"Oklahoma has minimum speed limits posted on more or less all interstate highways that are 25 mph below the maximum speed limit. For example, on the turnpikes, which have a maximum speed limit of 75 mph, they are nearly always accompanied by a sign stating a minimum speed limit of 50 mph."
http://en.wikipedia.org/wiki/Speed_limits_in_the_United_States#Oklahoma
"(a) No person shall drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with law.
(b) Whenever the State Highway Commission or local authorities within their respective jurisdictions determine on the basis of an engineering and traffic investigation that slow speeds on any part of a highway consistently impede the normal and reasonable movement of traffic, the Commission or such local authority may determine and declare a minimum speed limit below which no person shall drive a vehicle except when necessary for safe operation or in compliance with law."
http://www.oscn.net/applications/oscn/DeliverDocument.asp?CiteID=82324
Most 150cc scooters have a TOP speed of around 55 mph on level ground (slower on hills) . . a nice CRUISING speed is around 45-50 mph. Although technically legal, a 150cc scooter is realistically only suitable for very short duration interstate travel . . a 250cc or 400cc would be preferable.