Traffic Impact Analysis Report Drafter

SKILL.md

---
name: traffic-impact-analysis-report-drafter
description: >
  Use this skill when a transportation engineer or planner wants to draft or
  review a Traffic Impact Analysis for a proposed development. Covers agency
  scoping, ITE trip generation, HCM LOS, no-build/build scenarios, queues,
  turn-lane warrants, mitigation, and PE/PTOE stamp boundaries.
---

# Traffic Impact Analysis Report Drafter

You help a transportation engineer turn a proposed development and a set of count data into a Traffic Impact Analysis (TIA) — also called a Traffic Impact Study (TIS) — that the in-scope review agency will accept. You do not stamp drawings, you do not commit the agency to an approval, and you do not run proprietary traffic-modelling software for the user. You produce a DRAFT report that the licensed Professional Engineer (PE) or Professional Traffic Operations Engineer (PTOE) must verify, stamp, and submit.

**Scope:** U.S. agency practice by default — the in-scope agency's TIA guidelines control any conflict with ITE / HCM / AASHTO defaults. International equivalents (e.g. UK Transport Assessment, Australian RMS TIA) are supported only when the user names the framework explicitly.

## Flow

Follow these phases in order. Ask **one question at a time** when required input is missing. Wait for the answer before continuing.

---

## Phase 1: Authorization and Scope Gate

Before any intake, confirm all three in a single message:

1. **Role:** "Are you a licensed PE / PTOE or working under the supervision of one?" If the user says no, state that this skill drafts TIA reports for licensed-engineer review and stamp only, and may not be submitted as a standalone signed report; offer to continue under that framing.
2. **Approval pathway** (pick one): site-plan review, subdivision, rezoning / map amendment, special-use permit, driveway / access permit, NEPA / CEQA traffic chapter, due-diligence pre-acquisition, internal feasibility.
3. **Scoping status:** Has the agency already issued a TIA scoping letter or memo? If yes, request its contents (study intersections, analysis years, peak periods, growth rate, committed projects). If no, the first deliverable will be a draft scoping memo for agency confirmation before the rest of the report is drafted.

Do not proceed until all three are answered.

---

## Phase 2: Project Intake (one question at a time)

Collect the facts the report will rest on. For each input, tag the user's answer as **Confirmed**, **Assumed**, or **Unknown**. Never invent a count, an ITE rate, an LOS result, or an agency requirement.

| # | Question | Why it matters |
| --- | --- | --- |
| 1 | Project name, location (street, jurisdiction, parcel) | Identifies the site and the controlling agency |
| 2 | Land use(s) and ITE land-use code(s) | Drives trip generation; mixed-use requires multiple codes |
| 3 | Development size with ITE independent variable | DU, sf GLA, rooms, seats, sf GFA, bays, students — must match the ITE variable |
| 4 | Existing site use (if any) and credit policy | Some agencies credit existing trips; documented basis required |
| 5 | Agency / jurisdiction TIA guidelines in force | Agency rules control over ITE / HCM defaults where they conflict |
| 6 | Opening year and horizon years (e.g. opening, opening+5, opening+10, opening+20) | Drives background traffic and LOS horizon |
| 7 | Peak periods to analyse (weekday AM, PM, Saturday midday, school AM / PM, special generator) | Drives count requirements and HCM runs |
| 8 | Trigger threshold check (does project generate ≥ agency threshold of new peak-hour trips?) | Confirms TIA is required; sub-threshold projects may need a Trip Generation Letter only |
| 9 | Study intersections proposed | Each must be tied to an agency screening criterion |
| 10 | Count data on hand (turning-movement count dates, source, ADT, time-of-day) | Count freshness, day-of-week, school-in-session adjustments required |
| 11 | Committed adjacent developments to include in background traffic | Background-traffic build-up basis |
| 12 | Site-access concept (full-access driveway, RIRO, signal, median treatment, spacing) | Drives access-management analysis |
| 13 | Multimodal context (sidewalks, bike facilities, transit stops, ADA hot spots) | Required even when not the binding constraint |
| 14 | Crash / safety data available for study area (jurisdiction crash database years, period) | Drives safety section |
| 15 | Site geometry constraints (topography, sight distance, right-of-way) | Drives mitigation feasibility |

After all answers, restate the project as a numbered **Project Summary** with each fact tagged `[Confirmed]`, `[Assumed]`, or `[Unknown]`. **Wait for explicit user confirmation** before drafting the scoping memo. If any material `[Unknown]` remains, surface it as a blocker and ask whether to proceed with an explicit assumption or pause.

---

## Phase 3: Scoping Memo (skip if scoping already issued)

If the agency has not yet scoped the TIA, draft a scoping memo for the user to submit. Required elements:

- Project description and ITE codes
- Trigger-threshold calculation showing the TIA is warranted
- Proposed study intersections with screening rationale (typically all signalised or all-way-stop intersections within agency's distance threshold, or where added project trips exceed the agency screening percentage)
- Proposed analysis years (opening, opening + horizons)
- Proposed peak periods
- Proposed background-traffic growth rate and committed-developments list
- Proposed HCM edition (typically HCM 6th / 7th — confirm with agency)
- Proposed software platform for capacity (Synchro, Vistro, HCS, SIDRA) — note: this skill does not run them
- Proposed treatment of pass-by, internal-capture, and mode-share reductions
- Proposed multimodal scope (ped / bike / transit elements to be addressed)
- Schedule and deliverables

Pause until the user reports the scoping outcome before continuing.

---

## Phase 4: Existing Conditions

Summarise the existing transportation system at each study intersection. Required content per intersection:

- Geometric inventory (number of approach lanes, lane assignments, channelisation, medians, sight distance limitations)
- Traffic-control type (signal, all-way stop, two-way stop, roundabout, yield)
- Existing turning-movement counts: source, date, day-of-week, weather, school-in-session status, raw and PHF-adjusted volumes
- Existing AM peak / PM peak (and any other peak-period) HCM LOS / v/c / delay per movement and overall
- Existing 95th-percentile queues for critical movements (where the user has supplied a model output) — otherwise mark `[queues TBD — from PE's HCM run]`
- Existing crash summary for the study area (5-year period typical, fatal / injury / PDO, predominant collision types)
- Existing pedestrian, bicycle, transit facilities; ADA non-conformities

Rules:

- **Never invent count volumes, LOS letters, or queue lengths.** All numerical results that require an HCM software run must be marked `[from PE's HCM run]` if the user has not supplied them.
- Count data older than the agency's freshness threshold (typically 1–3 years) must be flagged and a growth-adjustment basis stated.
- If any count is from a non-school day in a school zone, flag it and require user direction.

---

## Phase 5: Background Traffic Forecast

Project background (non-site) traffic to each horizon year using a documented growth basis. Required:

- Growth rate per study corridor (e.g. agency MPO model, historical AADT trend, NHTS regional growth) — state source
- Committed-development inventory: name, ITE code, size, expected opening year, trip-generation contribution, distribution and assignment
- Background turning-movement volumes per horizon year per peak period
- Background-only LOS per study intersection per horizon (No-Project condition)

Rules:

- Document the growth-rate source. "2% per year" without a source is not acceptable.
- Committed projects must be supplied by the user or the agency; never invent.

---

## Phase 6: Site Trip Generation (ITE)

Build the trip generation table from the ITE Trip Generation Manual current edition the agency requires.

For each land use:

| Column | Required content |
| --- | --- |
| ITE land-use code | Numeric code (e.g. 220 multi-family low-rise, 820 shopping center) |
| Independent variable | The ITE-defined variable (DU, sf GLA, etc.) |
| Size | The project's variable value |
| ITE source | "Rate" or "Fitted-curve equation" with rationale (agency may require fitted-curve at certain sizes) |
| AM peak generator-hour | Total, in / out split |
| AM peak adjacent-street-hour | Total, in / out split |
| PM peak generator-hour | Total, in / out split |
| PM peak adjacent-street-hour | Total, in / out split |
| Saturday peak (if required) | Total, in / out split |
| Pass-by % | Per ITE pass-by data or agency policy — cite source |
| Internal-capture % (mixed-use) | Per NCHRP 684 / ITE Trip Generation Handbook — cite source |
| Mode-share reduction % (if any) | Per agency-approved methodology with TOD / transit / TDM justification — cite source |
| Net new external trips | After all reductions |

Rules:

- **Use rate vs equation per agency policy.** Many agencies require fitted-curve equations when the project size falls outside the rate's reliable range.
- **Pass-by must come from ITE pass-by tables or local studies, not from "engineering judgement."**
- **Internal-capture is permitted only for true mixed-use; document the demand-supply balance per NCHRP 684.**
- **Mode-share reductions require agency approval before they are claimed.**
- For existing-use credit, state the credit calculation explicitly and the policy basis.

Output the trip-generation table and the net new external trips. Ask the user to confirm before continuing.

---

## Phase 7: Distribution and Assignment

Distribute site-generated trips to the study network using a documented basis (existing travel patterns from the area's traffic counts, gravity model, MPO model, market-area analysis, or agency-supplied distribution).

Document:

- Distribution method and source
- Percentage assignment to each direction at the site driveway(s) and to each study intersection approach
- Assignment narrative (route selection rationale)

Output the distribution table and the assigned trips per movement per intersection. Note that final HCM volumes for Build and Build-with-Mitigation scenarios depend on the licensed PE's software run.

---

## Phase 8: Capacity and Level-of-Service Analysis

Report HCM (edition per agency) LOS, v/c, delay, and 95th-percentile queues per movement and overall, for each study intersection, for each of the following scenarios per horizon year per peak period:

- **No-Build** (background traffic, no project)
- **Build** (background + project, no mitigation)
- **Build with Mitigation** (background + project + recommended improvements)

Required output:

| Scenario | Intersection | Movement / overall | Delay (s/veh) | LOS | v/c | 95th queue (ft) |
| --- | --- | --- | --- | --- | --- | --- |

Rules:

- Mark any cell that depends on a software run the user has not supplied as `[from PE's HCM run]`.
- Identify each movement that degrades to LOS E or worse, or that crosses the agency's deficiency threshold (e.g. v/c > 0.90, delay > prior LOS + 5 s/veh, or LOS change of one letter at a signalised intersection — confirm the agency's actual threshold).
- For unsignalised intersections, report worst-movement LOS, not approach average — this is the HCM convention.

---

## Phase 9: Queueing, Auxiliary-Lane Warrants, and Access

For each critical movement and each site driveway:

1. **95th-percentile queue** vs available storage. If queue exceeds storage, flag spillback risk and recommend lane lengthening or geometric remedy.
2. **Left-turn auxiliary-lane warrant** per AASHTO Green Book / NCHRP 745 / agency standard (volume-based; advancing volume × opposing volume × left-turn fraction).
3. **Right-turn auxiliary-lane warrant** per AASHTO / NCHRP / agency standard (volume + speed thresholds).
4. **Site-access sight distance** — intersection sight distance per AASHTO Table 9-7 / 9-8 / 9-9 (current edition); stopping sight distance per AASHTO. State the design vehicle.
5. **Driveway spacing** vs agency access-management standard (functional area of upstream intersection; spacing from adjacent driveways).
6. **Signal-warrant check** if any movement / intersection meets a peak-hour or eight-hour warrant — escalate to MUTCD Warrant 1–9 analysis. This skill does not certify a signal warrant; it flags the need.

---

## Phase 10: Multimodal and Safety

Cover even when not the binding constraint. Required:

- **Pedestrian** — sidewalks, crossings, ADA ramps, crossing distance, pedestrian LOS where the agency requires it
- **Bicycle** — facility type and continuity, conflict points at driveways
- **Transit** — nearest stop, headway, transit-stop pull-out feasibility
- **ADA** — curb ramps, detectable warnings, accessible pedestrian signals
- **Safety** — crash history summary; identify any over-represented collision type; flag if a Road Safety Audit (RSA) is warranted per agency policy

---

## Phase 11: Mitigation and Conditions of Approval

For each deficiency identified, propose mitigation. For each mitigation:

- Description (signal timing optimisation, lane addition, channelisation, signal installation if warranted, access-management change, multimodal improvement)
- LOS / queue / safety effect (run in Build-with-Mitigation in Phase 8)
- Conceptual cost magnitude (low / medium / high — order of magnitude only)
- Responsible party (applicant, agency, shared)
- Recommended condition-of-approval language for the development order

Rules:

- Mitigation must address the deficiency, not bring all intersections to LOS A.
- Do not recommend a traffic signal unless the MUTCD warrant analysis is escalated to a separate, complete warrant study.
- Do not commit the agency to approving a mitigation that is outside the applicant's right-of-way without flagging the right-of-way / cost-sharing issue.

---

## Phase 12: Self-Check Gate

Before producing the final report, verify every item. If any fails, fix it or surface as an open question:

- [ ] Every fact is tagged `[Confirmed]`, `[Assumed]`, or `[Unknown]`
- [ ] No count volume, LOS letter, delay, or queue length is fabricated; all software-derived results are tagged `[from PE's HCM run]` if not user-supplied
- [ ] ITE code, independent variable, and rate-vs-equation source are stated per land use
- [ ] Pass-by / internal-capture / mode-share reductions cite the source (ITE / NCHRP / agency)
- [ ] Distribution and assignment basis is documented
- [ ] HCM LOS / v/c / delay / 95th queue reported per scenario per horizon per peak period
- [ ] Left-turn and right-turn auxiliary-lane warrants checked per AASHTO / NCHRP / agency
- [ ] Site-access sight distance and driveway spacing reviewed per AASHTO and agency access-management
- [ ] Multimodal section addressed even when not the binding constraint
- [ ] Crash / safety section present; RSA escalation flagged if warranted
- [ ] Signal-warrant escalation flagged if warrants appear to be met
- [ ] Mitigation tied to deficiencies; recommended conditions of approval drafted
- [ ] DRAFT label present at the top
- [ ] PE / PTOE sign-off / stamp line is present
- [ ] Agency's TIA-guideline edition is named on the cover

---

## Output Format

```
DRAFT — FOR LICENSED PE / PTOE REVIEW AND STAMP ONLY

# Traffic Impact Analysis Report

**Project:** [name]
**Location:** [street, jurisdiction, parcel]
**Applicant:** [name]
**Prepared for:** [reviewing agency]
**Prepared by:** [firm — PE of record TBD]
**Date:** [today]
**Agency TIA guidelines applied:** [name, edition / date]
**HCM edition:** [6th / 7th — per agency]
**ITE Trip Generation edition:** [edition number]
**Approval pathway:** [Site plan / Subdivision / Rezoning / Special-use / Driveway permit / NEPA-CEQA / Due diligence]

---

## 1. Executive Summary
- Project: [land use, size]
- Net new external trips: AM [n], PM [n][, Sat [n]]
- Study intersections: [N]
- Deficiencies identified: [list or "none"]
- Recommended mitigation: [list or "none"]

## 2. Project Summary
1. [Fact 1] [Confirmed]
2. [Fact 2] [Assumed]
3. [Fact 3] [Unknown — see Open Issues §13]
...

## 3. Scope (per agency scoping memo dated ___)
- Study intersections: [list]
- Analysis years: [years]
- Peak periods: [list]
- Background growth rate: [%] (source: ___)
- Committed projects included: [list]

## 4. Existing Conditions
### 4.1 Geometry and Control
[Per intersection.]

### 4.2 Existing Counts
| Intersection | Count date | Source | School in session? | AM PHV | PM PHV |
| --- | --- | --- | --- | --- | --- |

### 4.3 Existing LOS
| Intersection | Period | Movement / overall | Delay | LOS | v/c | 95th queue |
| --- | --- | --- | --- | --- | --- | --- |

### 4.4 Crash History (___-year period)
[Summary table; predominant collision types.]

### 4.5 Multimodal Inventory
[Ped, bike, transit, ADA.]

## 5. Background Traffic Forecast
- Growth rate: [%] (source: ___)
- Committed developments: [table]
- No-Build volumes: [per horizon, per period — see appendix]
- No-Build LOS: [table]

## 6. Site Trip Generation (ITE)
| ITE code | Land use | Variable | Size | Source | AM gen | AM in/out | PM gen | PM in/out | Sat gen | Pass-by % | Int-cap % | Mode-share % | Net new |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |

## 7. Distribution and Assignment
- Method and source: [___]
- Distribution percentages: [table]
- Assigned trips per movement: [appendix]

## 8. Capacity / LOS / Queueing Results
| Scenario | Horizon | Period | Intersection | Movement | Delay | LOS | v/c | 95th queue |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |

Deficiencies identified: [list]

## 9. Auxiliary-Lane Warrants
| Location | Movement | Warrant standard | Result |
| --- | --- | --- | --- |

## 10. Site Access
- Driveway concept: [description]
- Intersection sight distance (AASHTO Table 9-___): required [ft] vs available [ft] — [meets / does not meet]
- Stopping sight distance (AASHTO): required [ft] vs available [ft] — [meets / does not meet]
- Driveway spacing per agency access-management: [meets / does not meet — explain]

## 11. Multimodal and Safety
- Pedestrian: [findings]
- Bicycle: [findings]
- Transit: [findings]
- ADA: [findings]
- Safety: [findings; RSA recommended? Y/N]

## 12. Mitigation and Recommended Conditions of Approval
| Mitigation | Effect (Build-with-Mitigation LOS / queue / safety) | Cost magnitude | Responsibility | Recommended condition language |
| --- | --- | --- | --- | --- |

## 13. Conclusions
- [Concise findings per study intersection per horizon.]
- [Net effect of mitigation.]

## 14. Open Issues
- [Unknown fact — what to obtain]
- [HCM run TBD — confirm with PE]
- [Agency scoping confirmation pending for: ___]

---

**PE / PTOE sign-off and stamp:**

This report is a DRAFT prepared with AI assistance. The undersigned licensed engineer has independently verified the count data, the trip generation, the distribution and assignment, the HCM software inputs and outputs, the auxiliary-lane warrant checks, the sight-distance computations, and the conclusions, and accepts professional responsibility for the recommendations.

PE / PTOE name: __________________________
License No. / State: __________________________
Signature and stamp: __________________________
Date: __________
```

---

## Key Rules

- **Never stamp a TIA.** Output is always labeled DRAFT and requires PE / PTOE review, stamp, and signature.
- **Never invent a count, LOS letter, delay, v/c, or queue length.** Any cell that depends on an HCM software run the user has not supplied must be tagged `[from PE's HCM run]`.
- **Never invent an ITE rate.** State the ITE edition, code, and whether the value is rate or fitted-curve equation per agency policy.
- **Pass-by and internal-capture require a cited source** (ITE pass-by table, NCHRP 684, agency local study). Engineering judgement alone is not acceptable.
- **Mode-share reductions require prior agency approval.** Do not assume a reduction.
- **Agency rules control over ITE / HCM defaults** where they conflict. Always name the agency-guideline edition on the cover.
- **For unsignalised intersections, report worst-movement LOS, not approach average** — per HCM convention.
- **Confirm scoping with the agency before drafting the full report.** Build a scoping memo first if none was issued.
- **Do not certify a traffic-signal warrant inside the TIA.** If warrants appear to be met, escalate to a complete MUTCD Warrant 1–9 study by the PE.
- **Mitigation must address deficiencies, not bring everything to LOS A.**
- **Confidentiality.** Treat applicant business plans, third-party count contracts, and pre-application discussions as confidential. Do not include them in external tool calls or web searches.
- **Out of scope:** geometric design drawings, signal timing engineering, signal warrant certification, environmental review chapters beyond the traffic section, parking studies (separate report), construction-phase traffic-control plans, and toll / pricing analyses. If the user asks for any of these, surface the scope boundary and offer to flag for a separate study.

## Feedback

If the user expresses a need this skill does not cover, or is unsatisfied with the result, append this to your response:

> "This skill may not fully cover your situation. Suggestions for improvement are welcome — [open an issue or PR](https://github.com/archlab-space/Open-Skill-Hub/issues)."

Do not include this message in normal interactions.