I guess it is easy to see why few people use coaches. There is often little leg room, coach stations suffer from years of neglect, and journeys can be long and boring.
On the plus side, coach travel is probably the most environmentally friendly form of long distance travel. I say probably because there is some dispute about the figures. This carbon calculator uses these figures (PDF) which indicates that train travel is less emitting than coach travel. National Express, Britain's biggest coach firm suggests emissions for coach travel are much lower.
Either way, there are obviously many factors affecting the environmental efficiency of public transportation -- including how many people are on the coach/train, maintenance, type of fuel, etc. Buses and coaches which stop frequently are clearly going to be less efficient than non-stop city-to-city services.
The interesting thing about coach travel is that the demographic is completely different to train travel. Given the massive cost of train tickets in the UK, coaches have become a mass transportation system for the low paid, students and migrants. When I was a student, train prices were more reasonable. We would often be squeezed into trains like sardines, playing cards on the floor of the buffet carriage. In a lot of ways, coaches have taken over that role. Whilst trains are still often crowded, prices now preclude those who do not travel in suits with laptops and important business conversations on their cell phones.
There is no getting away from the fact that coach travel is not particularly comfortable, but then many train journeys were not either. An increase in demand from environmentally astute passengers would force companies to improve their vehicles and services and might make it a more enjoyable experience.
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