Traffic Impact Assessment (TIA)

Learning Objectives

  • Scope a TIA for the actual Philippine permitting authority and the development's transport effects.
  • Build reliable existing-condition counts and future no-build scenarios.
  • Estimate multimodal and motor-vehicle trips, distinguish primary, pass-by, and diverted trips, and conserve assigned flows.
  • Evaluate network capacity, delay, queues, access safety, and pedestrian/transit effects in build scenarios.
  • Compare mitigation and sensitivity scenarios and document assumptions, uncertainties, and recommendations.
A Traffic Impact Assessment is a reproducible comparison of transport conditions with and without a proposed project. The study's approval requirements are jurisdiction- and project-specific.

Traffic Impact Assessment (TIA)

A study of how a proposed project changes demand, operations, safety, and accessibility in its surrounding transport network, including possible mitigation.

Philippine regulatory applicability is local and project-specific

Do not say that every municipality requires a TIA for every development. Confirm the local building/planning office, relevant road authorities, development-permit conditions, and written terms of reference. For a specific example, Naga City's published subdivision development-permit requirements request a TIA for subdivision projects of 30 hectares and above; that page is not a nationwide rule. A 5% screenline-increase boundary, 2% annual background growth, a five-year horizon, or an assumed LOS target is a scenario input unless an applicable instruction explicitly adopts it. Check who reviews a mitigation obligation and which party bears implementation costs under the actual permit/contract.

Baseline, horizon, and network study area

From field observation to an auditable baseline

Select the peak periods appropriate to the project (weekday, weekend, school, loading, or event demand). Record 15-minute or other justified turning-movement counts; vehicle classes, buses, pedestrians, bicycles, transit waiting areas, signals, turning restrictions, queue lengths, travel times, roadway widths, and crash/conflict history. Check unusual days and event/weather effects. Reconcile approach and departure totals, distinguish counted traffic from estimates, and record dates and sources.

Network scoping by travel pathConceptual site entrance feeds two linked intersections; the study boundary should include potentially affected movements and sensitive links rather than an unsupported universal five-percent cutoff.SITEStudy boundaryEntrance + linked approaches

An illustrative count worksheet (not field evidence)

Peak-hour movementObserved / assumed for example (veh/h)Check
West approach through420Raw turning count
West approach right80Raw turning count
West approach total500Sum of movements
East approach through370Raw turning count
East approach left60Raw turning count
East approach total430Sum of movements

Data provenance: entirely synthetic classroom values. A report must show the complete movement matrix for each counted intersection and reconciliation of entering and leaving flows, not just two approach totals.

Peak-hour factor from quarter-hour counts

Classroom count diagnostic for four comparable consecutive 15-minute periods; not an acceptance criterion.

PHF=Vhour4V15,maxPHF=\frac{V_{\mathrm{hour}}}{4V_{15,\mathrm{max}}}

Peak-hour factor symbols

VhourV_{\mathrm{hour}} is the sum of four quarter-hour counts (vehicles in the hour), and V15,maxV_{15,\mathrm{max}} is the largest single 15-minute count (vehicles). PHFPHF is dimensionless. Use the same approach or movement and exact hour for every component of this ratio.

Illustrative no-build compound growth

Use only with a locally supported annual growth scenario and clearly defined forecast horizon.

VNB=V0(1+g)nV_{\mathrm{NB}}=V_0(1+g)^n

No-build is more than ambient growth

V0V_0 is counted demand (veh/h), gg is assumed annual decimal growth rate, and nn is years to the forecast horizon. No-build also requires separately identified committed developments, programmed road/transit changes, and network constraints. Avoid double-counting a known project inside both a trend forecast and an explicit project-addition estimate.

Generate trips before assigning traffic

Person-trips, modes, entrances, and trip purposes

Use locally collected analogous-site data where possible. External manuals such as ITE trip-generation references are estimation aids; test land-use, urban context, data age, mode share, units, directional split, and peak-hour alignment before transfer to a Philippine site. Person trips may be split among walking, cycling, transit, private cars, taxis/ride-hail, motorcycles, deliveries, and freight. A motor-vehicle trip is not automatically one person trip. Document assumed car occupancy and parking/access effects.

Mode shares before vehicle assignmentClassroom example partitions 1,000 person trips: 200 walking, 250 transit, 50 cycling, and 500 in vehicles. Vehicle counts additionally depend on occupancy.Synthetic 1,000 person tripsWalk 200Transit 250Cycle 50Vehicle persons 500Persons are not equal to vehicles; use a defensible occupancy estimate.

Convert vehicle-occupant person trips into vehicle trips

Simple occupancy-based estimate; all terms must refer to the same study period.

Tv=PvoT_v=\frac{P_v}{o}

Occupancy conversion terms

TvT_v is motor-vehicle trip ends (veh/h), PvP_v is the portion of person-trip ends traveling as vehicle occupants (persons/h), and oo is mean persons per vehicle (persons/veh). Account separately for non-passenger deliveries and other classes where appropriate.

Pass-by trip

A driveway visit by a vehicle already traveling on the adjacent road, normally returning to substantially the same route afterward; it still creates site access turning movements.

Diverted-link trip

A visit by a traveler already on the wider network who changes route to access the development; some links gain traffic and others lose it.

Keep trip adjustments spatially consistent

Primary site visits, pass-by trips, and diverted-link trips influence different screenlines and approaches. Do not automatically subtract every pass-by or diverted trip from every link. Apply rate/adjustment evidence appropriate to the land use; show gross driveway movements, new external trips, and diverted-route changes in separate columns.

Driveway effects do not vanish in a pass-by adjustmentSolid route represents an adjacent road; bent route shows an existing through vehicle entering the site and returning to the same road, increasing driveway turns without necessarily adding a new through trip.DevelopmentMain-road through demandOrange: pass-by entering, exiting

Site motor-vehicle trip estimation

A classroom rate model; the selected rate must match the time period and land-use unit.

T=rXT=rX

Trip rate meaning

TT is gross peak-period driveway vehicle trip ends (veh/h), rr is a matching trip rate (veh/h per land-use unit), and XX is the development size in those units. For person-trip estimates, replace vehicle units consistently and convert by documented mode and occupancy assumptions.

Distribute, assign, and test network operations

Conserving assigned site trips

For the same peak period and trip category, every directional share must be accounted for.

Ti=piT,∑ipi=1T_i=p_i T,\qquad \sum_i p_i=1

Directional shares and path assignment

A directional share pip_i is a fraction of the applicable trip total TT. Assign trips to feasible routes and turning movements based on observed origins/destinations, access restrictions, travel times, competing routes, and transit services. Check that split movements sum to the original total before rounding. Do not add site trips to movements that cannot physically reach the driveway.

Verify that assigned turning demands add to the site totalFor illustrative 300 trips per hour, 50% north is 150, 30% west is 90, and 20% east is 60; percentages total 100%.Site: 300/hN 50% = 150/hW 30% = 90/hE 20% = 60/h150 + 90 + 60 = 300 veh/h

No-build, build, and mitigated build are distinct tests

Compare the same design hour, common horizon, and network assumptions. Use the method accepted by the approving authority to test intersection/lane capacity, volume-to-capacity ratio, control delay, LOS where meaningful, turn-bay storage, 95th-percentile or representative queues, spillback, crossing exposure, transit reliability, and freight/loading conflicts. LOS categories are not universal pass/fail thresholds; show the actual criterion and why a mitigation addresses the failure mechanism.

Compare identical performance measuresSynthetic example delays are 42 seconds per vehicle without development, 67 with development, and 49 after mitigation; these are not regulatory limits or model outputs.426749No-buildBuildMitigatedDelay (s/veh)

Illustrative volume-to-capacity ratio

An operations screening quantity, not a substitute for a complete capacity or delay analysis.

x=Vcx=\frac{V}{c}

Demand and capacity terms

VV is applicable demand (veh/h) and cc is estimated effective capacity for that same movement, lane group, and hour (veh/h). The ratio xx is dimensionless. Ensure capacity assumptions are compatible with controls, saturation flow, and lane use.

Simplified queue storage estimate

Only a classroom conversion from queued vehicles to metres, not a percentile-based design method.

L=NqℓqL=N_q\ell_q

Queue length estimate terms

LL is estimated standing queue storage length (m), NqN_q is the queued vehicle count (vehicles), and ℓq\ell_q is assumed effective space per queued vehicle (m/vehicle). In real projects use vehicle mix, a defensible queue statistic, blockage risk, and an approved design method.

Reporting and sensitivity

Reproducible TIA workflow

  1. Agree on jurisdiction, triggers, study network, periods, horizons, output measures, and approval criteria.
  2. Count and quality-check baseline turning movements, queues, pedestrians, transit, and geometry.
  3. Establish supported no-build forecasts and separately identifiable committed developments.
  4. Estimate site person and vehicle trips, mode shares, pass-by/diverted trips, and directional distribution.
  5. Assign trips to feasible routes; check conservation; compare no-build and build operations and safety.
  6. Develop proportional mitigation; rerun with the identical evaluation method and report sensitivity to growth, trips, modes, and background projects.
  7. Provide raw data, assumptions, drawings, calculation/method files, limitations, and reviewer-ready conclusions.
TIA Scenario, Network, and Review Logic

An auditable project-specific process from permitting scope to mitigated evaluation.

Scope cutoffs, growth assumptions, LOS criteria, and implementation obligations require authority approval; no values here are universal mandates.

TIA Scenario, Network, and Review LogicAn auditable project-specific process from permitting scope to mitigated evaluation.. Scope cutoffs, growth assumptions, LOS criteria, and implementation obligations require authority approval; no values here are universal mandates.. Confirm permit scope and authority → Collect existing counts and multimodal data; Collect existing counts and multimodal data → Build future no-build network and demand; Build future no-build network and demand → Estimate mode and site trip movements; Estimate mode and site trip movements → Distribute and assign conserved trips; Distribute and assign conserved trips → Assess build delay, queues, safety and access; Assess build delay, queues, safety and access → Meets authority-agreed criteria?; Meets authority-agreed criteria? — Yes → Test growth, mode share, and trip sensitivity; Meets authority-agreed criteria? — No → Propose and test proportionate mitigation; Propose and test proportionate mitigation → Assess build delay, queues, safety and access; Test growth, mode share, and trip sensitivity → Report scenarios, uncertainty and obligations; Report scenarios, uncertainty and obligations → Submit for jurisdiction-specific review

Confirm permit scope and authority → Collect existing counts and multimodal data; Collect existing counts and multimodal data → Build future no-build network and demand; Build future no-build network and demand → Estimate mode and site trip movements; Estimate mode and site trip movements → Distribute and assign conserved trips; Distribute and assign conserved trips → Assess build delay, queues, safety and access; Assess build delay, queues, safety and access → Meets authority-agreed criteria?; Meets authority-agreed criteria? — Yes → Test growth, mode share, and trip sensitivity; Meets authority-agreed criteria? — No → Propose and test proportionate mitigation; Propose and test proportionate mitigation → Assess build delay, queues, safety and access; Test growth, mode share, and trip sensitivity → Report scenarios, uncertainty and obligations; Report scenarios, uncertainty and obligations → Submit for jurisdiction-specific review

  • Confirm permit scope and authority: terminator
  • Collect existing counts and multimodal data: input output
  • Build future no-build network and demand: process
  • Estimate mode and site trip movements: process
  • Distribute and assign conserved trips: process
  • Assess build delay, queues, safety and access: process
  • Meets authority-agreed criteria?: decision
  • Propose and test proportionate mitigation: process
  • Test growth, mode share, and trip sensitivity: process
  • Report scenarios, uncertainty and obligations: document
  • Submit for jurisdiction-specific review: terminator
Key Takeaways
  • TIA obligations and review criteria require actual Philippine authority and permit confirmation.
  • Growth rates and study-area percentage cutoffs are inputs, not universal legal defaults.
  • Keep counts, added trips, pass-by access turns, and diverted-link reassignment distinct.
  • Check queues, storage, pedestrians, transit, and safety as well as capacity or LOS.
  • State uncertainty and test reasonable alternative assumptions before recommending mitigation.

References