A similar confusion occurs in attempting to compute the elevation of the corner of the property, from the grade of the corner of the curb. For example in Fig. 85, assuming that the grade of the top of the curb is the same as that of the center of the street, and assum ing that the sidewalk has a downward slope away from the prop erty of 0.24 inch per foot (2 per cent), and also assuming that the grade of the corner of the curb, D, has been established as 80.00, then the elevation of the corner of the property, G, as computed from the grade of the curb D E is 80.30 feet, while the elevation of the same point computed from the grade of the curb D F is 80.80 feet.
Some engineers advocate the establishment of the grade of the corner of the property and the determination of the grades of the curb and of the street therefrom; while others advocate establish ing the grade of the corner of the curb and from that determining the grade of the corner of the property and also of the center of the street intersection. To be legal the grade must be fixed by ordinance. The courts generally hold that the "grade " is the top of the pavement in the center of the street; and therefore it is necessary to establish by ordinance the grade of the center of the street intersection. Further, to prevent misapprehension and error in computing the elevation of the corners of the curbs, and also to save the labor of computing them anew each time a lot is to be surveyed, it is wise to establish also the grade of the corner of the curb. The ordinance should distinctly state the method to be em ployed in computing the auxiliary grades, i. e., the grade of the sidewalk and of the corner of the property. Often the grades are established for only one street without due consideration of the intersecting street; and then when the second street is improved, the result is confusion, disputes, and sometimes suits for damages.
When the rate of grade of both streets is small, it is desir able that the entire street intersection from property line to prop erty line, should be level, a condition which permits the continua tion of the section of each roadway until they intersect, makes the top of the curb at the four corners of the same elevation, and also allows the sidewalks at the corners to be level. That is to say, in Fig. 86, the four points marked b and all the points marked a are in the same horizontal plane. Each street has its full crown on the line b b, and consequently there is a sight rise from b to c.
Where either or both streets have much inclination, it may not be wise to flatten out the intersection, and thereby increase the grade on the remainder of the street. Under these conditions, the best arrangement of the intersections is a matter requiring careful study and is one upon which there is much diversity of opin ion. If steep grades are continued across intersections, they introduce side slopes in the streets thus crossed, which are trouble some and possibly dangerous—particularly to vehicles turning the upper corners. Such intersections are also objectionable on account of the difficulty of properly caring for the storm water. In residence districts it is usual to make the intersection "level from curb to curb "; that is, in Fig. 86, the four points marked b are in the same horizontal plane. The level places serve as breathing places, and lessen the danger of collision at the intersection. However, if the street has a considerable grade, a level intersection appears to have a decided pitch toward the hill, which gives the street an unpleasing appearance; and there fore under these conditions, it is better to apply, even in residence districts, the principle of the succeeding paragraph and give, the intersection a moderate inclination down hill. If the intersection has only enough inclination to seem level, the general appearance of a series of such intersections is pleasing, having the effect of a succession of terraces.
The following rule * for adjusting the grades at street inter sections is frequently employed and apparently is the most com plete of any that has been proposed. " In the business section all the street grades of 3 per cent or less should be continued un broken over the intersection; and streets having a steeper grade than 3 per cent should have an intersection of 3 per cent between curb lines. The grade of the curb between the other curb line and the property line should in no case be greater than 8 per cent. The grade at the corner of the property should be determined by adding to each of the grades of the curb opposite the corner, the rise of the sidewalk and taking the mean." Fig. 87 shows the sev eral elevations of a street intersection adjusted according to the above rules, assuming the transverse slope of the sidewalk to be 2 per cent (practically inch per foot—the usual value).