In undeveloped areas, such as in new subdivisions, or for a type of project that occurs infrequently within City limits, such as Mulholland Drive, there is an opportunity to adhere closely to the theoretical design requirements previously presented. However, the characteristics of a grid and traffic pattern of most existing streets in urban areas tend to restrict or modify the use or the theoretical or desirable values of the elements of speed, horizontal curvature, and superelevation. For example, the speed is restricted by frequent intersection areas with traffic signal controls, constant turning and cross-traffic movements, dips due to cross-gutters, channelization and median islands with short turning radii, and congestion because of the heavy movement of pedestrians and vehicles.
The street alignment and the horizontal curvature, for the most part, are already existing, and the excessive right of way and construction cost may discourage or prohibit realignment for purposes of achieving a flat horizontal curvature. The values of superelevation that may be used are controlled by established street grades (meeting existing improvements) and drainage (extremely flat grades, excessive crossfall and crown sections, etc.). When the horizontal curves on a City street are of large enough radii to permit safe operation with a fully crowned section and at the proper design speed, superelevation is unnecessary and unduly complicates the design.
In general, values chosen from the figures should be tempered by the existing conditions rather than by the indiscriminate use of the theoretical. It should be kept in mind, however, that in no case should deviation from the standards of good design practice be so great as to render operation of pedestrian and vehicular movements unsafe, or to increase materially the City’s legal liabilities and maintenance responsibilities.
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