Mr. Edward D. Very, Sanitary Engineer, says that any attempt to estimate the amount of materials which accumulate on a city street must end in failure as the contributing elements vary in different localities in a city and in different cities, and where figures are given they do not really present any valuable data. Some general principles, however, have been determined. The Chicago Commission in its investigation declares that the quantity and volume of dirt attributed to horse drawn vehicle traffic is the most important source of street dirt. The loss of sand and coal and crushed stone, hay, manure and other loose material from poorly constructed vehicles or overloaded vehicles adds greatly to the quantity of street dirt to be removed. Important in a wet season is the dirt carried by moving vehicles through streets and alleys onto hard pavements, but the Commission says that the amount of dirt actually attributable to this source is considerably less than is usually believed. There is also considerable refuse in the form of leaves and grass which accumulates in the residential streets and along boulevards and parks, which has a tendency to lodge in catch basin inlets and stop the free flow of storm water. The quantity of leaves accumulating in the short leaf season on streets far exceeds that which naturally drops onto the surface of streets alone, because of the additional cleaning from lawns and parkway spaces.
Some reports express the belief that when a fixed standard is established of basing street cleaning schedules carefully on density of traffic, condition of pavement, character of frontage and kind of pavement, a definite relation will be found between the amount of street sweepings collected and the number of sweepers employed. In Chicago it has been found that different sweepers average daily collections of quantities varying from three-fourths of a cubic yard to three cubic yards. It has also been noted that street sweepings collected by regular block sweepers average about .0045 cubic feet per square yard. The weight of sweepings will, under ordinary conditions, approximate 36 pounds per cubic foot.
The paving and repair policy of a city is a very important factor in cleaning rates. Comparatively few cities as yet give any thought when selecting a particular pavement as to the relative cost of keeping it clean. It is also a fact that in many cities repair work is neglected at the expense of cleaning.