Road Location Elements

In Great Britain, the celebrated Holyhead road, built by Tel ford through the very mountainous district of North Wales, has an ordinary maximum of 1 in 30 (31%), with one piece of 1 in 22 ) and a very short piece of 1 in 17 (5.9%), on both of which pieces special care was taken to make the surface harder and smoother than on the remainder of the road.

On the National Pike over the Alleghenies, built before the intro duction of the railroad, the maximum was 7 per cent. At an early day the New York law limited the grades of turnpikes (toll roads) to 1 in 11 (9q).

In New York on state-aid roads the nominal maximum is 5 per cent, but grades as steep as 6i per cent have been found necessary in some cases. In New Jersey are a number of state-aid roads hav ing grades of 7 and 8 per cent, and one of lot percent. In Massa chusetts no hard-and-fast standard has been adopted for the state aid roads, but a few have 5 per cent grades and a considerable ber have 4 per cent. It is said that on some important roads the grade can not at reasonable expense be reduced below 7 per cent.

In improving city streets it is often impossible to make any radical change in the grade owing to the resulting damage to abut ting property, and it is always impossible to avoid the steep grade by a change of location; and consequently some city streets have very steep grades which are used with surprisingly good results. Newton, Mass., has a number of macadamized streets which have long stretches of 9 and 10 per cent grades, and has one 12 per cent grade 1,000 feet long. Waltham, Mass., has one 400-foot stretch of macadam on a 12 per cent grade, and another on a 13 per cent grade. In the Borough of Richmond (Staten Island), New York City, are several pieces of 10, 11, and 12 per cent grades, and 100 feet of 14 per cent, two stretches of 200 feet each of 16 per cent, and one piece 200 feet long of 20 per cent grade.

For mountain roads where the bulk of the traffic is down hill, the maximum grade is often 8 per cent and sometimes as much as 12 per cent. "Experience in heavy freighting has shown that wagons can be satisfactorily controlled in all weathers on 12 per cent grades, but they can not be safely controlled on steeper grade." For pleasure driving, the grade of a good gravel or broken stone road should, if practicable, not exceed 4 per cent. A good horse with a light buggy and two persons will easily trot up this grade, and also trot down without a brake; but with a steeper grade the strain in either direction is unduly great.

For bicycle travel, a 2 per cent grade can be ascended with com parative ease and descended with but little effort. Heavier grades, up to 5 per cent, can be ascended by the average bicycle rider with out extreme effort and descended without serious danger. A 5 per cent grade should be used only when unavoidable, and steeper grades can not be ascended with reasonable effort or descended with assured safety.

Minimum Grade.

Considering only the cost of transporta tion, a perfectly level road is the best; but it costs less to maintain a road upon a slight grade than one perfectly level. All roads should be higher in the center than at the sides, so as to shed the rain to the side ditches, but on any road longitudinal ruts are lia ble to form and interfere with the surface drainage; and therefore if the road is perfectly level in its longitudinal direction, its surface can not be kept free from water without giving it so great an incli nation transversely as to expose vehicles to the danger of overturn ing. On a perfectly level road, every rut will hold water, which will soak into the road and soften it whether it be earth or broken stone: whereas with even a slight longitudinal grade, every wheel track becomes a channel to carry off the rain water. It is a common ob servation that earth roads running up hill and down dale are better to travel upon than more level ones. This is largely due to the better longitudinal surface drainage.

The harder the road material the less the necessity for longitudi nal drainage of the surface. An earth road-surface is certain to wear into ruts, and hence is greatly benefited by having a longi tudinal slope. Gravel and broken-stone roads are liable to wear into longitudinal ruts, and hence need longitudinal drainage. Broken-stone roads built with the hardest limestones or trap are not easily worn into ruts, and therefore the necessity for a longi tudinal grade is least with this class of construction.

A longitudinal grade decreases the cost of maintenance, and the advisability of introducing a grade for such a purpose depends upon the relative cost of constructing it and upon the capitalized value of the cost of maintaining it. With earth roads the expenditures for maintenance are ordinarily too slight to justify much expense in securing a longitudinal grade; but with high class broken-stone roads, which naturally have a heavy traffic, a considerable expense to secure a slight longitudinal grade is usually justifiable. Engi neers whose experience has been largely upon railroads and canals are prone to spend money to secure an absolutely level road, where a slight grade could be secured at a less expense. In filling up a hollow or cutting down a hill, the employment of a light longitudinal grade may decrease the cost of construction and also the cost of maintenance without increasing the cost of transportation (§ 68-70). The important principle to remember is that a slight longitudinal grade is an advantage; although over a long stretch of level country it may not be practicable to secure it.

The following is the minimum grade adopted by leading engi neers for broken-stone roads: in England 1 in 80, or 1+ per cent; in France, by the Corps des Ponts et Chaussees, 1 in 125, or 0.8 per cent; in the United States 1 in 200, or 0.5 per cent.