Construction Program Guide
From 1988 to 1998, the FHWA's policy required the use of metric units in the Federal-aid highway program; however, the 1998 Transportation Equity Act for the 21st Century made the use of metric units optional.
The 1988 Omnibus Trade and Competitiveness Act (Public Law 100-418, signed into law 8/23/88), Section 5164(b) amended the 1975 Metric Conversion Act to require that Federal agencies use the International System of Measurements (SI) in their procurement, grants, and other business related activities. On July 25, 1991, President Bush signed Executive Order 12770 requiring that all Federal agencies develop timetables for their transition to the metric system.
Section 205(c) of the National Highway System Designation (NHS) Act of 1995 prohibited the Federal Highway Administration (FHWA) from mandating that the State Departments of Transportations (DOT) place metric legend signs, and which prohibited FHWA from mandating the use of metric units in the highway construction program before October 1, 2000.
However, Section 1211(d)) of the 1998 Transportation Equity Act for the 21st Century (TEA-21), removed the target date for metric conversion, thus allowing the State DOTs the option of converting to SI.