An Applicant's Trip Generation and a Corridor That Fails Without the Project
A developer proposes 250 single-family detached units, 40,000 sq ft of neighborhood retail, and 60,000 sq ft of office on 42 acres at the suburban edge of a city. Locally calibrated vehicle trip rates are: single-family 9.5 daily / 0.75 AM peak / 0.95 PM peak per unit; retail 45 daily / 1.2 AM / 3.5 PM per 1,000 sq ft; office 11 daily / 1.5 AM / 1.5 PM per 1,000 sq ft. The applicant's engineer applied a 20 percent internal capture to all trips and a 40 percent pass-by reduction to retail, producing 240 PM peak trips. The adjacent arterial is projected to carry 1,780 vph in the peak direction in the build year without the project; the adopted LOS D threshold is 1,700 vph. Existing volume is 1,610 vph, and background growth alone accounts for the 170-vph increase. The site is 0.4 mi from a planned bus rapid transit station opening in three years.
1. Recompute the net new external PM peak trips and evaluate the applicant's internal capture and pass-by assumptions.
Read the worked answer
Gross PM peak trips (both directions)
- Single-family: 250 x 0.95 = 237.5
- Retail: 40 x 3.5 = 140.0
- Office: 60 x 1.5 = 90.0
- Total = 467.5
Critique of the applicant's adjustments
- 20 percent internal capture applied to all land uses: internal capture is generated by complementary pairs - residential-to-retail and office-to-retail. Retail here is only 40,000 sq ft against 60,000 sq ft of office and 250 homes; the retail base is too small to absorb that share. A 10 percent blended rate is defensible; 20 percent is not.
- 40 percent pass-by applied to retail: pass-by is legitimate for convenience retail but must be applied to the retail trips only, never to residential or office. A 35 percent retail pass-by rate is representative for this size of center.
Recomputation
- Internal capture 10%: 467.5 x 0.90 = 421
- Remaining retail = 140 x 0.90 = 126; pass-by 35% = 44
- Net external PM peak = 421 - 44 = about 377 trips
Judgment. The applicant's 240 understates the project by roughly 57 percent. At a 60/40 directional split, the heavier direction carries about 226 vph - close to three times the 80-vph no-build deficiency on the arterial. The gap is not a rounding difference; it changes the concurrency conclusion.
2. Determine the project's proportionate share of the corridor deficiency and assess whether the applicant can argue the impact is de minimis.
Read the worked answer
Build condition. 1,780 (no-build) + 226 (project, peak direction) = 2,006 vph against a 1,700-vph LOS D threshold.
Who causes the failure
- Total volume above the threshold = 2,006 - 1,700 = 306 vph
- Of that, 80 vph is background growth and 226 vph is the project
- Project share of the deficiency = 226/306 = about 74 percent
De minimis argument. The applicant will note that the link fails without the project, so the project is not the sole cause. That is true but does not help much here: the project is the dominant cause. A de minimis threshold is typically on the order of a few percent of capacity - the project is contributing roughly 11 percent of total build traffic and 74 percent of the excess. Both numbers point the same direction.
Practical framing. Two defensible positions exist, and the planner should state which is being used:
- Proportionate share of the improvement cost: 74 percent of the cost of the capacity improvement that restores LOS D. This is the harsher reading.
- Share of total build-year volume: 11 percent. Some jurisdictions use this for areawide improvements rather than site-specific links.
Judgment. The recommendation should not be 'deny on concurrency' alone. The project can proceed if mitigation is packaged: a proportionate-share payment toward the corridor widening, a signalized site access rather than a full-movement driveway, and a TDM commitment that is enforceable - with the 0.4-mi BRT station as the credible basis for it, but only if the station opens before the project's first certificate of occupancy. If the timing does not line up, do not bank the transit credit.
3. The applicant offers a TDM program (transit passes, vanpool, telework) claiming a 20 percent trip reduction. How would you treat that claim in the traffic study and what conditions would you attach?
Read the worked answer
Treat it as unproven until it is measured. Trip reduction claims of 20 percent from a voluntary TDM package are at the optimistic end of what the literature supports for suburban residential development without pricing or parking constraints; typical realized reductions are in the single digits to low teens and decay over time.
How to handle it in the study
- Show two runs: a concurrency run with no TDM credit and a sensitivity run with the claimed credit. The concurrency determination should be made on the no-credit run when the corridor is already failing.
- If credit is allowed, cap it at something defensible (5 to 10 percent) and apply it only to the residential and office components, not retail.
Conditions to attach
- Measurable commitment: annual mode-split or vehicle-trip survey reported to the city, with a defined methodology.
- Trigger and consequence: if measured trips exceed the study assumption by more than a stated margin, a mitigation payment is triggered automatically - not renegotiated.
- Funded and staffed: transit passes and a transportation management association budget secured for the life of the development, not a one-year launch.
- Physical support: site design that makes the BRT station reachable (direct pedestrian connection, no walled perimeter) - a pass program cannot overcome a 1.5-mi walking route.
Judgment. The most common failure is allowing a large TDM credit to resolve a concurrency failure and then never measuring it. Either the credit is small enough that the project still mitigates, or the credit is backed by a monitoring and enforcement mechanism. Do not accept a number that exists only in the applicant's narrative.