Interesting question... why indeed did mass transit fade away? At least in a number of cities (East San Francisco Bay, San Jose, Fresno, Stockton, Sacramento, San Diego and Los Angeles), it was because the automotive industry did to the rail industry, what the rail industry had done to the canals in the East - they bought them up and closed them down. Some evidence is presented in this web-site - http://www.moderntransit.org/ctc/ctc06.html. It is possible that they would have died anyway, but this is not certain, and they probably would not have disappeared as quickly or as completely, so ramboy's comment loses a lot of its force.

It is true that personal vehicles are more convenient, but only because they are also cost-effective. This may well be changing, so arguments that are currently quite valid may not be true in the near future. In fact we are already beginning to see the exurbs losing market value, and this is beginning to lead to a move back to the towns - http://www.iht.com/articles/2008/06/24/business/exurbs.php. This will make mass transit more viable.

So, is public transport more energy-efficient? There is still some debate on this, but increasing the amount that people walk (or cycle) is definitely a good idea. Does public transportation have to be inconvenient? I have mainly lived in cities, so I can only really speak to that, but often it was quicker for me to cycle to places in San Francisco than to drive, simply because I didn't have to worry about parking. Public transportation was best if heading to the East Bay by BART, but waiting for a bus could take anything between 10-40 mins. I now live in Hong Kong and here public transport is very convenient. Trains come on average every one to two minutes, which means that the average wait time is half that, and there are buses everywhere (with an average wait time of around 5 mins). Once you get close to your destination, you can either walk or take one of the many taxis that run on liquefied gas.

Yes, China is buying cars, but the government is already looking to the future and is investing in alternative sources of energy. They may just be the people arriving late at the party. Building more freeways may reduce wasted fuel due to idling, but if we can reduce the number of cars it would be a much better approach. It still costs a lot of energy to build freeways, even if they are cheaper than rail, and this may not be the best investment for the future. Bus rapid transit may be one way to go (since roads are cheaper than rail). Maybe we just need to change the image of buses. If they can be made clean and efficient, then ridership will rise. Evidence already exists for this - http://www.iht.com/articles/2007/12/28/business/wbspot29.php.

So the question becomes, if the cost of energy is going to keep rising (and I know that this is not yet proven, but most accept that the trend is upwards), then is there anything that we should start doing now? One of the concerns is that the later we leave it, the greater the cost of energy generation (e.g. extraction costs), and the greater the pressures of demand. Unfortunately all forms of energy generation require an "investment" of energy, and for most that investment is much higher that the cost of an oil gusher, which is what we have been relying upon. Thus it would be best to spend energy now in moving to more energy-efficient living, rather than leaving it later.

In conclusion, I think that all societies are going to have to rely more on public transport, cities are going to get denser, and everyone is going to take fewer trips, but some sort of private vehicles are going to remain. We should probably start preparing now for a future with less energy.

Please note - all this is quite independent of the whole green-house gases issue, and I shall leave it that way!