As a matter of fact, with the widths of vehicles which now prevail in Pittsburgh, if standing and slow-moving vehicles are compelled to keep in contact with the curb, it is possible to keep open a line of travel on each side of the car tracks, with only occasional blockades, where the width between curbs is 50 feet, or, at a pinch, even 48 or 47 feet. That is to say, the difference in traffic capacity between a thoroughfare 50 feet from curb to curb and one 45 feet is enormous; while the difference between 45 feet and 40 feet is very slight.
Since a main thoroughfare is apt in time to become a retail trading street, wide sidewalk space is important. It is a common rule to make the distance of the curb from the property line one-third the width of the roadway.
A total width of 90 feet, with a 54-foot roadway and 18-foot sidewalks, is a satisfactory minimum for meeting the practical requirements of an ordinary main traffic street; a width of 100 feet is preferable, and 80 feet may be regarded as a rather niggardly irreducible minimum.
In this connection it is interesting to note the standard widths adopted in European cities. The standard in London is 48 feet[5] between curbs and 80 feet between buildings for secondary avenues, and 100 feet over all for principal arteries; and 140 feet over all is proposed for two great main arteries, the cutting of which, through the midst of the city, is being considered. In German cities of the second size, such as Leipzig, Frankfort and Hanover, the standards are as follows: for strictly local streets, 33 to 47 feet; for secondary thoroughfares, 50 to 80 feet, and for main thoroughfares, 85 to 118 feet. A Prussian law, in force since 1875, and apparently drawn up to meet the requirements of Berlin with its heavier traffic, requires the following dimensions for the laying out of new streets and for the alteration of old ones: local streets, 40 to 65 feet; secondary thoroughfares, 65 to 95 feet; main thoroughfares, over 95 feet.[6]
Special Types of Thoroughfares
Park treatment of hillside street junction at Stuttgart
The above considerations apply only to the ordinary main thoroughfares of normal character. In most of the great cities of the world, there has been a considerable development of special thoroughfares of much greater width, including, for example, locations for transportation lines (surface or elevated), on separate rights of way decorated with trees; and including tree-shaded promenades and garden strips. These have usually been laid out in suburban sections before they were much built up; or, if within the built-up districts, on the sites of old fortifications, canals, or other abandoned engineering works. The latter opportunities are lacking at Pittsburgh, except in connection with the river banks. In the suburban localities of Pittsburgh, so much of the available building land is topographically divided into narrow strips that it would be cut to pieces in an exceptionally uneconomical manner by any boulevards, of the type usual in flatter cities, where a substantially uniform width of 150, or 200, or 300 feet is not infrequently carried through for considerable distances. As a general rule, any width to be secured for esthetic purposes in connection with Pittsburgh suburban thoroughfares, over and above that needed for handling the expectable future street traffic, must not be in the form of a general and continuous widening. But occasional pieces here and there may be taken for park purposes, as, for instance, a steep sidehill adjacent to the line and unavailable or difficult for building. Or a narrow ridge, on which the thoroughfare runs, may have at some point so little available building land fronting upon it that the whole can reasonably be parked for a short distance, thus keeping open the distant views.
Public resting place and outlook spot on a one-sided hillside street in Heidelberg