What has contributed more to the advancement of society, computers, cars or communication devices?

Cars are only essential as a means of transportation in the US because the state of public transportation is pathetic. You said cars so I take it that your system allows for the use of trucks, railroads, motorbikes, bicycles etc.Computers and communication devices have contributed mightily to the advancement of mankind but to the detriment of the average teenagers knowledge of language due to their neglect of books.

Communications devices, of which computers are some.

Instead of cars, I would say transportation greatly advanced society. Transportation was the begining of the industrial revolution and it allowed people to spend less time travelling to places and more time concentrating on doing things that would make life easier. Hours spent working were greatly increased, time it took to get foodstuffs, and the distances they could go meant that fewer people had to work for food and more people could then look at doing other types of activities including education. The flow on effect always comes about due to the increase in free time that people have. By the way I'm an IT consultant so its not a biased answer.

Cars, in the generic sense of improved transportation. Before mass transportation, communities were isolated. By being able to travel cheaply, ideas can be shared, goods can be made cheaper (think Florida oranges available in New York) ditto for health, same example. Without that ability, computers could not have been invented, plus you can't lay cables around the world for the internet without oil powered ships to lay them. In reality constant improvement is the biggest thing that has moved society forward - improvements in one area aid improvements in another. The key is to allow individuals freedom to innovate.