For the most part, most of the rides you will see require the guest to transfer to the ride seat under their own power or with assistance. Several parks and carnival owners do not let their employees offer any more than the minimal amount of assitance, e.g. holding someone’s hand to steady them getting in and out of the ride. This is not only for fear of liability for injury to the guest (most people are not trained on how to lift a person properly, and the chances of dropping someone or exacerbating a pre-existing medical condition increase) or the possibility of sexual harassment (purposely or even accidentally holding someone ‘the wrong way’ can lead to a very uncomfortable situation for both parties), but also to protect the employee against their own injury or harassment.
Most of the travelling carnivals and lots of the stationary parks use pre-fab rides. [I don’t have a copy of [*Amusement Business*](http://www.amusementbusiness.com/amusementbusiness/index.jsp) available at the moment, so I can’t give any impressive names of the big manufacturers.] Ride design takes a lot into consideration: physics of the ride itself, the physics of the human body (what it can, cannot and should not do), and the general enjoyment of the ride (thrill factor, nausea inducement). Lots of successes, lots of failures, lots of ‘back to the drawing board’.
[The big manufacturers have engineers and designers working at this. There are trade shows for park and unit owners and operators, displaying the lastest in rides and ride technology. My old carnival unit used the side yard in winter quarters to set up and display new rides since there was a problem setting them up in the convention hall (driving stakes into the floor for a 90,000 lb. ride was frowned upon). We had fun setting them up, and most of the carnies volunteered to test ride.]
Redesigning a ride to now accomodate a wheelchair takes a whole new set of physics into consideration: new space allotments, entering and exiting the ride, safety considerations for wheelchair brakes that may not completely work (happens a lot with the theme parks, considering the amount of use they get), safety belt redesign for both the occupant and the chair, and generalizing this to accommodate the wide variety of manual wheelchairs available (straight wheel vs angled (racing wheel) wheels, different heights of chairs, foot rest size and angles), and whether electric wheel chairs can be accomodated at all. Most of the travelling shows and smaller amusement parks cannot afford the cost that would go into these redesigns.
Larger parks and theme parks (Disney, Universal Studios, Busch Entertainment Corp, not sure about Six Flags) often have their own set of engineers and designers, and can afford to absorb the cost of redesign, since they are going to be accomodating a larger and more diverse group.
However, with theme park attendance dropping somewhat and a lot of belt-tightening going on in the industry itself, many of the theme-parks are probably going to be using more pre-fabbed rides with new bells and whistles to save money. Case in point: stip down both Disney’s “Dumbo” ride and Universal Studios Islands of Adventure’s “One Fish, Two Fish, Red Fish Blue Fish” ride (phew!) to their bare minimums and what do you have? A circular ride around a central post with XX number of sweeps (radial arms) and a rider’s option of elevation.
The rides that can accomodate the wheelchair and rider are usually the stationary rides found at the larger theme parks; however, even these have limitations - in most cases, someone in an electric wheelchair must transfer to a regualr wheelchair before boarding.
Lest you get the wrong idea, ride designers are making changes to rides to accomodate a larger audience, so to speak. Roller coasters are now designed with seats for larger and overweight people - safety belt extensions, double belts instead of single belts hooking into the overshoulder harness, larger overshoulder harnesses (very much appreciated by us large-busted women - no more holding my breath for the duration of the ride!).
We just haven’t gotten to the point where everything can be accomodated.