Highway lines and mile markers come from FHWA’s Highway Performance Monitoring System (HPMS, 2024 data year): every mainline section a state signed as Interstate or U.S. highway. Several states leave that signing blank in HPMS (Texas, California, Maryland and Delaware among them), so a section’s signing also comes from FHWA’s National Highway System dataset, which records the posted route for the same HPMS route IDs. Sections of one highway in one state are joined end to end; a second carriageway or couplet drawn beside the first is dropped. Roads FHWA designates that are not Interstate or U.S. highways (state routes, parkways, turnpikes) use FHWA’s own corridor lines.

Mile markers come from the route measure each state reports to FHWA in HPMS, where it runs continuously along the line (state_route_measure). It often matches the posted mileposts (it does for I-70 in Colorado) but not always: some states measure from a different starting point or by section. Where the measure does not run continuously, such as one that restarts by county, the markers are miles from the south or west end of the line within the state (from_south_or_west_end).

Chargers are AFDC stations listed as open and public with at least one DC fast port, any network and any plug, including Tesla. A charger serves a highway when it is within 1 mile of it in a straight line. Chargers just across a state line count.

We walk each highway from end to end and record every stretch longer than 50 miles between consecutive chargers, or between a charger and a state line, the Canada or Mexico border, or the end of the highway. Gap length is measured along the highway.

Why 50 miles: FHWA’s February 10, 2022 NEVI guidance treated a state’s corridors as fully built out when chargers were installed every 50 miles along its Interstates. FHWA’s August 11, 2025 interim final guidance superseded it and leaves spacing to each state; Colorado’s NEVI plan (CDOT and the Colorado Energy Office, 2025 update) keeps a 50-mile target. 23 CFR part 680 itself sets no spacing. We use 50 miles everywhere so states can be compared.

Corridor designation comes from FHWA’s Alternative Fuel Corridors map (Round 8), matched to the highway lines within 0.1 mile. A corridor named for another road counts only where that road has no line of its own there.

Known limits

• Straight-line distance to the highway, not driving distance to the exit.
• Where two highways share the road, the line is drawn once, under one of them, so the shared miles appear once.
• Chargers in Canada and Mexico are not in AFDC’s U.S. listing, so a gap ending at a border may be shorter in practice.
• A charger listed as open in AFDC may be down on a given day; the gaps assume it works.