National Highway System Designation Act of 1995
The National Highway System Designation Act of 1995, as amended, includes two sections dealing with the metric system.
Metric road signs are a state option: The law gives states the option to install metric road signs, but doesn't allow the federal government to require installation of metric signs.
Metric road construction is a state option: The law gives states the option to metricate road construction projects (plans, specifications, materials, construction work, etc.), but doesn't allow the federal government to require a switch to metric road construction.
Although this restriction nixed the original plan for a nationwide conversion to metric road construction by the end of 2000, many states switched anyway, and the federal government itself continues to use metric measurements. For more information on this topic, read the FHWA Memorandum to all State Chief Executive Officers regarding metrication conversion at state option, issued in October 1998, and the June 2001 Update on metric use requirements.
On November 6, 1978, the Federal-Aid Highway Act of 1978, Pub. L. 95-599, outlawed metric road signs. (Technically, it outlawed the use of federal funds for installing metric signs.)
On November 9, 1979, Pub. L. 96-106 amended the restriction to allow metric road signs in Puerto Rico.
On December 18, 1991, the Intermodal Surface Transportation Efficiency Act of 1991 ("ISTEA"), Pub. L. 102-240, repealed the restriction, making metric signs legal.
Finally, on November 28, 1995, the National Highway System Designation Act of 1995, Pub. L. 104-59, imposed the existing restriction, which allows states to install metric signs, but doesn't allow the federal government to require them.
On November 28, 1995, the National Highway System Designation Act of 1995, Pub. L. 104-59, delayed the imposition of nationwide metric road construction until September 30, 2000. In other words, the restriction mentioned above (and in the law's subpar. 2 below) was originally set to expire on September 30, 2000, after which the federal government could require the switch to metric construction projects.
On June 9, 1998, the Transportation Equity Act of 1998 ("TEA-21"), Pub. L. 105-178, removed the expiration date, making the restriction permanent, i.e., leaving metrication an option for each state. Regarding the change, the Federal Highway Administration issued the memo mentioned above, FHWA Memorandum to all State Chief Executive Officers regarding metrication conversion at state option, as well as the later Update on metric use requirements.
The law is codified as a note to 23 USC 109.
(1) Placement and modification of signs. The Secretary shall not require the States to expend any Federal or State funds to construct, erect, or otherwise place or to modify any sign relating to a speed limit, distance, or other measurement on a highway for the purpose of having such sign establish such speed limit, distance, or other measurement using the metric system.
Here and below, "The Secretary" refers to the Secretary of Transportation.
(2) Other actions. The Secretary shall not require that any State use or plan to use the metric system with respect to designing or advertising, or preparing plans, specifications, estimates, or other documents, for a Federal-aid highway project eligible for assistance under title 23, United States Code.
(3) Definitions. In this subsection, the following definitions apply:
(A) Highway. The term "highway" has the meaning such term has under section 101 of title 23, United States Code.
(B) Metric system. The term "metric system" has the meaning the term "metric system of measurement" has under section 4 of the Metric Conversion Act of 1975 (15 U.S.C. 205c).
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