Off-road passenger vehicles such as golf carts typically have safety or emergency brake systems patterned after road vehicles, while slower moving other types of these passenger vehicles, such as self-propelled and motor powered vehicles for the physically disadvantaged including wheelchairs, so-called wheeled travel chairs and the like, typically have a simple friction type brake acting directly against one or more of the vehicle's wheels or tires to provide all braking operation. Examples of the latter are disclosed in U.S. Pat. Nos. 2,782,870; 4,204,588; 4,320,818, 4,350,227, 4,623,163 and 4,846,295. There are almost endless types of road vehicle brake systems but they are not well suited to providing safety brake operation for the slower moving self-propelled or motor powered shopping/travel carts, wheelchairs, wheeled travel chairs and the like, where the operator is disabled and may have limited or even no ability to operate a brake in an emergency situation. Nor do they offer a type of safety brake system particularly suited to the latter type of vehicles in some of the typical situations in which they operate. For example, there are situations with the latter vehicles where it would be highly desirable to over power the motor powering the vehicle and very rapidly lock up the wheels in advance of an impending impact either automatically or under the control of the operator, but provide a much milder wheel braking action in the reverse direction. Moreover, a safety brake system for the physically disabled should require little manual effort and otherwise be readily activated. In the above identified patents, various friction brake systems are disclosed that offer various types of braking operations to different degrees, but they do not offer a cost effective safety or emergency brake system that can provide emergency braking in a relatively simple and positive manner, let alone accomplish this automatically and with self-energization with little or no effort required of the vehicle operator and in advance of an impending impact with an obstacle in the path of the vehicle.