15.3 Traffic Impact Study (TIS) Scoping, Horizon Years, & Mitigation
Key Takeaways
- A Traffic Impact Study (TIS) identifies site access, on-site circulation, and off-site roadway network improvements necessary to maintain acceptable traffic operations and safety upon proposed development buildout.
- Under ITE Recommended Practice, a comprehensive TIS is typically required when a development generates 100 or more peak-hour vehicle trips (inbound + outbound) during the adjacent street or generator peak hour.
- TIS analysis requires multi-scenario modeling across distinct horizon years: Existing Baseline, Opening Year No-Build, Opening Year Build, and Long-Term Horizon (Buildout + 5 to 20 years).
- Background traffic forecasting combines historical ambient trend growth compounding with explicit trip generation and routing of approved pipeline developments.
- Developer off-site mitigation cost responsibilities are determined using proportional fair-share impact formulas based on the project's percentage contribution to total horizon traffic growth.
15.3 Traffic Impact Study (TIS) Scoping, Horizon Years, & Mitigation
PTOE Exam Focus: Traffic Impact Studies (TIS) represent the core regulatory intersection of transportation planning and traffic operations engineering. Candidates must understand ITE TIS preparation thresholds ($100\text{ peak-hour trips}$), define study area boundaries, construct multi-horizon scenario matrices (Existing, No-Build, Build, Long-Term), factor ambient background growth alongside approved pipeline developments, evaluate Level of Service (LOS) degradation thresholds, and calculate proportional fair-share mitigation fee contributions.
1. Purpose & Legal Framework of Traffic Impact Studies
A Traffic Impact Study (TIS) (also called a Traffic Impact Analysis [TIA] or Transportation Impact Assessment) is a specialized engineering study that evaluates the effects of a proposed land development or redevelopment project on the surrounding transportation system.
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| PRIMARY OBJECTIVES OF A TIS |
| |
| 1. Forecast additional traffic generated by the proposed land development. |
| 2. Determine geographic extent of traffic impacts on surrounding roadways. |
| 3. Assess multi-modal operational performance (LOS, delay, v/c, queues). |
| 4. Identify safety deficiencies, sight-distance constraints, and turn hazards. |
| 5. Formulate geometric and operational mitigation improvements (turn bays, |
| signal optimization, modern roundabouts, corridor widening). |
| 6. Establish developer fair-share financial contributions for off-site upgrades. |
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2. Study Warrants & Scoping Agreements
A. ITE Trip Generation Thresholds
According to the ITE Recommended Practice (Transportation Impact Analyses for Site Development), a comprehensive TIS is warranted when:
- The proposed development generates $100\text{ or more new peak-hour vehicle trips}$ (inbound plus outbound combined) during either the adjacent street peak hour or the generator's peak hour; OR
- The site is located in an environmentally sensitive corridor or existing high-congestion bottleneck (operating at LOS E or F); OR
- A proposed access driveway connects to a high-speed arterial or creates potential safety/weaving hazards.
B. Scoping Meeting & Memorandum of Understanding (MOU)
Prior to initiating analysis, the engineer must conduct a formal scoping meeting with reviewing agency officials (City, County, and State DOT) to formalize a binding Scoping MOU covering:
- Study Intersections & Roadway Segments: All access driveways plus all signalized and major unsignalized intersections where site traffic exceeds $5%\text{ to }10%$ of approach capacity.
- Count Data Requirements: Fresh turning movement counts (TMCs) collected within the past $12\text{ months}$ during typical mid-week periods (Tuesday through Thursday).
- Peak Hours: AM peak (7:00–9:00 AM), PM peak (4:00–6:00 PM), and Saturday midday peak (for major retail/entertainment).
- Approved Pipeline Developments: Specific unbuilt or partially built projects whose traffic must be explicitly accounted for.
- Annual Growth Rate: Agreed ambient background traffic compounding rate ($r$).
3. Horizon Years & Analysis Scenarios
A legally defensible TIS evaluates four standardized scenario combinations:
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| TIS SCENARIO MATRIX ARCHITECTURE |
| |
| 1. Existing Conditions (Year 0): Baseline field counts + geometry. |
| 2. Opening Year No-Build (Year 0 + N): Existing + Ambient Growth |
| + Approved Pipeline Developments (without Project Site). |
| 3. Opening Year Build (Year 0 + N): Opening Year No-Build + Project Site |
| Generated Trips. |
| 4. Long-Term Horizon Build (Year + 10/20): Long-range regional growth |
| + full buildout of multi-phase project. |
| 5. Build with Mitigation: Operational analysis demonstrating that |
| proposed roadway improvements restore acceptable LOS and safety. |
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4. Background Traffic Growth Forecasting
Background traffic at the horizon year consists of two distinct components:
A. Ambient Background Growth ($V_{\text{ambient}}$)
Accounts for regional travel growth not tied to a specific local project. It is compounded over the horizon duration ($n$ years) using an agreed annual growth rate ($r$, typically $1.0%\text{ to }3.0%$):
B. Pipeline / Approved Developments ($V_{\text{pipeline}}$)
Specific developments that have received site plan or zoning approval but are not yet fully built or occupied. Their trips are generated, distributed, and assigned individually to the study network and added directly to $V_{\text{ambient}}$:
5. Site Traffic Development & Trip Assignment
Project site traffic is developed in four sequential steps:
- Trip Generation: Estimate gross inbound/outbound trips using ITE Trip Generation Manual rates/equations.
- Trip Adjustments: Deduct internal capture in mixed-use developments (MXD) and pass-by trips to yield net new external vehicle trips.
- Directional Trip Distribution: Determine the percentage of traffic arriving from and departing to various geographic origins/destinations using gravity models, existing traffic flow patterns, or journey-to-work demographic data.
- Trip Assignment: Route distributed trips through the specific turning movements of the study network along the shortest or lowest-impedance travel paths.
6. Impact Assessment & Proportional Fair-Share Mitigation
A. Level of Service (LOS) Impact Criteria
Jurisdictions establish threshold criteria defining a "significant traffic impact" requiring developer mitigation:
- Intersection drops from acceptable LOS (e.g., LOS C or D) to unacceptable LOS (LOS E or F).
- If the intersection already operates at LOS E or F in the No-Build condition, any project addition causing an increase in control delay $> 5.0\text{ seconds}$ or increase in $v/c > 0.02$ triggers a mandatory mitigation requirement.
B. Proportional Fair-Share Cost Allocation
When a major regional improvement (e.g., a $600,000 signalization and turn-lane project) is required to mitigate cumulative growth, the developer is legally obligated to pay only their proportional fair share rather than the full capital cost:
Example Calculation:
• Existing Baseline Entering Volume = 1,000 veh/h
• Opening Year Build Total Volume = 1,500 veh/h (Total Growth = 500 veh/h)
• Project Site Trips Added = 150 veh/h
• Total Intersection Improvement Cost = $600,000
Fair-Share Fraction = 150 / (1,500 - 1,000) = 150 / 500 = 0.30 (30%)
Developer Payment = 0.30 × $600,000 = $180,000
Traffic Impact Study (TIS) Horizon Analysis Scenario Matrix
| Analysis Scenario | Time Horizon | Traffic Components Included | Roadway Network Geometry | Core Analytical Purpose |
|---|---|---|---|---|
| 1. Existing Baseline | Current Year (Year 0) | Existing counted traffic volumes | Existing physical roadway geometry and signal timings | Establishes baseline operational and safety benchmark |
| 2. Opening Year No-Build | Buildout Year (Year 0 + N) | Existing traffic + Ambient growth (1+r)^n + Approved pipeline projects | Existing geometry + Funded/committed public capital improvement projects | Isolates non-project background traffic congestion |
| 3. Opening Year Build | Buildout Year (Year 0 + N) | Opening Year No-Build + Project site generated traffic | No-Build geometry + Proposed site access driveways | Identifies project-specific operational and safety impacts |
| 4. Long-Term Build | Design Year (Year + 10/20) | Long-range regional growth + Full multi-phase site buildout | Future long-range transportation plan (LRTP) network | Evaluates long-range corridor capacity and access preservation |
| 5. Build with Mitigation | Buildout Year (Year 0 + N) | Opening Year Build traffic volumes | Opening Year Build geometry + Recommended developer mitigation upgrades | Demonstrates that proposed mitigation restores acceptable LOS/delay |
According to the ITE Recommended Practice for Traffic Access and Impact Studies, what is the standard vehicular trip generation threshold that typically warrants a comprehensive Traffic Impact Study (TIS)?
An intersection currently handles 1,000 entering vehicles per hour. At the 5-year buildout horizon, the Opening Year Build volume is forecast at 1,500 veh/h, of which 150 veh/h are generated directly by a new commercial development. To maintain acceptable LOS D, a signal and turn-lane improvement costing $600,000 is required. Using the proportional fair-share methodology, what is the developer's required financial contribution?
In a formal Traffic Impact Study, which of the following combinations correctly defines the traffic volume components evaluated in the 'Opening Year Build' analysis scenario?