That certain plan for the construction of automobile runways extending from the roadways of the improved public streets in the City and County of San Francisco to and upon a portion of the sidewalk area adjacent to the curb line of the roadway of any such streets prepared by the Bureau of Engineering of the Department of Public Works and on file under File No. 14733 in the office of the Clerk of the Board of Supervisors and entitled and designated as "Plan for Construction of Automobile Runway, File L-6964.1" is hereby approved and adopted as the plan in conformity with which any automobile runway extending from the roadway of an improved public street in said City and County to and upon a portion of a sidewalk area adjacent to the curb line of the roadway of such street is required hereafter to be constructed and maintained; provided, however, that when special conditions exist and this plan is not the most feasible the Director of Public Works is authorized to allow deviations, and any such automobile runway hereafter constructed and maintained in any manner not strictly conforming to the design and measurements indicated on such approved and adopted plan, unless modified as herein provided by the Director of Public Works, shall be unlawful.