The National Highway System Designation Act of 1995 repealed the national m
aximum speed limit. As a result, approximately 70% of the United States has
increased posted speed limits to 104.7 km/h (65 mi/h) for select multilane
highways. This study evaluates the application of the Highway Capacity Man
ual multilane highway rules-of-thumb for free-flow speed to both an 88.6 an
d a 104.7 km/h (55 and 65 mi/h) posted speed limit condition. The paper fur
ther quantifies the observed relationship between the posted speed limit an
d observed free-flow speed on rural multilane highways in Georgia. Specific
issues evaluated include heavy vehicle influence, traffic volumes, access
point density, and vertical grade. The research indicates that the current
Highway Capacity Manual rule-of-thumb free-flow estimation technique based
on posted speed limit does not adequately estimate free-flow speed for the
higher speed limit condition.