- Scope-matched to light-duty vehicles only, NHTS 2022's weighted personal-vehicle-driver trip mileage captures 82.8% of FHWA's Table VM-1 light-duty total (a 17.2% undercount), not the approximately 71% figure that results from comparing NHTS against FHWA's full all-vehicle total.
- That ~71%/29%-undercount figure, which this site's own internal documentation previously used as a rough note, conflates a real survey-methodology effect with a scope mismatch: NHTS personal-vehicle trip codes never capture buses, single-unit trucks, combination trucks, or motorcycles, so comparing against FHWA's all-vehicle total systematically overstates the apparent undercount by about 9.7 percentage points.
- The scope-matched, reproducible correction factor is 1.207: multiplying an NHTS-derived personal-vehicle mileage total by 1.207 approximates the equivalent FHWA VM-1 light-duty figure.
01 Introduction
Researchers using the National Household Travel Survey (NHTS) for mileage estimates routinely need to account for a known property of self-reported trip-diary data: weighted NHTS mileage totals run lower than the Federal Highway Administration's administratively-measured Vehicle Miles Traveled (VMT), reported annually in Table VM-1. A commonly cited approximate figure for this gap is that NHTS captures around 70-71% of FHWA's total. This note shows that figure is not wrong so much as imprecise: it compares NHTS's personal-vehicle-only mileage against FHWA's entire vehicle fleet, including bus, truck, and motorcycle travel that NHTS's personal-vehicle trip codes were never going to capture in the first place. Once the comparison is restricted to the same vehicle population on both sides, the real gap is smaller, and a reproducible correction factor can be computed directly.
NHTS 2022 Public Use (csv/tripv2pub.csv); FHWA Office of Highway Policy Information, Table VM-1 (2022, updated March 2025).02 The Scope-Matching Problem
NHTS's trip file records each respondent's mode of travel via the TRPTRANS variable. The personal-vehicle codes are 01 (Car), 02 (Van), 03 (SUV/Crossover), 04 (Pickup truck), and 06 (RV) (there is no code 05). A trip recorded under any other mode, including when a personal vehicle trip is driven by someone other than the respondent, is correctly excluded from a personal-vehicle-driver mileage total.
FHWA's Table VM-1, by contrast, reports VMT across the entire national motor vehicle fleet, broken into Light Duty Vehicles (short- and long-wheelbase, i.e., cars, vans, SUVs and pickups, columns confirmed directly from the table's own header rows), motorcycles, buses, single-unit trucks, and combination trucks. Comparing NHTS's personal-vehicle total against VM-1's all-vehicle total (2,822,663.9 + 23,764.9 + 18,490.2 + 684,858.6 + 136,223.8 + 195,048.6 million miles ≈ 3,196,191.4 million miles) necessarily understates NHTS's apparent capture rate, since NHTS was never attempting to measure bus, truck, or motorcycle mileage through its personal-vehicle-driver trips.
FHWA Table VM-1 (2022), "Total Rural and Urban" row, all nine vehicle-type/subtotal columns.| Comparison | FHWA Total (millions of miles) | NHTS ÷ FHWA |
|---|---|---|
| vs. All motor vehicles (scope-mismatched) | 3,196,191.4 | 73.1% |
| vs. Light-duty vehicles only (scope-matched) | 2,822,663.9 | 82.8% |
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Use the Calculator03 Method and Reproducibility
The NHTS-side figure is computed from csv/tripv2pub.csv: every trip record where DRIVER = 01 (the respondent was the driver, not a passenger, so the trip record corresponds to that vehicle's movement rather than double- or triple-counting it across multiple occupants) and TRPTRANS is one of the five personal-vehicle codes above, with trip distance (TRPMILES) weighted by the trip weight WTTRDFIN and summed. Of 24,224 qualifying trip records, 3 (0.012%) had a missing-value code (-9, "not ascertained") and were excluded; this has no material effect on the result.
The FHWA-side figure is Table VM-1's "Total Rural and Urban" row for 2022, column "ALL LIGHT DUTY VEHICLES" (confirmed from the table's own multi-row header as the short-wheelbase plus long-wheelbase subtotal, distinct from the "ALL MOTOR VEHICLES" grand-total column used in the scope-mismatched comparison).
FHWA Table VM-1 (2022), "Total Rural and Urban" row: Light Duty Vehicles total 2,822,663.9 million miles.04 The Correction Factor
The scope-matched capture ratio is 82.8% (2,337,676.6 ÷ 2,822,663.9), meaning an NHTS-derived personal-vehicle mileage estimate can be scaled toward FHWA's administrative total using a correction factor of 1 ÷ 0.828 = 1.207. This factor is specific to the 2022 NHTS and 2022 VM-1 vintage used here; a different survey year would need its own factor recomputed the same way, since the gap is not guaranteed to be constant across years.
Calculated: 1 / 0.828181 = 1.207466 (shown to 6 significant figures; see Section 03 for the unrounded source totals).05 Why a Real Gap Still Remains
Even after scope-matching, NHTS still captures only about 83% of the comparable administrative total, a real, if smaller, gap. This is a known and expected property of self-reported travel diaries generally: trip under-reporting (especially of very short local trips respondents don't think to log), recall error, and the voluntary nature of diary completion are all documented limitations of this survey design, distinct from the scope issue addressed above. This note does not attempt to further decompose that remaining 17.2%; it isolates the scope-mismatch component from the genuine survey-methodology component, which a researcher correcting for the "NHTS undercount" should treat as two separate issues, not one.
Calculated: 100% - 82.8% = 17.2% (scope-matched remaining gap), vs. 100% - 73.1% = 26.9% (scope-mismatched figure, 9.7 percentage points larger).06 Data Sources
- FHWA: Highway Statistics, Table VM-1, "Annual Vehicle Distance Traveled in Miles and Related Data," 2022 (updated March 2025). fhwa.dot.gov/policyinformation
- Federal Highway Administration / U.S. DOT: 2022 National Household Travel Survey, Public Use Data. nhts.ornl.gov