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.