Looking for a design of a exterior platform turntable capable of
rotating a 40 foot bus 180 degrees. Needs to be pedestrian friendly
and functional year round in Nebraska weather.

In the UK, there would be a lot of hoops to go through for a project
like this but maybe things are easier in Nebraska.

A Consulting Engineering firm would be my bet.

About 4 years ago the San Francisco Muni put in a powered turntable
for the cable cars.  It basically uses the same technology used in
Railroad turntables and mining equipment.  The turntable is supported
on the perimeter by a circular rail, which takes the load of the unit.
 Actual rotation can be accomplished by a center post driven by a
really big gear reduction, or by a smaller motor that contacts a rack
around the perimeter.  For Nebraska, I would tend to go for the center
post and gear, which would be more protected from the winter weather.

Snow and ice removal are a problem that might be best accomplished by
heating the area.

The bus would drive onto a set of guideways, similar to what you see
on an automobile hydraulic hoist.  A stop block would be at the end,
and the driver would drive his wheels against it, and put on the
parking brakes.  The unit would then turn slowly, and when the unit
had moved 180 degrees, the stop block would retract downward (and one
at the other end would have raised up, probably at the start of the
rotation to keep the bus from moving backwards).

Typically the driver would control the device using a remote similar
to a garage opener.

Pedestrians are kept away by means of a fence.  The turntable itself
would have a totally covered (round) top, to keep people from falling
into the pit.

There are several companies that specialize in this area.  One that
can fill the needs you are looking for is British Turntable.  This
specific link goes to just what you are looking for.
http://www.british.turntable.co.uk/frames/masterframereunion.cgi?http%3A//www.british.turntable.co.uk/vehicleaccess.htm

For more sites that can help use the key words: Vehicle Access Turntable