Observe problem areas and find the cause. Some connections to the city create bottlenecks that block traffic, so these can be corrected by careful observation and data collection. If more cars are coming from the north than ten years ago, then perhaps a new lane is necessary.
Control the flow of traffic through certain areas, city centers for example, with toll roads or congestion charges.
Support the existing public transportation, while at the same time trying to expand the network. Having a smooth, efficient transport service is a way to effectively reduce the number of cars on the road significantly, especially in large urban areas where congestion will always be a problem.
Monitor city development patterns by looking at where new workplaces are being created and where the population is living. If company headquarters are all being built in one area of the suburbs then more traffic will start heading there, potentially from many areas of and around the city itself. If there isn't any public transportation in the area, the infrastructure must be ready to support the new influx.
Balance out improvements, while being flexible and realistic with your goals. Some areas can be easily improved using better information and efficiency tactics. Other areas do not have the space or resources to truly solve the congestion problems.
Study a variety of locations with traffic problems and judge what can realistically be changed in the short and long term.
