STREETS AND HIGHWAYS The first work requiring the skill of the engineer is to lay out town sites properly, especially with reference to the future requirements of a large city where any such possibility exists. Few if any of our large cities were so planned. The same principles, to a limited extent, are applicable to all towns or cities. The topography of the site should be carefully studied, and the street lines adapted to it. These lines should be laid out systematically, with a view to convenience and comfort, and also with reference to economy of construction, future sanitary improvements, grades, and drainage.
Arrangement of City Streets. Generally, the best method of laying out streets is in straight lines, with frequent and regular inter secting streets, especially for the business parts of a city. When there is some centrally located structure, such as a courthouse, city hall, market, or other prominent building, it is very desirable to have several diagonal streets leading thereto. In the residence portions of cities, especially if on hilly ground, curves may with advantage replace straight lines, by affording better grades at less cost of grading, and by improving property through avoiding heavy embankments or cuttings.
Width of Streets. The width of streets should be proportioned to the character of the traffic that will use them. No rule can be laid down by which to determine the best width of streets; but it may safely be said that a street which is likely to become a commercial thorough fare should have a width of not less than 120 feet between the building lines—the carriage-way 80 feet wide, and the sidewalks each 20 feet wide.
In streets occupied entirely by residences a carriage-way 32 feet wide will be ample, but the width between the building lines may be as great as desired. The sidewalks may be any amount over 10 feet which fancy dictates. Whatever width is adopted for them, not more of it than 8 feet need be paved, the remainder being occupied with grass and trees.
Street Grades. The grades of city streets depend upon the topography of the site.. The necessity of avoiding deep cuttings or high embankments which would seriously affect the value of adjoining property for building purposes, often demands steeper grades than are permissible on country roads. Many cities have paved streets on 20 per cent grades. In establishing grades through unimproved property, they may usually be laid with reference to securing the most desirable percentage within a proper limit of cost. But when improve ments have already been made and have been located with reference to the natural surface of the ground, giving a desirable grade is fre quently a matter of extreme difficulty without injury to adjoining property. In such cases it becomes a question of how far individual
interests shall be sacrificed to the general good. There are, how ever, certain conditions which it is important to bear in mind: (1) That the longitudinal crown level should be uniformly sustained from street" to street intersection, whenever practicable.
(2) That the grade should be sufficient to drain the surface.
(3) That the crown levels at all intersections should be ex tended transversely, to avoid form ing a depression at the junction.
Arrangements of Grades at Street Intersections. The best ar rangement for intersections of streets when either or both have much inclination, is a matter requiring much consideration, and is ore upon which much diversity of opinion exists. No hard or fast rule can be laid down; each will require special adjustment. The best and sim plest method is to make the rectangular space aaaaaaaa, level, with a rise of one-half inch in 10 feet from AAAA to B, placing gulleys at AAAA and the catch basins at ccc. When this method is not practicable, adopt such a grade (but one not exceeding 22 per cent) that the rectangle AAAA shall appear to be nearly level; but to secure this it must actually have a considerable clip in the direction of the slope of the street. If steep grades are intersections, they introduce side slopes in the streets thus crossed, which are trouble some, if not dangerous, to vehicles turning the corners, especially the upper ones. Such intersections are especially objectionable in rainy weather. The storm water will fall to the lowest point, concentrating a large quantity of water at two receiving basins, which, with a broken grade, could be divided between four or more Fig. 48 shows the arrangement of intersections in steep grades adapted for the streets of Duluth, Minn. From this it will be seen that at these intersections the grades arc flattened to three per cent for the width of the roadway of the intersecting streets, and that the grade of the curbs is flattened to eight per cent for the width of the intersecting sidewalks. Grades of less amount on. roadway or sidewalk are con tinuous. The elevation of block-corners is found by adding together the curb elevations at the faces of the block-corners, and 21 per cent of the sum of the widths of the two sidewalks at the corner, and dividing the whole by two. This gives an elevation equal to the average eleva tion of the curbs at the corners, plus an average rise of two and one half per cent across the width of the sidewalk.