Traffic Impact Studies
A Traffic Impact Study (TIS) is a specialized engineering study that evaluates the effects of a proposed land development on the surrounding transportation network. On the PE Civil Transportation exam, TIS questions test your ability to perform Build vs. No-Build comparisons, calculate background traffic growth, assign development trips to specific turning movements, and identify necessary mitigation measures.
The Traffic Impact Study Process
A TIS follows a logical progression of traffic demand forecasting:
[ Existing Traffic ] + [ Background Growth + Approved Projects ]
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v
[ No-Build Condition ]
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+ [ Site-Generated Trips ] (Assigned to network)
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v
[ Build Condition ]
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Is LOS acceptable? ---> Yes ---> [ Approve Project ]
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No
v
[ Identify Mitigations ]
- Define the Study Area and Horizon Year: Identify which intersections and driveways are likely to be affected. The horizon year is usually the year of project completion or 5-10 years post-opening.
- Determine Existing Traffic Volumes: Collect count data for peak hours (usually AM and PM commuter peaks).
- Project Future Background Traffic (No-Build Condition): Project existing traffic to the horizon year by applying an annual growth rate and adding traffic from other approved (but not yet built) developments.
- Forecast Site-Generated Traffic: Use ITE methods to calculate trips, adjusting for pass-by and internal capture.
- Distribute and Assign Site Trips: Allocate site trips to the study area roadways and intersections based on regional travel patterns, market studies, or gravity models.
- Analyze Future Traffic Operations (Build Condition): Add the assigned site trips to the No-Build volumes. Analyze the Levels of Service (LOS), delays, and queue lengths for both the Build and No-Build scenarios.
- Develop Mitigation Measures: If the development causes the LOS to fall below the local agency threshold (typically LOS D in urban areas, LOS C in rural areas), design mitigations (e.g., adding turn lanes, installing signals, or re-striping).
Projecting Background Traffic
Background traffic at the horizon year is modeled using either linear or compound growth models.
1. Linear Growth Model:
$$V_{\text{horizon}} = V_{\text{existing}} \times (1 + r \times n)$$2. Compound Growth Model:
$$V_{\text{horizon}} = V_{\text{existing}} \times (1 + r)^n$$Where:
- $V_{\text{existing}}$ = current traffic volume
- $r$ = annual growth rate (expressed as a decimal)
- $n$ = number of years between existing counts and the horizon year
Worked Example: Build vs. No-Build Projection
Problem:
A developer plans to build an office park that will generate $400\text{ vehicles/hour}$ during the PM peak hour (inbound and outbound combined). The study focuses on an adjacent signalized intersection.
- Existing Eastbound Left-Turn (EBLT) Volume: $150 \text{ vehicles/hour}$
- Annual Background Growth Rate: $3.0\%$ (compounded annually)
- TIS Horizon Year: $5\text{ years}$ from today
- Trip Distribution: Market analysis indicates that $35\%$ of the site-generated traffic will perform the Eastbound Left-Turn (EBLT) movement at the intersection.
Calculate:
- The Eastbound Left-Turn (EBLT) volume under the 5-year No-Build Condition.
- The Eastbound Left-Turn (EBLT) volume under the 5-year Build Condition.
Solution:
Step 1: Calculate the 5-Year No-Build Volume
Apply the compound growth formula to the existing EBLT volume:
- $V_{\text{existing}} = 150 \text{ veh/hr}$
- $r = 3.0\% = 0.03$
- $n = 5 \text{ years}$ $$V_{\text{No-Build}} = V_{\text{existing}} \times (1 + r)^n$$ $$V_{\text{No-Build}} = 150 \times (1 + 0.03)^5 = 150 \times (1.03)^5 = 150 \times 1.1593 \approx 173.9 \text{ veh/hr}$$
Rounding to the nearest whole vehicle:
$$V_{\text{No-Build}} \approx 174 \text{ veh/hr}$$Step 2: Calculate Site-Generated Trips Performing the EBLT Movement
Determine how many office park trips will be added to this specific movement:
- $\text{Total Site Trips} = 400 \text{ veh/hr}$
- $\text{EBLT Split} = 35\% = 0.35$ $$V_{\text{site-EBLT}} = 400 \times 0.35 = 140 \text{ veh/hr}$$
Step 3: Calculate the 5-Year Build Volume
Add the site-generated EBLT trips to the projected No-Build EBLT volume:
Rounding to the nearest whole vehicle:
$$V_{\text{Build}} \approx 314 \text{ veh/hr}$$Intersection Mitigation Decisions
A TIS is used to determine if mitigations are necessary. Standard triggers include:
- LOS Degradation: The Level of Service drops from an acceptable level (e.g., LOS C) in the No-Build to an unacceptable level (e.g., LOS E) in the Build.
- Delay Increase: The average control delay at an intersection increases by a significant margin (e.g., more than 5 or 10 seconds), even if the LOS letter grade remains the same.
- Queue Spillover: The projected queue length for a turn lane exceeds the physical storage length of the bay, causing it to block through traffic.
Common Mitigations:
- Adding Turn Lanes: A dedicated right-turn or left-turn lane removes turning vehicles from the through-traffic stream, increasing intersection capacity.
- Signal Optimization: Adjusting cycle lengths, green splits, or offsets to better accommodate the new traffic pattern.
- Adding Signal Phases: Introducing a protected left-turn phase (green arrow) if left-turn delay and conflict rates increase.
Crucial Pitfalls and Exam Traps
- Double-Counting Site Trips: When calculating No-Build traffic, do not include the proposed site trips. The No-Build condition represents the future without your project. Site trips are only added in the Build condition.
- Growth Model Selection: Read the problem carefully to determine if the growth rate is linear or compound. Compounded growth yields higher volumes over longer periods.
- Applying Growth to Site-Generated Trips: Do not grow the site-generated trips over time unless the project is built in phases over several years. Site trips are added as a static block representing full build-out.
- Turning Movement vs. Approach Volumes: Verify if a percentage is applied to the total approach volume or to a specific turning movement.