--------------------------------+------------- |Maximum Grade Kinds of Roadway | 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. =Noisiness.=--Noisiness is a real source of disease, especially mental disorders. The less noisy types of pavement are usually laid in front of hospitals. =Acceptability.=--The acceptability of a roadway depends in addition to the things mentioned on its looks, appearance, esthetics; on the degree of heat and light which it reflects; upon its springiness and comfortableness to travel over as well as its easiness upon horses’ feet and rubber tires. [Illustration: © _Underwood and Underwood_ CROWNING A CALIFORNIA DIRT ROAD WITH TRACTOR DRAWN GRADER]