Graphical representation of the effect of road resistance on the load that may be drawn.
Slipperiness.
—Road surfaces which become slippery not only decrease the tractive effort of horses and motors but are very dangerous also. Non-slipperiness ought then to be given weight in the selection of the type of roadway. Observations in London in 1873 by Heywood on slipperiness of pavements indicated granite-block most slippery, then asphalt and wood-block. Greene, in 1885, analyzing a series of observations made in the principal cities of the United States, gave the order of slipperiness as wood-block, granite-block, and sheet-asphalt.
Slipperiness increases with grade. A special committee upon road materials of the American Society of Civil Engineers[183] recommend the following maximum grades for various kinds of pavements:
| Kinds of Roadway | Maximum Grade Per Cent |
|---|---|
| Gravel | 12 |
| Broken stone | 12 |
| Bituminous surface | 6 |
| Bituminous macadam | 8 |
| Bituminous concrete | 8 |
| Sheet asphalt | 5 |
| Cement concrete | 8 |
| Brick, cement grout filler | 6 |
| Brick, bituminous filler | 12 |
| Stone-block, cement grout filler | 9 |
| Stone-block, bituminous filler | 15 |
| Wood-block | 4 |
This would indicate that in the belief of the committee slipperiness is about in the inverse ratio of the grades. Those on which the steepest grades are allowed being the least slippery.
Climatic conditions affect slipperiness. Roads which are non-slippery in dry weather may be very slippery in wet weather. Pavements having a small amount of clay or earth on them are quite slippery when dampened, but after a hard rain may be much less slippery. Earth roads that have been thoroughly dragged are much more slippery immediately after a small shower than after a hard or soaking rain. Stone blocks and brick are worse after they have worn turtle-backed. Ice and sleet render all pavements slippery, but some more than others.
Sanitariness.
—The sanitariness of a road is the measure of the effect it has on the health of its users and the dwellers along its side. A dusty road is ordinarily an unsanitary one because of the germs of disease carried on the dust particles and which may be widely spread by the wind. An earth or gravel road when not dry or dusty is a sanitary road. A concrete or asphalt pavement when clean is very sanitary, but because dirt and debris brought upon it soon becomes ground into dust may become more unsanitary than an earth road. Mud, when clean, if that expression may be allowed, is sanitary, but when mixed on the road with the droppings of animals, sputum and other unclean things may become very unsanitary.