The book that sets the geometry of American highways is published by a private nonprofit and sold from an online store. AASHTO describes itself as a nonprofit, nonpartisan association representing highway and transportation departments in the 50 states, the District of Columbia and Puerto Rico, and as an international leader in setting technical standards for all phases of highway system development. None of that is a grant of legal authority. The authority arrives separately, in a paragraph of federal regulation that names the book by edition and folds it into the Code of Federal Regulations. Incorporation by reference is the mechanism, and it explains both what confuses people about highway standards and where the gaps are, because a book and a regulation revise on different schedules and only one binds.
The federal design rule reaches less road than its reputation suggests
Part 625 of title 23 designates the standards, policies and standard specifications that FHWA finds acceptable for the geometric and structural design of highways. Its operative reach is narrower than the reputation. Design and construction standards for new construction, reconstruction, resurfacing other than maintenance resurfacing, restoration or rehabilitation of a highway on the National Highway System are to be those approved by the Secretary in cooperation with the state departments of transportation.
Off that network the regulation hands the pen back. Federal-aid projects not on the National Highway System are to be designed, constructed, operated and maintained in accordance with state laws, regulations, directives, safety standards, design standards and construction standards. The rule even contemplates a local jurisdiction designing to a roadway design publication its own state does not use, under four conditions that include local ownership and a route off the Interstate System. Federal geometry and federal money follow different maps. The reverse holds too, since part 625 sets federal standards for work on the National Highway System regardless of funding source. Where the listed documents conflict, the latest listed standard, policy or standard specification governs.
What is actually incorporated, and how the edition gap gets handled
The list at 23 CFR 625.4 opens with A Policy on Geometric Design of Highways and Streets, the volume the profession calls the Green Book, and names it precisely as AASHTO GDHS-7, seventh edition, 2018. Beside it sit A Policy on Design Standards-Interstate System, May 2016, and the AASHTO LRFD Bridge Design Specifications eighth edition of 2017.
Each is incorporated with the approval of the Director of the Federal Register under 5 U.S.C. 552(a) and 1 CFR part 51. Approved material is available for inspection at the Department of Transportation’s National Transportation Library and at the National Archives, and is otherwise available from AASHTO’s store. Binding federal design criteria therefore sit behind either a purchase or a trip to Washington. A controlling text that is not free to read is the least advertised fact in research resources coverage.
Because AASHTO republishes on its own timetable while the regulation changes only by rulemaking, the two drift apart. The regulation anticipates the drift at 625.3(f)(2), where approval may be given on a programmatic basis to use a more recent edition of any standard incorporated by reference. A state can be designing to a newer edition than the one with legal force, entirely properly, under an approval nobody outside the process ever sees. The project exception in the same subsection covers experimental features and projects where conditions warrant, and it is the ancestor of the design exception table that appears on plan title sheets, described in reading a plan set.
The MUTCD is the one that genuinely is a national standard
Traffic control devices work differently, and the difference is worth stating exactly. Under 23 CFR 655.603, the manual approved by the Federal Highway Administrator is the national standard for all traffic control devices installed on any street, highway or bicycle trail open to public travel.
That last phrase reaches further than most readers expect. For the purpose of applicability, open to public travel takes in toll roads and roads within shopping centers, airports, sports arenas and other similar business or recreation facilities that are privately owned but where the public may travel without access restrictions. It stops at roads within private gated properties where access is restricted at all times, with gated toll roads carved back in, and at parking areas, the driving aisles inside them, and private highway-rail grade crossings.
States may publish their own manuals, and the conformance test has teeth. A state manual or supplement must conform as a minimum to the Standard statements in the national manual, and Guidance statements have to appear as well unless the reason for omitting one is satisfactorily explained on engineering judgment, specific conflicting state law, or a documented engineering study. No state manual may contain Standard, Guidance or Option statements that contravene or negate the national Standard or Guidance statements, and the prohibition runs past the manual to state policies, directives, standard drawings and specifications. Where a state manual cannot conform to a Standard statement because of a state law in effect before January 16, 2007, FHWA may grant an exception, but only on a determination that the non-conformance does not create a safety concern.
Updates run entirely through the Federal Register. FHWA dates the arrangement to 1979, when it assumed responsibility for updating the manual and announced that all future changes would be processed through Federal Register notices, in two steps: a notice of proposed amendments inviting comment, then a final rule that adopts a proposal, modifies it, or defers it pending further research. Section 655.601 requires publication in the Federal Register before FHWA can enforce any edition other than the one that section specifies.
Reading the current state of that machinery takes two documents rather than one. The annual printed Code of Federal Regulations, revised as of April 1, 2025, incorporates the eleventh edition of December 2023. The live regulation as of August 2026 incorporates that edition with Revision 1 incorporated, dated December 2025, following an amendment at 91 FR 10773 on March 5, 2026. FHWA’s own pages date the final rule adopting the eleventh edition to December 19, 2023, effective January 18, 2024, and the Revision 1 rule to March 5, 2026, effective the same day. FHWA prints no copies, so the PDF is the publication.
Roadside hardware is governed by an agreement, not by part 625
The crash test standard for guardrail, terminals, crash cushions and work zone devices is not among the documents part 625 incorporates. It runs instead on a joint implementation agreement between AASHTO and FHWA, elaborated in a question and answer document approved by both and developed by a joint technical working group drawn from state transportation departments, FHWA and accredited crash testing laboratories.
Its compliance dates attach to letting dates on National Highway System contracts and arrived in stages. New permanent installations and full replacements of w-beam barrier and cast-in-place concrete barrier in standard installations had to be MASH 2016 compliant for lettings after December 31, 2017. Tangent w-beam terminals followed after June 30, 2018, and crash cushions by December 31, 2018. Cable barriers and their terminals were moved into the December 31, 2019 date covering all other longitudinal barriers and all other terminals. Letting date here means the owner agency’s definition of bid opening date.
Four documented exceptions survive that last date and are routinely misread as non-compliance, among them the case where a compliant device exists but does not meet the state’s needs given project or regional conditions. Temporary work zone devices manufactured on or before December 31, 2019 and tested successfully to NCHRP Report 350 or the 2009 edition of MASH may also stay in use throughout their normal service lives, which is why hardware from an older test regime is still legitimately on the road, and what that means for the barrier a driver passes is worked through in guardrails and barriers.
The state manual is where a criterion becomes a decision with a name attached
Federal documents set floors. State manuals decide who signs when a floor is not met, and the taxonomy can be elaborate. California’s design manual sorts its own contents into absolute requirements phrased in words that admit no exception, controlling criteria, boldface and underlined standards, decisions requiring other approvals, and permissive guidance.
The controlling criteria are the federal ones, and the manual lists all ten: design speed, lane width, shoulder width, horizontal curve radius, superelevation rate, stopping sight distance, maximum grade, cross slope, vertical clearance, and design loading structural capacity. It also records how far they reach. All the geometric criteria apply on high-speed roadways, meaning Interstate highways, other freeways and roads designed for 50 mph or more. On low-speed non-freeways below 50 mph, only the design speed criterion does.
Approval authority is then encoded typographically. Boldface standards carry the word shall, and deviations require the approval of the Chief, Division of Design, delegated in defined circumstances to district directors and project delivery coordinators. Underlined standards carry the word should, and their deviations go to district directors outright. The federal layer above this is thinner than it looks: a stewardship and oversight agreement signed under MAP-21 generally delegates approval of deviations from the ten controlling criteria on all Interstate projects to Caltrans whether or not FHWA has oversight responsibility, and FHWA approval is not required at all for deviations from the standards Caltrans has identified as its own. The manual is equally direct about the past. Current standards are not intended to be applied retroactively to all existing state highways, because doing so would be neither warranted nor economically feasible. That sentence accounts for most of the geometry a driver meets on any given day, and the sequence in which the remaining decisions get made is set out in how highway projects are planned.
The adoption window in 655.603 puts dates on the rest. States had two years from the eleventh edition’s January 18, 2024 effective date, and the clock on Revision 1 started March 5, 2026. Inside any such window two different signing treatments can both be lawful in neighboring states, with neither state out of compliance, and what the regulation asks of roads already built is a program of systematic upgrading rather than replacement.