Interstates make up 48,830 of the 4,208,454 miles of public road in the 50 states and the District of Columbia, a little over 1 percent of the network. In 2024 they carried 854 billion vehicle miles, about 26 percent of all driving. Local roads run the other way: 2.9 million miles, 69 percent of the network, and 13 percent of the travel. Both figures come from Federal Highway Administration tables (HM-20 and VM-2) organized around one seven-way sort of every public road.
That sort is functional classification. Each road segment is labeled by the job it does in the network, from long-distance through traffic down to driveway access. Federal law hangs money on the label. Whether a road can receive most federal-aid highway funds, which design criteria an engineer starts from, and which statistics a state reports all depend on the class assigned.
The governing guidance is FHWA’s Highway Functional Classification: Concepts, Criteria and Procedures, issued in February 2023. It replaced a 2013 edition. Much of what the road-design coverage on this site assumes about arterials and federal money traces back to this document and to 23 CFR Part 470.
Seven classes and one tradeoff
The guide describes two road functions at opposite ends of a continuum. Mobility means few places to enter or leave, so traffic moves with little friction. Access means many entry points: driveways, side streets, crosswalks. Most roads do some of each, and the class reflects the road’s primary purpose.
FHWA illustrates the extremes with two places named Eisenhower. The Eisenhower Tunnel on I-70 west of Denver is pure mobility, since nothing is reachable from inside it. Eisenhower Court in North Platte, Nebraska, is pure access, driven almost only by the people who live on it.
Roads that lean toward mobility are arterials, roads that lean toward access are locals, and collectors sit in the middle, gathering traffic from locals and feeding it to arterials. The federal scheme splits these into seven classes:
- Interstate: Routes designated by the Secretary of Transportation as part of the Eisenhower Interstate System. The guide says there is no ambiguity in this category.
- Other freeways and expressways: Divided, limited-access roads that look and work like Interstates without the designation.
- Other principal arterials: High-mobility roads that still allow driveways and at-grade intersections.
- Minor arterials: Moderate-length trips; in rural areas they link cities and larger towns.
- Major collectors: Longer routes, fewer driveways, higher speeds and more traffic than minor collectors.
- Minor collectors: Shorter routes that bring developed areas within reach of the collector network.
- Local: Whatever remains after arterials and collectors are identified. The guide says local roads are often classified by default.
The first three together are the principal arterials. Every class exists in both urban and rural form, which yields 14 combinations such as urban minor arterial.
Functional class is separate from ownership. A footnote in the guide points out that local roads are not always under local jurisdiction, and that cities and counties can own arterials. Traffic volume alone does not decide the class either. US 290 through Giddings, Texas, has many intersections, dense commercial frontage and speed limits as low as 35 mph, yet it is classified as an arterial because it is one of the two most direct routes between Austin and Houston and carries a large share of long-distance trips.
Ramps take the highest class among the mainline roads they connect. States classify roads by how they function in the current year, though a planned route can be classified if it is in the approved Statewide Transportation Improvement Program and expected to be under construction within four years.
The classes by the numbers
The table combines HM-20 mileage and VM-2 travel for 2024, excluding Puerto Rico. Class totals are sums of the rural and urban columns. They add to 4,208,453 miles, one mile short of the table’s printed total of 4,208,454, which looks like rounding. Travel is in millions of annual vehicle miles.
| Functional class | Rural miles | Urban miles | Total miles | Share of miles | Annual VMT (millions) | Share of VMT |
|---|---|---|---|---|---|---|
| Interstate | 29,462 | 19,368 | 48,830 | 1.2% | 854,413 | 25.9% |
| Other freeways and expressways | 6,580 | 12,289 | 18,869 | 0.4% | 303,271 | 9.2% |
| Other principal arterial | 92,319 | 65,671 | 157,990 | 3.8% | 690,695 | 21.0% |
| Minor arterial | 135,392 | 111,000 | 246,392 | 5.9% | 558,661 | 17.0% |
| Major collector | 408,309 | 131,341 | 539,650 | 12.8% | 386,658 | 11.7% |
| Minor collector | 258,695 | 23,693 | 282,388 | 6.7% | 68,048 | 2.1% |
| Local | 2,025,860 | 888,474 | 2,914,334 | 69.3% | 432,286 | 13.1% |
| Total | 2,956,618 | 1,251,836 | 4,208,454 | 100% | 3,294,031 | 100% |
All four arterial classes together account for 11.2 percent of mileage and 73.1 percent of travel. Interstates and other freeways together are 1.6 percent of the miles and carry 35.1 percent of the vehicle miles. Divide travel by length and the average Interstate mile carries about 48,000 vehicle miles a day, against roughly 400 on the average local road.
The rural column holds 70 percent of the mileage but only 31 percent of the travel. State-by-state mileage, which varies widely with geography and settlement pattern, is broken out in the road mileage by state data page.
The travel figures do not all have the same pedigree. A VM-2 footnote says travel on rural minor collectors and on all local roads is estimated by the states “based on a model or other means” and reported to FHWA as summaries. The rest comes from state data in the Highway Performance Monitoring System. The 13 percent local share is a modeled number in a way the Interstate share is not. The caveat applies wherever FHWA’s main data series report local road travel.
Where urban ends
Because every class comes in urban and rural versions, the urban boundary matters as much as the class. Federal law sets the thresholds. Under 23 U.S.C. 101(a)(35), an area of 5,000 or more people can qualify as urban, and under 101(a)(36), an area of 50,000 or more is urbanized. FHWA calls areas between 5,000 and 49,999 small urban areas. For the 2020 Census, the Census Bureau began defining urban areas by housing unit density, with a floor of 2,000 housing units or 5,000 people.
States do not have to use the census lines as drawn. Working with local officials, they can push the boundaries outward for transportation planning, provided the adjusted boundary takes in the entire census urban area and the Secretary of Transportation approves it. Until new adjusted boundaries are approved, the old ones stay in effect. If a state never adjusts, the unadjusted census boundaries apply, and the guide warns that a road previously counted as urban could become rural and lose federal-aid eligibility.
The guide also tells states to stop a practice some had followed: automatically changing a route’s functional class where it crosses an urban boundary. Classes are supposed to follow function, with the urban or rural label attached separately.
Who draws the map
Under 23 CFR 470.105, the state transportation agency has primary responsibility for developing and updating the statewide classification, in cooperation with local officials. The results must be mapped and submitted to FHWA for approval, and the approved map becomes the official record of which roads are federal-aid highways.
In practice the 2023 guide describes a chain. Metropolitan planning organizations are the main local contact in urbanized areas and forward change requests from member cities with a recommendation. Counties outside MPO areas can start requests for their own roads. The guide recommends that a single unit inside the state DOT own the official classification, approve changes on the state side and send them to the FHWA Division Office for review and approval.
The schedule is tied to the census. The guide recommends finishing adjusted urban boundaries within one year of the Census Bureau’s release and the functional classification update within two years after the boundaries are approved, with a full review at least every 10 years. Classification feeds the same planning systems discussed in how state DOTs rank roads for reconstruction, since condition targets and reporting are broken out by class.
What the label decides: federal-aid eligibility
The regulatory definition is short. 23 CFR 470.103 defines federal-aid highways as public roads not classified as local roads or rural minor collectors. Every Interstate, freeway, arterial and major collector qualifies, as do urban minor collectors. Rural minor collectors and all locals do not.
Applied to the 2024 mileage, that line puts about 1,035,000 miles, roughly 25 percent of public road length, inside the federal-aid system. Those roads carry about 86 percent of vehicle miles.
The rule bites through program statutes. Under 23 U.S.C. 133(c), a Surface Transportation Block Grant project may not be built on a road classified as a local road or rural minor collector unless that road was on a federal-aid system on January 1, 1991. The statute lists exceptions, among them bridge and tunnel work other than new crossings at new locations, several categories of project defined elsewhere in title 23, replacing a low-water crossing with a bridge, and anything the Secretary approves.
Safety money is drawn more broadly. The Highway Safety Improvement Program in 23 U.S.C. 148 lets states spend on projects on any public road, local roads included.
The guide calls the choice between major and minor collector one of the hardest in classification work. In rural areas the same choice decides whether a road is in or out of the federal-aid system.
The National Highway System is a separate list
The National Highway System is often confused with the principal arterial classes because the two were merged once. On October 1, 2012, section 1104 of MAP-21 added to the NHS every road then classified as a principal arterial, and FHWA’s question-and-answer guidance put the expanded system at about 230,000 miles.
That was a one-time event. The 2023 guide states that it did not create a standing link, and that a change to the principal arterials does not automatically change the NHS. The approvals differ too: the FHWA Division Office approves a functional class change, while FHWA headquarters approves NHS changes under 23 CFR 470.
FHWA’s Our Nation’s Highways 2026 puts the NHS at 221,568 miles, 5 percent of public road miles carrying 55 percent of all travel. The regulation has not caught up with that figure. Section 470.107(b) still says the NHS shall not exceed 178,250 miles, a limit the current network passed after MAP-21.
Design standards follow the class
Class drives geometry as well as funding. The guide quotes the AASHTO Green Book, which tells designers to first “define the function that the facility is to serve,” and then select design speed and geometric criteria from the range for that classification. The standards documents themselves are covered in the guide to AASHTO and MUTCD.
The guide’s own reference tables show how far apart the classes sit. Typical Interstate lanes are 12 feet with 10 to 12 feet of outside shoulder, fully controlled access and urban daily traffic of 35,000 to 129,000 vehicles. Typical local roads have 8 to 10 foot lanes, 0 to 2 feet of shoulder and rural daily volumes of 15 to 400.
The guide notes that designers keep some flexibility on design speed after the class is set. It cites FHWA’s livability guidance cautioning that classification-based design may not respond to context, The guide shows how some states have responded: Oregon DOT uses its own roadway classification system, and Idaho DOT has proposed redefining its street classes.
Where the guidance and the data part ways
The 2023 guide gives typical mileage and travel ranges for each class and asks states to adjust when their networks fall far outside them. National numbers from 2024 mostly land in range. Rural arterials carry 66 percent of rural travel, above the guide’s description of approximately half, and urban collectors carry 10.5 percent of urban travel, inside its 5 to 15 percent range.
One statement in the guide does not match the national inventory. It says total major collector mileage is typically lower than minor collector mileage. HM-20 shows the reverse: 539,650 miles of major collectors against 282,388 of minor collectors, nearly two to one, and the rural split alone is 408,309 to 258,695. The guide says collector mileage is typically one third of local mileage, and the 2024 ratio is closer to 28 percent.
The reference tables have their own loose ends. The narrative says the ranges come from 2008 HPMS data, while the table footnotes say 2011. The urban minor arterial mileage range for all states is printed with an upper bound of 114 percent, an apparent typo. Urban collector figures carry a note that they rest on a small number of reporting states. States are told to measure their networks against these benchmarks, which FHWA says it intends to review periodically.