What does a transportation engineer on retainer typically do?

A transportation engineer (PE with PTOE or traffic engineering experience, or AICP-certified transportation planner) on monthly retainer typically provides ongoing advisory across traffic impact analysis review, intersection and highway design advisory, travel demand modeling, and traffic safety analysis. In traffic impact analysis review, the retained transportation engineer evaluates ITE Trip Generation (11th Edition) land use code selection, reviews pass-by and internal capture reduction applications, checks signal timing assumptions in HCM 7th Edition LOS analyses, and evaluates background growth rate assumptions against local turning movement count data. In intersection and highway design advisory, the retained engineer reviews intersection sight distance against AASHTO Policy on Geometric Design stopping sight distance requirements, evaluates signal timing plans for minimum pedestrian phase timing per MUTCD Table 4E-1, reviews roundabout capacity analyses using NCHRP Report 672 entry capacity equations, and evaluates horizontal curve superelevation and transition lengths against AASHTO design speed requirements. In travel demand modeling, the retained engineer reviews travel demand model calibration against base year screenline count data, evaluates assignment error against RMSE acceptance criteria, and reviews future year mode split assumptions for consistency with adopted transit service plans. In traffic safety analysis, the retained engineer conducts crash frequency and rate analysis using the Highway Safety Manual Part C predictive methodology, evaluates the safety performance function (SPF) selection for the roadway facility type, and applies FHWA Crash Modification Factors (CMFs) from the CMF Clearinghouse to estimate countermeasure effectiveness. The retainer scope typically specifies the types of projects under review (residential, commercial, institutional, highway), the jurisdictional context, and whether expert witness services for contested development approvals are included in the monthly retainer.

What transportation engineering work is most commonly underlogged?

The most systematically underlogged categories in transportation engineer retainers are: ITE Trip Generation data source review and adjustment (reviewing the ITE database observations underlying the selected regression equation or average rate to confirm applicability to the local land use context — checking sample size, geographic distribution of observations, and rate vs. regression equation applicability — takes 4 to 10 hours and is invisible to the client until the TIA review letter is issued); HCM LOS calculation input verification (reviewing signal timing plans, saturation flow rate assumptions, and PHF against field-measured conditions rather than assuming HCM default values takes 6 to 14 hours per intersection and produces no visible outcome until an LOS discrepancy is documented); background growth rate validation (pulling historical turning movement count data from the study area, fitting a growth trend, and comparing the trend against the TIA's assumed background growth rate takes 4 to 8 hours and is invisible until a methodology objection is filed in the project record); signal warrant analysis documentation (running MUTCD Chapter 4C eight-hour vehicular volume, peak hour, and pedestrian volume warrants from turning movement count data and documenting the warrant evaluation methodology takes 5 to 12 hours and is invisible to the agency until the warrant analysis memorandum is provided); and crash data analysis (pulling five-year crash data from the state crash database, calculating crash rate per million entering vehicles for the subject location, and comparing to the statewide average crash rate for the facility type under HSM methodology takes 8 to 18 hours and produces no visible deliverable until the safety analysis memorandum is issued).

What should a transportation engineer retainer agreement include?

Transportation engineer retainer agreements should specify: the transportation engineering services included in the retainer scope (TIA review, intersection design advisory, signal timing, travel demand modeling review, traffic safety analysis, or a defined combination); the project context including the jurisdictional setting, the type of development projects or transportation improvements under review, and the applicable design standards (AASHTO Green Book, MUTCD, HCM 7th Edition, ITE Trip Generation 11th Edition, NCHRP reports); the specific deliverables (TIA review memorandum, LOS analysis spreadsheet, signal warrant analysis, sight distance calculation, safety performance analysis, travel demand model review comments); the applicable standards governing the advisory (AASHTO Policy on Geometric Design 2018, MUTCD 2009 with revisions through 2012, HCM 7th Edition 2022, ITE Trip Generation 11th Edition 2021, HSM 1st Edition 2010 with supplements); whether expert witness services for contested development hearing testimony, deposition in highway design litigation, and transportation safety expert opinion are included in the monthly retainer or require separate fee arrangements; and the hours tracking mechanism that gives the agency or developer visibility into transportation engineering advisory work between TIA submission dates, planning commission hearing dates, and traffic signal installation milestones. Monthly retainer amounts for transportation engineering advisory typically range from $4,000 to $14,000 per month depending on project volume, complexity, and whether expert witness testimony services are included.

What are typical retainer rates for transportation engineers?

Retainer rates for transportation engineers vary by experience level, specialization, and engagement type. Staff transportation engineers and traffic analysts (1 to 5 years, primarily conducting turning movement counts, HCM analyses, and ITE trip generation calculations) typically bill at $90 to $160 per hour. Senior transportation engineers and project leads with PTOE certification or traffic engineering PE (5 to 15 years, responsible for TIA preparation, signal design, and agency coordination) typically bill at $150 to $280 per hour. Principal transportation engineers specializing in travel demand modeling (CUBE, TransCAD, VISUM), microsimulation (VISSIM, Synchro, SimTraffic), or transportation safety analysis under HSM methodology typically bill at $225 to $400 per hour. Transportation engineers with established expert witness records in highway design standard of care cases, contested development approvals, or traffic signal warrant disputes typically command rates of $300 to $550 per hour for expert opinion and testimony work. Monthly retainer amounts typically range from $4,000 to $14,000 per month depending on project volume and complexity.

How should transportation engineer retainer hours be logged?

Transportation engineer retainer work log entries should capture the project, the specific transportation task, and the finding, technical issue, or advisory decision. A useful format is: [Project/Location] + [Specific transportation task] + [Finding, issue, or decision]. For example: 'Riverside Commons Mixed-Use TIA review — ITE LUC selection: developer used LUC 220 (Multifamily Housing – Mid-Rise) at 0.44 PM peak trips/unit for a 12-story building; reviewed ITE Trip Generation 11th Edition LUC definitions — LUC 220 is specified for 3-10 story buildings, LUC 221 (High-Rise) at 0.36 PM peak trips/unit applies to 11+ story buildings; trip generation revised from 179 to 147 PM peak-hour trips; transit proximity reduction of 15% was applied with nearest transit stop at 0.28 miles — agency guidelines allow reduction only for stops within 0.25 miles; revised trip estimate 147 trips without transit reduction: 7 hours.' Or: 'Route 18 at Commerce Drive signal timing review — PHF used in HCM 7th Edition analysis was 0.95 (default) vs. 0.87 measured from turning movement count data; reducing PHF from 0.95 to 0.87 increases the adjusted turning movement demand by 9%, shifting the eastbound approach from v/c 0.78 (LOS C) to v/c 0.85 (LOS D) in the PM peak hour; signal timing review also identified minimum pedestrian phase of 7 seconds used for a 48-foot crossing distance — MUTCD Table 4E-1 requires 3.5 ft/sec walk speed design, yielding minimum 14 seconds for that crossing: 9 hours.' Entries that name the project, the specific calculation reviewed, and the numerical finding make the work log legible as a concrete technical record for the agency or developer.

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Transportation engineer on retainer: traffic impact analysis review, intersection and highway design advisory, travel demand modeling, and traffic safety analysis on monthly retainer

July 31, 2026 · ~22 min read

A regional municipality is reviewing a traffic impact analysis submitted for a proposed 320-unit multifamily residential development at a constrained urban infill site. The developer’s traffic engineer has used ITE Land Use Code 220 (Multifamily Housing — Mid-Rise) at a PM peak-hour trip rate of 0.44 trips per unit to produce a total PM peak-hour trip generation estimate of 141 trips before pass-by adjustment. The TIA’s Level of Service analysis shows the adjacent signalized intersection operating at LOS C in the PM peak hour with the project trips added to background traffic — within the agency’s LOS D minimum acceptable level.

The municipality’s retained transportation engineer reviews the TIA and identifies two errors. The proposed building is 12 stories — ITE Trip Generation 11th Edition defines LUC 220 (Mid-Rise) as applicable to buildings of 3 to 10 stories; the correct classification for an 11-or-more-story multifamily building is LUC 221 (High-Rise) at 0.36 PM peak-hour trips per unit. The 12-story building’s trip generation should be 115 PM peak-hour trips, not 141 — a 19% overestimate in this case, since the developer actually benefits from the correct LUC producing fewer trips. The second error runs the other direction: the TIA applied a 15% transit proximity reduction to all trips per the agency’s TIA guidelines, which allow up to a 20% transit reduction for sites within 0.25 miles of a transit stop with 15-minute or better headways. The nearest qualifying transit stop is 0.28 miles from the project driveway — 3 feet beyond the agency’s 0.25-mile threshold. The uncorrected TIA has underestimated both the applicable adjustment factors and the applicable land use code, and the errors partially offset: the corrected trip generation without the transit reduction is 115 trips, versus 120 trips in the original submittal after applying the transit reduction to the LUC 220 rate.

Reviewing the ITE land use code definition, confirming the building height against the LUC specifications, locating the nearest transit stop on the agency’s GIS layer, and measuring the walking distance required 7 hours of retained advisory time. That review produced a formal comment letter identifying the LUC error and the transit proximity threshold error that would have otherwise been entered into the public project record as unchallenged.

Traffic impact analysis review advisory

Traffic impact analysis review advisory is the transportation engineering retainer function that evaluates submitted TIAs for accuracy in trip generation methodology, background traffic growth assumptions, level of service calculation methodology, and mitigation measure adequacy before the TIA is accepted by the reviewing agency as technically complete. The retained transportation engineer reviewing TIAs does not typically collect the underlying turning movement count data — that is done by the applicant’s traffic engineer — but reviews the methodology, assumptions, and calculations for errors that would affect the LOS findings, the significance thresholds, and the need for mitigation improvements.

ITE trip generation methodology review

ITE trip generation methodology review evaluates whether the selected land use code accurately describes the proposed development, whether the appropriate estimating technique (fitted curve regression equation vs. weighted average rate) was selected for the observation count in the ITE database, whether internal capture and pass-by reductions were applied with supporting documentation, and whether locally calibrated trip rates from the agency’s own count database were available and should have been used in lieu of national ITE rates. Trip generation methodology errors that most frequently require transportation engineering review are: land use code selection that does not match the ITE LUC definitions for the proposed building type or tenant mix (ITE LUC 820 Shopping Center is defined for enclosed mall and strip center configurations with anchor tenants; a power center, lifestyle center, or open-air specialty retail development is better characterized by LUC 826 (Specialty Retail) or LUC 823 (Factory Outlet Center), with materially different trip rates — LUC 820 averages 42.70 PM peak trips per 1,000 SF GLA versus LUC 826 at 9.21 PM peak trips per 1,000 SF GLA for fitted curve at 100,000 SF, a 4.6× difference); regression equation vs. average rate selection for small dataset sizes (when the number of ITE database observations is below 20, the fitted curve regression equation can have a coefficient of determination r² below 0.50, meaning the regression explains less than half the variance in the data set; in those cases the average rate may provide a more stable estimate than the regression equation, and the TIA should document which estimating technique was selected and why); and transit and pedestrian mode reduction applications without site-specific survey or agency-adopted methodology support (transit proximity reductions that are applied at a fixed percentage without measuring the actual transit access quality — stop proximity, headway, service span, and transfer requirement — at the subject site produce reductions that may be unsupported by the observed travel behavior at similar sites in the local context).

A retained transportation engineer reviewed a TIA for a proposed 480,000 SF mixed-use lifestyle retail development. The developer’s traffic engineer had used ITE LUC 820 (Shopping Center) to estimate PM peak-hour trip generation at 42.70 trips per 1,000 SF × 480 KSF = 20,496 PM peak trips before pass-by reduction, then applied a 34% pass-by reduction per ITE to arrive at 13,527 new PM peak-hour trips. The retained engineer reviewed the development program and found the project contained no anchor tenant, no enclosed mall, and was configured as a series of single-story specialty retail and restaurant buildings in an open-air village format. The applicable ITE LUC was LUC 826 (Specialty Retail) at 9.21 PM peak trips per 1,000 SF using the fitted curve equation at 480 KSF, producing 4,421 new PM peak trips before pass-by reduction. The corrected estimate with pass-by reduction was 2,918 new PM peak trips — 78% fewer than the LUC 820 estimate. The LUC misclassification was identified before the TIA was entered into the planning commission record.

Level of service analysis and signal timing review

Level of service analysis review evaluates whether the HCM 7th Edition (Highway Capacity Manual) signalized intersection analysis used the correct saturation flow rate, peak hour factor, phase timing, and lost time inputs, and whether the resulting v/c ratios and LOS designations are supported by the input data. Signal timing review evaluates whether the cycle length, effective green times, and minimum pedestrian phase durations used in the HCM analysis reflect the actual or proposed signal timing plan, or whether default values were substituted without verification against field conditions. Signal timing analysis errors that most frequently require transportation engineering review are: peak hour factor (PHF) selection using HCM default values instead of the value calculated from the 15-minute turning movement count data collected at the study intersection (a PHF of 0.95 is frequently used as a default; measured PHF values at suburban intersections typically range from 0.82 to 0.92, and a 0.92 PHF inflates the adjusted hourly demand by 3.2% relative to the 0.95 default, shifting intersection operations toward higher v/c ratios that may cross LOS thresholds); saturation flow rate assumption that uses the HCM base saturation flow rate of 1,900 pcphpl without local adjustment for lane width, truck percentage, or turning movement deceleration (a lane with 11-foot width rather than 12-foot width carries a 3% saturation flow reduction per HCM adjustment factors, and a 5% truck percentage with equivalency factor of 2.0 produces an additional 4.8% reduction, corrections that are not applied when the base rate of 1,900 pcphpl is assumed without verification); and minimum pedestrian phase timing that does not meet MUTCD Section 4E.06 requirements (minimum pedestrian phase must provide at least 7 seconds of walk time plus a pedestrian clearance interval calculated at 3.5 feet per second for the crossing distance; a 48-foot crossing requires a 14-second minimum crossing time, and a 7-second pedestrian phase without a flashing don’t walk interval does not provide the minimum crossing time for a slower pedestrian walking at 2.5 ft/sec per ADA pedestrian design guidelines).

A retained transportation engineer reviewed a TIA’s LOS analysis for a signalized study intersection. The TIA showed the intersection operating at LOS C (v/c = 0.78) in the PM peak hour with the project trips added. The retained engineer reviewed the signal timing plan on file with the agency and found the TIA had used a PHF of 0.95 (HCM default) and a cycle length of 100 seconds for the analysis. The agency’s signal timing log showed the controller was operating on a 90-second cycle in the PM peak period. Re-running the HCM 7th Edition analysis with the measured PHF of 0.87 from the turning movement count data and the correct 90-second cycle length produced v/c = 0.93 (LOS E) on the critical eastbound approach — over the agency’s LOS D significance threshold. The LOS finding shifted from “no significant impact” to “significant impact requiring mitigation” based on the PHF and cycle length correction. A left-turn channelization improvement on the eastbound approach reduced v/c to 0.88 (LOS D) — within the acceptable threshold.

Background growth and trip distribution review

Background growth rate review evaluates whether the annual growth rate applied to existing turning movement count volumes to project background traffic at the design year is supported by historical count data at the study locations or consistent with the adopted regional travel demand model projections. Trip distribution review evaluates whether the directional distribution of project-generated trips to and from the study intersections is consistent with the land use pattern, the highway network configuration, and the trip purposes of the proposed land use. Background growth and distribution errors that most frequently require transportation engineering review are: background growth rates applied uniformly across all movements and time periods without verification against historical count data (applying a 1.0% annual growth rate at an intersection where historical count data shows eastbound PM peak volume declining at −0.3% per year overstates the future background volume on that approach, producing a conservative LOS analysis that may overstate the project’s incremental contribution to intersection delay); trip distribution based on a gravity model calibrated to the regional travel demand model for a different land use type than the proposed project (distributing multifamily residential trips using a gravity model calibrated to suburban retail trips can produce directional distribution percentages inconsistent with the residential commute pattern, overstating the proportion of project trips assigned to one approach and understating the proportion assigned to another); and committed development traffic included in background volumes from a development that has not yet received entitlement approval (projects included in the background traffic that are subsequently denied entitlement or substantially reduced in scope will not produce the traffic volumes assumed in the background, producing an overstated background volume that artificially understates the project’s relative contribution to the degraded LOS condition).

A retained transportation engineer reviewed background growth assumptions in a TIA for a proposed warehouse distribution center. The TIA applied a 1.5% annual background growth rate to all movements at study intersections, producing a 2028 background volume on the southbound approach of 680 vehicles in the PM peak hour. The retained engineer pulled 10 years of AADT data from the state DOT count station at the subject highway segment and found a compound annual growth rate of −0.8% per year — the segment had been declining in volume since a parallel arterial bypass was opened in 2019. Correcting the background growth rate from +1.5% to −0.8% per year reduced the projected 2028 southbound PM peak volume from 680 to 540 vehicles, producing a southbound approach v/c that was 0.12 lower at the project’s design year — a material difference in the mitigation conclusions for an approach already operating near the LOS D threshold.

Intersection and highway design advisory

Intersection and highway design advisory is the transportation engineering retainer function that reviews geometric design plans for proposed intersections, highway improvements, and access configurations for conformance with AASHTO design standards, MUTCD signing and marking requirements, and the agency’s adopted design criteria. The retained transportation engineer advising on intersection and highway design does not typically produce the primary design drawings — that is done by the project design engineer — but reviews the design for stopping sight distance adequacy, geometric element compliance, turning movement accommodation, and signal warrant documentation before the plans are submitted for agency approval.

Stopping sight distance and intersection sight distance review

Stopping sight distance (SSD) review evaluates whether the available sight distance at each approach to a proposed intersection or access point meets the AASHTO minimum SSD for the posted speed limit per AASHTO Policy on Geometric Design of Highways and Streets (Green Book) Exhibit 3-1. Intersection sight distance (ISD) review evaluates whether the available sight lines for crossing and turning movements from the minor road approach meet the AASHTO ISD requirements for time gap acceptance (Case B1 through B5 depending on the intersection configuration and the turning movement type). Sight distance review errors that most frequently require transportation engineering correction are: SSD calculation using posted speed limit rather than the 85th percentile operating speed when the 85th percentile speed exceeds the posted limit by more than 5 mph (AASHTO SSD criteria use design speed as the basis for deceleration and perception-reaction time calculations; using the posted speed as a proxy for design speed when driver operating speeds are materially higher produces a shorter calculated SSD than the actual design condition requires); profile grade correction omitted from crest vertical curve sight distance calculations (the available sight distance across a crest vertical curve is a function of the algebraic difference in grades and the vertical curve length; if the designer uses the horizontal clearance as the sight line measure without correcting for the vertical profile, the available sight distance can be overstated at locations with significant crest vertical curves); and intersection sight triangles that assume vegetation trimming will maintain clear sight lines without a maintenance agreement or right-of-way easement protecting the sight triangle (sight lines that depend on ongoing maintenance of private landscaping in the sight triangle are not reliably available for the design life of the intersection without a maintenance easement or roadway right-of-way extending to the back of the sight triangle).

A retained transportation engineer reviewed the intersection sight distance at a proposed commercial access drive connecting to a rural two-lane highway posted at 45 mph. The geometric design plan showed an available intersection sight distance of 495 feet to the left from the minor road approach, evaluated at the 15-foot setback from the edge of the travel lane. The AASHTO ISD requirement for Case B1 (crossing a two-lane undivided highway) at 45 mph posted speed is 435 feet for passenger vehicles per Green Book Exhibit 9-54 using the 85th percentile speed of 50 mph (as measured by the agency’s speed study at this location). The design appeared compliant by 60 feet. The retained engineer reviewed the vertical profile at the sight line location and found the available sight line passed over a crest in the highway alignment approximately 320 feet from the access drive. Correcting the sight distance calculation for the crest vertical curve geometry reduced the available sight distance to 410 feet — 25 feet below the 435-foot ISD requirement. A supplemental grading easement was required to lower the crest profile within the sight triangle before the access permit was issued.

Roundabout capacity and geometry review

Roundabout capacity and geometry review evaluates whether the proposed roundabout geometry — inscribed circle diameter, entry width, circulating lane width, number of entry lanes, and entry angle — is consistent with NCHRP Report 672 (Roundabouts: An Informational Guide) design parameters for the design vehicle, the design speed, and the projected entry and circulating volumes. Roundabout capacity analysis using the entry capacity equation from NCHRP Report 672 relates the approach entry capacity to the conflicting circulating volume per lane, with entry capacity declining as circulating volume increases. Roundabout geometry and capacity errors that most frequently require transportation engineering review are: entry capacity calculations that use the single-lane equation for a two-lane entry without adjusting for the lane use factor that reflects the unequal distribution of approaching vehicles across the entry lanes (a two-lane roundabout entry where the left entry lane carries approximately 40% of approaching volume and the right entry lane carries 60% will produce a different entry capacity per lane than an analysis assuming 50/50 distribution, and applying the single-lane equation to the combined approach volume rather than each lane independently overstates total entry capacity); inscribed circle diameter selection for multi-lane roundabouts that does not accommodate the design vehicle swept path (a 140-foot inscribed circle diameter is typically the minimum for a two-lane roundabout with design WB-67 (67-foot semi-trailer) swept path, and an under-diameter circle will result in the design vehicle requiring the circulating lane to complete the turning maneuver, a conflict with circulating vehicles); and splitter island geometry that does not provide at least 6 feet of refuge width for a bicyclist waiting to enter the circulating roadway per NCHRP Report 672 guidance.

A retained transportation engineer reviewed a single-lane roundabout design at a rural highway intersection. The entry capacity analysis showed each approach operating at LOS A in the design year peak hour. The retained engineer reviewed the entry capacity calculation and found the designer had used a circulating volume of 180 pcph as the conflicting flow for each approach — the total circulating volume — without distributing it to the conflicting lanes per approach. For a single-lane roundabout, this approach is correct. However, reviewing the projected 2038 design year volumes, the retained engineer found the northbound approach entry volume of 420 pcph exceeded the single-lane NCHRP Report 672 entry capacity at the projected circulating volume of 340 pcph: the entry capacity equation e = 1,130 e⊃(−0.0019 Vc) produced an entry capacity of 698 pcph at Vc = 340, which appeared adequate. The error was in the unit: the 340 pcph circulating volume was in passenger vehicles per hour per lane but should have been adjusted for heavy vehicle percentage. Applying a 12% heavy vehicle factor on the rural highway reduced the adjusted circulating volume to 391 passenger car equivalents, reducing entry capacity to 640 pcph. At the 420-pcph entry demand, v/c = 0.66 (LOS C), still within threshold, but the analysis was materially different from the originally reported LOS A finding.

Travel demand modeling advisory

Travel demand modeling advisory is the transportation engineering retainer function that reviews regional or subarea travel demand model development, calibration, validation, and application for technical adequacy and appropriateness of model outputs for the intended planning application. The retained transportation engineer advising on travel demand modeling does not typically build the primary travel demand model — that is done by the MPO, state DOT modeling team, or project consultant — but reviews the model structure, calibration targets, validation results, and scenario outputs for errors in network coding, trip generation rates, mode split assumptions, and traffic assignment methodology.

Model calibration and validation review

Travel demand model calibration and validation review evaluates whether the model reproduces observed base year traffic conditions within the FHWA and NCHRP-acceptable error tolerances, whether the validation has been performed against independent count data not used in calibration, and whether the model is suitable for the planning application at hand given its geographic scale and temporal resolution. Model calibration and validation errors that most frequently require expert review are: screenline validation with RMSE exceeding 25% for links carrying more than 1,000 daily vehicles (the FHWA model validation guidelines recommend maximum 25% RMSE for high-volume links; exceeding this threshold indicates that the model trip distribution, mode split, or assignment parameters are not adequately reproducing observed travel patterns in at least some corridors); base year loaded link volume comparison with observed counts that shows systematic bias in a particular direction (a model that consistently underpredicts volume by 15 to 20% on all north-south freeway segments and overpredicts by a similar margin on east-west arterials indicates a network impedance calibration error in the north-south freeway coding, not a random calibration error that averages out across the network); and design year mode split assumptions that project transit market share in the future based on the existing transit service level without accounting for planned service expansions (applying the base year transit mode split to the design year without modeling the ridership effect of the planned BRT or light rail extension produces a mode split forecast that understates the transit ridership increase and overstates the highway volume increase in the project corridor).

A retained transportation engineer reviewed the validation statistics for a metropolitan area travel demand model used to support a major interchange improvement project environmental impact study. The model documentation reported an overall RMSE of 22% for all validated links, within the FHWA guideline. The retained engineer reviewed the validation results disaggregated by facility type and found the RMSE for freeway links was 38% — substantially above the guideline for the highest-volume facility class. The high RMSE on freeway links was traced to a network coding error in the ramp capacity parameters: on-ramp capacity was coded at 1,800 pcph for all on-ramps, regardless of ramp geometry, producing uniform assignment behavior that did not reproduce the observed ramp metering delay at the two controlled freeway entrance ramps in the study area. Recoding the ramp capacity parameters at 900 pcph for the two metered ramps reduced freeway RMSE from 38% to 19%, within the FHWA guideline, and changed the design year mainline volume forecast on the subject freeway segment by 8%.

Traffic safety analysis advisory

Traffic safety analysis advisory is the transportation engineering retainer function that evaluates crash history at study locations, identifies overrepresented crash patterns, and recommends countermeasures with estimated effectiveness using the Federal Highway Administration Crash Modification Factor (CMF) methodology and the Highway Safety Manual Part C predictive method. The retained transportation engineer providing traffic safety analysis does not typically administer the safety improvement program — that is done by the agency’s safety program office and HSIP funding coordinator — but evaluates the crash data, identifies the statistical significance of observed crash patterns, and estimates the expected crash reduction from candidate countermeasures.

Crash frequency and rate analysis

Crash frequency and rate analysis evaluates whether the observed crash frequency at a study location is statistically elevated above the expected frequency for similar locations on the same roadway facility type, using the Highway Safety Manual Part C safety performance functions (SPFs) calibrated to the local jurisdiction to define the expected crash frequency. Crash analysis errors that most frequently require transportation engineering expert review are: crash rate calculation that uses a fixed mileage denominator without considering the entering vehicle volume at the study location (calculating crash rate as crashes per million vehicle miles traveled (MVMT) produces a higher apparent crash rate for lower-speed, lower-AADT facilities where crash exposure is better expressed as crashes per million entering vehicles (MEV) for intersection locations); comparison of observed crash rates to statewide average crash rates published in the state crash manual without confirming that the comparison benchmark uses the same facility type classification (comparing an urban five-lane undivided arterial crash rate to the statewide average for “urban arterials” without matching the specific roadway type to the appropriate comparison class within the statewide averages can produce a misleading “above average” or “below average” conclusion); and crash pattern identification that does not distinguish between rear-end crashes associated with signal timing deficiencies, angle crashes associated with sight distance limitations, and single-vehicle run-off-road crashes associated with lane departure conditions (each crash type has different candidate countermeasures and CMF effectiveness estimates, and a crash analysis that groups all crashes together without identifying the dominant crash type pattern will not support targeted countermeasure selection).

A retained transportation engineer analyzed five-year crash data at a signalized intersection with 14 total crashes over the study period. The safety study submitted by the agency’s consultant calculated a crash rate of 0.82 crashes per million entering vehicles (MEV) and concluded the location was “below the statewide average of 1.04 crashes per MEV for urban signalized intersections.” The retained engineer reviewed the crash type breakdown and found 9 of 14 crashes were rear-end crashes on the northbound approach, with 7 occurring in the afternoon peak period. The HSM Part C SPF for urban four-leg signalized intersections predicted 9.4 total crashes per year at the subject intersection AADT; the observed 2.8 crashes per year was below the predicted frequency, apparently confirming the below-average conclusion. However, the rear-end crash concentration on one approach — 64% of all crashes at one movement in one peak period — represented a pattern inconsistent with the overall intersection crash distribution. The retained engineer recommended a targeted countermeasure evaluation for the northbound approach: the signal timing showed only 3.2 seconds of yellow change interval for a 45 mph approach speed, below the ITE recommended minimum of 4.5 seconds. Extending the yellow to 4.5 seconds carried a CMF of 0.65 for rear-end crashes per the CMF Clearinghouse, estimating a 35% reduction in rear-end crashes on the affected approach.

HSIP countermeasure selection and benefit-cost analysis

Highway Safety Improvement Program (HSIP) countermeasure selection review evaluates whether the candidate safety improvements have been selected using CMFs from the CMF Clearinghouse with an appropriate star quality rating, whether the crash reduction calculation correctly applies the CMF to the crash type and crash severity distribution at the subject location, and whether the benefit-cost analysis uses the current FHWA unit crash cost values by KABCO injury severity level. HSIP benefit-cost analysis errors that most frequently require transportation engineering review are: CMF application to all crash types at a location when the CMF has been validated only for the specific crash type being addressed (a CMF for turn lane addition reduces angle crashes and left-turn crashes; applying it to the total crash count rather than only to the angle and left-turn crash types overstates the expected crash reduction); benefit calculation using outdated crash cost values (FHWA publishes unit crash cost guidance that is updated periodically to reflect inflation in medical, lost productivity, and quality-of-life cost components; using 2010 crash cost values in a 2026 analysis will understate the monetized crash reduction benefit by 40 to 60%); and countermeasure cost comparison that uses construction cost estimates without including the present value of ongoing operations and maintenance costs for signal timing updates, pavement marking maintenance, and signing replacement (a countermeasure with a 25-year life requires maintenance cost present value to be included in the cost denominator for an accurate benefit-cost ratio).

A retained transportation engineer reviewed HSIP countermeasure recommendations for a rural highway corridor with an elevated run-off-road crash rate. The agency had proposed shoulder widening from 4 feet to 8 feet along a 3.2-mile segment as the primary safety countermeasure, estimating a benefit-cost ratio of 4.2:1 based on a CMF of 0.74 from the CMF Clearinghouse for shoulder widening from 4 to 8 feet applied to total crashes. The retained engineer reviewed the CMF 3-star rating and found it was specific to single-vehicle run-off-road crashes, not all crash types. Of the 22 crashes in the 5-year study period at the subject corridor, 14 were single-vehicle run-off-road crashes and 8 were multi-vehicle head-on or sideswipe crashes not addressed by shoulder widening. Applying the CMF to only the 14 SVROR crashes rather than all 22 crashes reduced the estimated crash reduction from 5.3 to 3.4 crashes per year, reducing the benefit-cost ratio from 4.2:1 to 2.7:1 — still above the minimum 1.0:1 threshold for HSIP eligibility but materially different from the 4.2:1 initially reported, and appropriately scoped to the crash problem the countermeasure addresses.

Why transportation engineering retainer hours are invisible between project milestones

Transportation engineering retainers generate most of their value between visible project approval and construction milestones. Planning commission hearing dates are visible. Traffic signal installation dates are visible. Highway improvement construction completion dates are visible. What is invisible to the planning director or public works director are the hours the retained transportation engineer spent verifying the ITE land use code definition before responding to the TIA review request, the hours checking the agency’s signal timing log against the cycle length and phase splits assumed in the LOS analysis, the hours pulling historical AADT trend data to verify the background growth rate applied in the design year projections, and the hours analyzing the CMF applicability to the specific crash type distribution before including a safety improvement in the HSIP project list.

The invisibility problem is particularly acute in transportation engineering retainers because the advisory work is specifically designed to catch methodology errors and assumption deviations before they become part of the public project record. When the retained transportation engineer identifies a PHF input error before the TIA is accepted as complete, the LOS finding is corrected before it is cited in the conditions of approval as the basis for the required mitigation improvements. When the retained engineer identifies a background growth rate that overstates traffic increase at a declining-volume location, the project’s incremental contribution to intersection delay is fairly evaluated rather than attributed to background traffic growth that will not occur. When the retained engineer identifies a CMF applied to total crashes rather than the targeted crash type, the HSIP project list reflects the realistically achievable crash reduction rather than an overstated benefit estimate that will underperform expectations when the improvement is constructed.

Transportation engineers on retainer who use a structured work log can show agencies and developers what the invisible advisory hours produced. The 7-hour ITE land use code review becomes a work log entry documenting the LUC definition checked, the building height confirmed, the transit stop distance measured, and the corrected trip generation estimate that replaced the submitted figure in the project record. The 9-hour LOS calculation review becomes a record of the PHF measured from the turning movement count, the cycle length confirmed from the agency signal timing file, and the corrected v/c ratio that changed the LOS finding from C to E on the critical approach. HourTab is a retainer hours dashboard built for advisory relationships like transportation engineering retainers where the client value — TIA methodology errors corrected before the project record is established, signal timing inputs verified before LOS findings are cited in conditions of approval, safety countermeasures scoped to the crash types they actually address — is created between project hearing dates and construction milestones. The transportation engineer logs time against specific review, analysis, and advisory tasks with technical notes, and shares a public URL that gives the agency or developer a running view of hours balance and work log between TIA submission dates and planning commission approvals.

Setting up a transportation engineering retainer agreement

Transportation engineer retainer agreements should define the scope with enough specificity to distinguish routine TIA review and intersection design advisory included in the monthly retainer from expert witness services for contested development hearings, traffic impact litigation testimony, and travel demand model development that require separate scoping and fee arrangements. A retainer structured as “transportation engineering advisory” without specifying the project types, the review deliverables, and the applicable design standards creates scope ambiguity about whether the retainer covers TIA review only, includes intersection geometric design review, and whether hearing testimony and deposition services are included or additional.

A well-structured transportation engineering retainer specifies: the transportation engineering services covered (TIA review, intersection design advisory, signal timing review, roundabout capacity analysis, travel demand model review, traffic safety analysis, or a defined combination); the project context including the jurisdictional setting, the volume and type of development projects under review, and the applicable design standards (AASHTO Green Book 7th Edition 2018, MUTCD 2009 with revisions, HCM 7th Edition 2022, ITE Trip Generation 11th Edition, NCHRP Report 672 for roundabouts, HSM 1st Edition 2010 for safety analysis); the specific deliverables (TIA review memorandum, LOS analysis check spreadsheet, sight distance calculation, signal warrant analysis, safety performance function application, HSIP benefit-cost review); whether expert witness services for contested development hearings, deposition testimony in highway litigation, and transportation safety expert opinions are included in the monthly retainer or require separate fee arrangements; and the hours tracking mechanism that gives the agency or developer visibility into transportation engineering advisory work between project submission dates, planning commission hearing dates, and traffic signal installation milestones. Monthly retainer amounts for transportation engineering advisory typically range from $4,000 to $14,000 per month depending on project volume, complexity, and whether expert witness testimony is included in the retainer scope.

HourTab turns a time-tracker CSV into a public retainer-hours URL your client can bookmark. No client login. No portal setup. Start free →