Traffic and transportation remain a problem in urban areas. Solutions are implemented, but inevitably the implementation expense and the time required to plan, approve, and build infrastructure can't keep pace with the population growth. There have been some improvements. Carpool lanes have become common in urban areas, which are intended to encourage people to drive together rather than in individual automobiles. San Jose is gradually building a light rail system (often over routes of an original turn-of-the-century electric railroad line that was torn out and paved over to encourage the advent of the automobile age). None of the implemented solutions are without their critics. The sprawling nature of the Bay Area and of the Los Angeles Basin makes it difficult to build mass transit that can reach and serve a significant portion of the population.