I finally found a picture of the Kansas Turnpike’s 80 MPH speed limit signs:
(This image is on http://www.route56.com/highways/photobrowse.cgi?photo=KTA1. The image is from page 31 of the Kansas Turnpike Authority’s Fifth Annual Report (1957), so any copyright is probably owned by the state.)
The Kansas Turnpike’s former 80 MPH speed limit is the highest numeric speed limit ever been posted on a U.S. road. The second highest numeric speed limit is the 75 MPH limit currently used in many midwest and western states.
Before the federal government’s 1973 arbitrary 55 MPH speed limit cap, Nevada and Montana had no numeric speed limit. After the 1995 repeal of arbitrary federal speed limit caps, Montana reverted to its non-numeric daytime limit until May 1999 when a legislated 75 MPH went into effect.
Kansas currently allows speed limits up to 70 MPH, but its legislature is trying to allow 75 to be posted on certain highways.
Until 1973, speed limits gradually rose to keep pace with roadway and vehicular technology improvements. Even though vehicles and roads are now profoundly more safe than 1973, most current speed limits are lower than 1973 limits due to arbitrary and capricious speed policies. The sad truth is that insurance corporations, the hysterical buffoons they fund (e.g., Joan Claybrook, IIHS, Ralph Nader), and inane politics have far more influence on speed limit policy than sound traffic engineering principles.