[Sidenote: The old and the newer methods of determining widths.] =How Wide Ought a Street to be?=—Until very recent years in all American cities, and even yet in some of them, the practice has been to lay out streets in widths of forty, sixty, or eighty feet,—always using multiples of ten. This is a mere rule-of-thumb method and bears no direct relation to the needs of traffic. The downtown streets of the older cities are, for the most part, too narrow; in the newer suburbs they are often a good deal wider than they need be. “But what harm is done by having more street space than is necessary?” it may be asked. Well, every square foot of street space costs money; it has to be paved, kept in repair, cleaned, and lighted. The proper policy in laying out streets is to adapt their width to the probable needs of future traffic. This cannot always be done with mathematical accuracy, because the density of traffic changes from decade to decade; but with careful study a fairly dependable estimate can usually be made. [Sidenote: The traffic “zones”.] The best practice nowadays is to fix the width of new streets in terms of _traffic zones_, not merely in multiples of ten feet. A stream of traffic—motor cars, trucks, and other vehicles following one another—requires a certain sluiceway or zone to move in. This zone is ordinarily reckoned as ten feet in width. A zone of parked vehicles alongside the curb uses about eight feet. In order to allow full parking privileges and still have space for two streams of traffic to flow along easily (one in each direction) a street should be thirty-six feet in width from curb to curb. Anything less than this usually means that parking must be restricted or the thoroughfare must be made a one-way street. Anything more than this is useless unless a full zone of ten feet is added, and it is of relatively little value unless two additional zones are put on.[83] ------------------------------------------------------------------------ [Illustration: For 4 lines of traffic All moving |—13’—|———40’———|—13’—| |————--———66’—————————| For 6 lines of traffic 4 moving & 2 standing |—15’—|—————56’—-———|—15’—| |———————————86’————-——————| For 8 lines of traffic 6 moving & 2 standing |—22’—|———————76’———————|—22’—| |————————————120’—————————————| ] The normal roadway width is based on an allowance of 10 feet for each line of moving vehicles and 8 feet for each line of standing vehicles. Where space is absolutely limited this unit allowance may be reduced to 9 feet for moving and 7 feet for standing vehicles. The sidewalk width above suggested may be increased or reduced to meet special requirements as to pedestrian traffic or special demands for a tree and lawn strip. TRAFFIC ZONES