At no time in history has there been such a mass  movement of people from the countryside to the city as is happening now. By the year 2030, it's estimated  that more than two thirds of the world's population will be living in cities, twice as many as today. This means that the problems faced by cities today--overcrowding, poor housing, unemployment, poverty  and lack of food and water--will be twice as bad, unless we find solutions soon. Another serious issue is how to provide good transportation for their citizens. Many of the world's major cities are already struggling with out-of-date transport infrastructures . How can they deal with the additional demands? London is a good example. Its enlargement was made possible by the invention of the steam engine, which powered the world's first underground railway. But its transport system are now hopelessly out-of-date and need urgent modernization. London's future success depends very much on developing better public transport. Over a million people travel into central London every day from outside the city. _ , together with the people who live in London, want a public transport system that is efficient, safe and _ . What they often get, however, falls far short of that ideal. Passengers complain about cost and pollution, while businesses worry about the problems their staff have in getting to work on time. Yes, the proportion of London households that own a car grew from just over ten percent in the early 1950s to over sixty percent today. As the city has become increasingly crowded and polluted, there has been a growing realization that action must be taken soon. The passage is mainly written to tell us that   _  .
it's difficult to solve the problem faced by cities today overpopulation causes the problems in cities like London possible solutions to cities' transport problems will be found soon public transport plays an important role in developments in major cities