughly 100 or more vehicle trips during a single peak hour. The data it produces drives real decisions: whether a developer has to build a turn lane, pay an impact fee, fund a shuttle, or redesign the project before permits issue.
When One Is Required
Most traffic studies are triggered by a proposed development. A new shopping center, apartment complex, hospital, or office park generates trips that didn’t exist before, and the jurisdiction needs to know whether nearby roads and intersections can absorb them. The common trigger is a project expected to produce 100 or more new vehicle trips entering or exiting during any single peak hour, though some agencies also set daily thresholds.
Development isn’t the only reason one gets ordered. Government agencies initiate studies when an area draws persistent congestion complaints, when a corridor shows an unusually high crash rate, or when they’re planning major roadway upgrades. Safety audits focused on crash patterns often lead to targeted studies of specific intersections. The purpose is the same across all of them: collect hard data before spending public money or granting private approvals.
Who Prepares It and Who Pays
Traffic studies are engineering work, and virtually every jurisdiction requires them to be prepared and sealed by a licensed Professional Engineer. State licensing laws define the practice of engineering to include investigation, evaluation, planning, and studies related to transportation systems, which places traffic impact analysis within work that requires a PE license. The engineer’s seal means a licensed professional stands behind the methodology and conclusions.
When a study is triggered by a private development, the developer hires and pays the engineering firm. The local government usually sets the scope, specifying which intersections to study, what time periods to analyze, and what future growth rates to use. The developer’s engineer then collects the data, runs the analysis, and submits the report for agency review. When the study is initiated by the government for a public project or safety audit, the agency funds it through its transportation budget.
What Engineers Measure in the Field
Every traffic study begins with field data collection to establish baseline conditions. You can’t model what a new development will do to a road without first knowing how that road performs today.
Vehicle Counts
The most basic measurement is average daily traffic (ADT), the average number of vehicles passing a point over a counting period of several days to a week. The FHWA defines ADT as the total of all daily volumes during a specified period divided by the number of days in that period.1Federal Highway Administration. Traffic Data Computation Method Pocket Guide Engineers commonly collect this using pneumatic road tubes stretched across travel lanes, which send an air-pressure pulse to a counter each time a tire rolls over them and can also classify vehicles by axle count and spacing and measure speed.2Federal Highway Administration. Pneumatic Road Tube Video camera analysis and drone footage are increasingly used alongside tubes and can distinguish cars, trucks, pedestrians, and cyclists.
Turning Movements at Intersections
At intersections, engineers perform turning movement counts to track how many vehicles turn left, turn right, or continue straight from each approach during peak morning and evening hours. Counts are recorded in vehicles per hour and typically broken into 15-minute intervals to capture fluctuations within the rush period. Video monitoring and automated tracking software handle much of this work today, though manual counts remain common for smaller studies.
Speed
Speed data is collected to determine the 85th percentile speed, the speed at or below which 85 percent of drivers travel on a given road segment. It’s a key input for setting speed limits because it reflects the speed most drivers consider reasonable for the road’s conditions. Engineers typically collect speed data during a 24-hour weekday period and exclude segments with unusual curves or grade changes that would skew the results.3Federal Highway Administration. Speed Information
Where Trips Come From
For larger studies, engineers track where traffic is coming from and where it’s going. License plate recognition cameras, Bluetooth readers, and cellphone-based travel data can map trip patterns across a study area. This origin-destination data helps engineers understand whether a proposed development will primarily attract local trips or pull traffic from highways and distant corridors, which affects which roads need improvement.
How the Numbers Get Analyzed
With field data in hand, engineers model current performance and project future performance under different scenarios, usually comparing conditions with and without the proposed development’s traffic added.
Level of Service
The most widely used performance metric is Level of Service (LOS), which grades traffic flow on an A-to-F scale. LOS A is free-flowing. LOS D describes conditions near capacity where speeds drop and lane changes become difficult. LOS F means breakdown: demand exceeds capacity and queues form. Engineers calculate LOS using methodologies from the Highway Capacity Manual, published by the Transportation Research Board, which provides standardized procedures for evaluating roads, intersections, and interchanges.4Federal Highway Administration. Scoping and Conducting a Traffic Study to Meet Community Needs A significant impact is typically flagged when a project would cause an intersection’s LOS to drop below the jurisdiction’s minimum standard, which many communities set at LOS D.
Trip Generation
To predict how much new traffic a development will create, engineers rely on the Institute of Transportation Engineers’ Trip Generation Manual, now in its 12th edition. It compiles trip generation data from thousands of studied sites, organized by land use type. A 200-unit apartment complex, a 50,000-square-foot grocery store, and a 300-room hotel each produce very different trip volumes at very different times of day. The manual provides rates and equations that let engineers estimate expected vehicle, pedestrian, and bicycle trips based on a development’s specific characteristics.5Institute of Transportation Engineers. ITE Trip Generation Manual
The Shift Toward Vehicle Miles Traveled
LOS measures delay at a specific point, but it doesn’t capture the broader picture. A jurisdiction could “solve” congestion by widening a road, improving LOS at that intersection while encouraging more driving overall. That tension led California to adopt Vehicle Miles Traveled (VMT) as the required metric for evaluating transportation impacts under its environmental review process, effective July 2020. VMT measures the total distance driven by all users of a proposed project, capturing greenhouse gas and land-use effects that LOS ignores. Under a VMT framework, mitigation focuses on reducing total driving rather than just reducing delay at one intersection. The concept is influencing planning discussions in other states as agencies look beyond intersection-level delay.
What the Study Can Require a Developer to Do
When a study shows a project will push road performance below acceptable standards, the developer has to address those impacts as a condition of approval. These mitigation measures are binding. Permits don’t issue until the developer commits to and funds the required improvements.
Physical Road Improvements
The most direct fix is adding road capacity. That might mean a dedicated left-turn lane to clear turning traffic out of through lanes, a right-turn deceleration lane, additional through lanes on an approach, or a new traffic signal where only stop signs exist today. Signal timing adjustments at existing signals are a lower-cost option that can meaningfully improve flow by giving more green time to the heaviest movements during peak hours.
Transportation Demand Management
Not every fix involves pouring concrete. Transportation demand management (TDM) strategies aim to reduce the number of vehicle trips a development generates in the first place. The FHWA defines TDM as a set of strategies aimed at maximizing traveler choices.6Federal Highway Administration. Transportation Demand Management A developer might be required to subsidize transit passes for residents or employees, install bike parking and shower facilities, run a carpool-matching program, provide preferential parking for carpools, or fund a shuttle to the nearest transit station. For office developments, supporting remote work can measurably reduce peak-hour trips. These measures are especially common under VMT-based frameworks.
Transportation Impact Fees
When a development’s traffic burden extends beyond the intersections right next to the site, developers may be required to pay transportation impact fees. These are one-time charges levied by local governments on new development to help pay for off-site infrastructure improvements like road widening, new signals, and pedestrian facilities throughout the broader network.7Federal Highway Administration. Development Impact Fees Fee amounts are calculated differently for residential and commercial development based on the projected traffic impact each land use type generates.8Federal Highway Administration. Frequently Asked Questions – Impact Fees The idea is that new development should pay its fair share of the infrastructure costs it creates rather than shift the burden to existing taxpayers.
Limits on What Can Be Demanded
Impact fees and other exactions aren’t unlimited. The U.S. Supreme Court has established two constitutional tests any development exaction must satisfy. The first, from Nollan v. California Coastal Commission, requires an “essential nexus,” a logical connection between the government’s interest and the condition imposed on the permit. The second, from Dolan v. City of Tigard, requires “rough proportionality,” meaning the fee amount must bear a reasonable relationship to the projected impact of the development. A jurisdiction can’t charge a small coffee shop the same traffic impact fee as a regional shopping mall.
In 2024, the Supreme Court extended these protections in Sheetz v. County of El Dorado, holding that the Takings Clause applies equally to fees imposed by legislation and those imposed by individual administrative decisions.9Supreme Court of the United States. Sheetz v. County of El Dorado Before that decision, many jurisdictions treated legislatively adopted fee schedules as immune from the nexus and proportionality requirements. Fee schedules based on outdated or unsupported assumptions are now more vulnerable to legal challenge.
Review, Public Input, and How to Weigh In
Once the developer’s engineer submits the completed study, the local government’s transportation staff reviews the methodology, data, assumptions, and conclusions. Review timelines vary widely with project complexity and agency workload, but two to six months is common for moderately complex studies. The agency may request revisions, challenge trip generation assumptions, or require analysis of additional intersections.
Neighbors and community members usually get a chance to comment during the public hearing process for the underlying development approval. The traffic study is a public document. If you’re a resident concerned about a proposed project, look at the trip generation assumptions, the study area boundaries, and whether the engineer actually analyzed the intersections you use. Studies that define too narrow a study area, or rely on trip generation rates that seem low for the type of development, are the most common points where challenges gain traction. The strongest objections come with data: your own counts, crash reports from the local police department, or documentation of existing congestion the study may have underestimated.