National Maximum Speed Law
 "Photograph of 55 mph speed limit replacing a 70 mph limit". February 12, 1974.
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"Photograph of KSDOT workers changing a 75 mph sign to 55 mph". 1974.
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The National Maximum Speed Law (NMSL) in the United States was a provision of the Federal 1974 Emergency Highway Energy Conservation Act that prohibited speed limits higher than 55 miles per hour (90 km/h). It was drafted in response to oil price spikes and supply disruptions during the 1973 oil crisis.
While Federal officials hoped gasoline consumption would fall by 2.2%, actual savings were estimated at between 0.5% and 1%.
The law was widely disregarded by motorists, and some states opposed the law, but many jurisdictions discovered it to be a major source of revenue. Actions ranged from proposing deals for an exemption to de-emphasizing speed limit enforcement. The NMSL was modified in 1987 and 1988 to allow up to 65 mph (105 km/h) limits on certain limited access, rural roads. Congress repealed the NMSL in 1995, fully returning speed limit setting authority to the states.
The law's safety benefit is disputed as research found conflicting results.
Historically, the power to set speed limits belonged to the states. Immediately before the National Maximum Speed Law became effective, speed limits were as high as 75 mph (120 km/h). (Kansas had lowered its turnpike speed limit from 80 before 1974.) Montana and Nevada generally posted no speed limits on highways, limiting drivers to only whatever was safe for conditions.
As of November 20, 1973, several states had modified speed limits:
As an emergency response to the 1973 oil crisis, on November 26, 1973, President Richard Nixon proposed a national 50 mph (80 km/h) speed limit for passenger vehicles and a 55 mph speed limit for trucks and buses. That, combined with a ban on ornamental lighting, no gasoline sales on Sunday, and a 15% cut in gasoline production, were proposed to reduce total gas consumption by 200,000 barrels a day, representing a 2.2% drop from annualized 1973 gasoline consumption levels. Nixon partly based this on a belief that cars achieve maximum efficiency between 40 and 50 mph (65 and 80 km/h) and that trucks and buses were most efficient at 55 mph (90 km/h).
The California Trucking Association, the then-largest trucking association in the United States, opposed differential speed limits on grounds that they are "not wise from a safety standpoint."
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