Traffic Impact Assessment (TIA)
Learn the fundamental principles, methodologies, and regulatory requirements for conducting a Traffic Impact Assessment to evaluate how new developments affect the surrounding transportation network.
Traffic Impact Assessment (TIA)
A specialized engineering study that determines the potential traffic impacts of a proposed new development or expansion on the existing surrounding transportation infrastructure, and identifies the necessary mitigation measures to maintain acceptable levels of service.
Core Objectives of a TIA
- Evaluate Existing Conditions: Determine the current baseline traffic volumes and Level of Service (LOS) at critical nearby intersections.
- Forecast Future Demand: Predict the amount of new traffic the proposed development will generate (Trip Generation).
- Distribute and Assign Trips: Model where the new traffic is coming from and going to (Trip Distribution), and which specific roads they will use (Traffic Assignment).
- Analyze Impacts: Calculate the future LOS of the road network with the newly added development traffic.
- Develop Mitigation Strategies: Recommend physical improvements (e.g., adding turn lanes, new traffic signals) or policy changes (e.g., staggering work hours) to offset the negative impacts of the development.
The TIA Methodology
The standard step-by-step process required by municipal planning boards.
Procedure
1. Establish the Study Area and Horizon Year
The study area typically includes all major intersections where the new development will increase traffic volumes by 5% or more. The horizon year is usually the anticipated opening year of the development, plus sometimes a 5- or 10-year future forecast.
2. Collect Existing Baseline Data
Engineers conduct manual or automated turning movement counts at study intersections during peak hours (usually AM and PM commuter peaks). They also collect data on crash history, transit routes, and pedestrian facilities.
3. Forecast Background Traffic Growth
Even without the new development, regional traffic will naturally grow. Engineers apply an annual ambient growth rate (e.g., 2% per year) to the existing counts to estimate "No-Build" traffic volumes in the horizon year.
4. Generate Site-Specific Trips
Using standard empirical data (most commonly the ITE Trip Generation Manual), engineers calculate exactly how many new trips the development will create based on its size (e.g., trips per 1,000 sq ft of retail, or trips per dwelling unit).
5. Distribute and Assign New Trips
The new trips are directionally distributed (e.g., 60% from the north, 40% from the south) based on census data or gravity models, and assigned to specific turning movements at the study intersections.
6. Capacity Analysis (Build Condition)
The new site-generated trips are added to the future No-Build traffic volumes. A capacity analysis (often using Highway Capacity Manual methodologies) is performed to determine the "Build" Level of Service (LOS) and queuing delays.
7. Mitigation and Recommendations
If the Build LOS falls below the municipality's acceptable threshold (e.g., drops from LOS C to LOS E), the engineer must propose mitigation measures to restore the intersection to an acceptable state.
Trip Generation Considerations
Factors that adjust the raw trip generation numbers.
Pass-By vs. Diverted vs. Primary Trips
- Primary (New) Trips: Trips made for the specific purpose of visiting the destination (e.g., driving from home directly to a new office building). These are the only trips that truly add new traffic to the regional network.
- Pass-By Trips: Trips made by vehicles that are already on the roadway directly adjacent to the site, which briefly pull in and then continue on their original route (e.g., stopping for gas on the way home from work). These add turning movements at the driveway but do not increase the overall volume on the adjacent arterial.
- Diverted Link Trips: Trips made by vehicles that were already on the regional network but alter their route to visit the site (e.g., taking a different highway exit to visit a new mall).
Mitigation Strategies
Methods to offset the negative impacts of development-generated traffic.
Common Engineering Countermeasures
- Geometric Improvements: Adding exclusive left-turn or right-turn lanes at the site driveway or nearby intersections to remove slowing vehicles from the through-lanes.
- Signalization: Installing a new traffic signal or modifying the phasing/timing of existing signals to accommodate new turning volumes.
- Access Management: Restricting driveway movements (e.g., "Right-In/Right-Out" only) to prevent dangerous left turns across multiple lanes of heavy traffic.
- Transportation Demand Management (TDM): Implementing policies to reduce single-occupancy vehicle trips, such as subsidizing transit passes for employees, providing secure bicycle parking, or mandating staggered shift times.
Key Takeaways
- A Traffic Impact Assessment (TIA) is legally required by municipalities to ensure new developments do not degrade the surrounding road network.
- The process compares future "No-Build" conditions against future "Build" conditions.
- Trip generation must account for Pass-By and Diverted trips, especially for retail developments, so as not to over-penalize the developer.
- If a development causes the Level of Service (LOS) to drop below acceptable municipal standards, the developer is responsible for funding and constructing physical Mitigation Strategies.