(a) The grades of streets shall not exceed the following; except, that where unusual or exceptional conditions exist, the Commission may modify these regulations:
(1) Highways and primary thoroughfares, four percent.
(2) Secondary thoroughfares and parkways, ten percent; local streets, minor streets, service drives and alleys, fifteen percent.
(3) Pedestrian ways or crosswalks, twenty percent, unless steps of an acceptable design are to be constructed.
(b) All changes in street grades in excess of one percent shall be connected by vertical curves of a minimum length equal to fifteen times the algebraic difference in the rate of grade for highways, thoroughfares and parkways, and one-half of this minimum for all other streets.
(c) The radii of curvature on the centerline shall not be less than the following:
(1) Highways, thoroughfares and parkways, four hundred feet.
(2) Local streets, minor streets and service drives, one hundred feet.
(11-4-85, appx. II.)