The Segway Personal Transporter (PT) is renowned for safety that has been designed in, right from the start. This has been achieved by what engineers called 'redundant subsystems.' You can read more about what this means on this page at the Segway Safety website. In brief, every component of the Segway PT that is critical to the device retaining it's self-balancing function is "doubled up" so that if one part fails the other can take over completely. For example, there are two batteries, two identical but independent computer control boards, a pair of double-wound electric motors, and more than twice as many "gyroscopes" than the minimum required to know which way is up (there is triple-redundancy in the pitch axis). "Redundancy" is a key feature of equipment used in mission-critical applications. It is common in medical equipment (Dean Kamen, the inventor of the Segway PT is famous for his inventions in the medical field). It is also found in military machines, public utilities such as electric power generation, and in corporate computer networks. Returning our discussion to vehicles, and to electrical vehicles in particular, perhaps the most famous electric vehicle of all (well, after the Segway PT) is the Lunar Rover. Four of these were built by Boeing in 1970-72, of which three made it to the moon on Apollos' 15, 16 and 17 respectively (Apollo 18 was cancelled). When you're 400,000 km from home you can't just call a tow-truck, so the engineers built in redundant subsystems not dissimilar to those used in the Segway PT. Here is a description by writer Paul Charman, as featured in NZ Herald's Driven section (19 June 2013):

Each wheel was individually powered by a 190 watt motor, for a total power of 750 watts, or one horsepower (the same power output as a Segway PT). The vehicle's top speed was 13 kph on a relatively smooth surface, and the range 65km. Two 36 volt batteries provided the power, although either battery could power all vehicle systems if required. The front and rear wheels had separate steering systems but, if one steering system failed, it could have been disconnected and vehicle would have operated with the other. Centralised controls allowed either astronaut to drive the vehicle.