The congestion of traffic in the downtown streets of our large cities is due to four causes: (a) the increased number of vehicles, (b) the different rates of speed at which different types of vehicles desire to move (horse-drawn vehicles, street cars, motor trucks, trucks, motor cars, motorcycles, etc.), (c) the inadequate width and faulty arrangement of streets due to the lack of planning, and (d) the improper driving and parking of vehicles. The older streets cannot now be widened except at very great expense because the private property which fronts on them is so valuable. But new streets can be planned to take care of future traffic growth. No city street should be laid out with less than fifty-six feet between curb and curb. The sidewalk space varies with the width of the street, as the accompanying diagram shows. ------------------------------------------------------------------------ [Sidenote: Types of pavement.] =Street Pavements.=—Apart from good planning and adequate width, the usefulness of a street depends to a considerable extent on its paving. The qualities of an ideal pavement are easy enough to specify, but not so easy to secure. To reach perfection a street pavement should be cheap to construct, durable, easy to repair, easy to keep clean, smooth, safe for traffic, noiseless, and attractive in appearance. Unfortunately there is no type of pavement possessing all these qualities. A pavement of granite blocks will last for many years under heavy traffic, but it is expensive to build, noisy, and hard to keep clean. The asphalt pavement is cheaper, cleaner, and easier to drive upon; but it is slippery in wet weather and breaks down very quickly when heavy traffic is allowed on it. Wood blocks have come into favor in many cities during recent years because they are believed to make a pavement which is sufficiently strong to stand the burden now placed on the streets by truck traffic and yet afford a surface which is easy to drive over, not difficult to keep clean, and relatively noiseless. There is no one best form of pavement for all sections of the city.[84] It would be absurd to lay asphalt in the dock and shipping districts where the streets are filled with five-ton trucks, and just as absurd to put a granite-block surface on the streets of fine residential districts. The nature of the pavement should be adapted to neighborhood conditions. [Sidenote: The contract and direct labor systems.]