FACTORS AFFECTING SELECTION OF LANE WIDTH AND SHOULDER WIDTH ON URBAN FREEWAYS

Over the years a set of desirable design standards has evolved for lane width and shoulder width on freeways. These standards have been applied extensively, and a level of comfort has developed in which the use of full design standards results in roadways that are safe and operate satisfactorily. The use of these full standards is preferred and, all things equal, should form the basis for roadway design. However, especially in the upgrade and reconstruction of existing roadways--many of which were not originally built to full standards--a variety of competing factors begin to require attention. Some of the issues that justify consideration are (a) traffic operations, (b) traffic safety, (c) maintenance, (d) enforcement, (e) project cost, (f) public acceptance of the project, and (g) environmental issues. In effect a complicated trade-off analysis begins to take place. Although it is clearly desirable to construct a roadway that will be safe to operate and maintain, frequently other issues such as cost and environmental impacts can be greatly mitigated if something less than full design standards are used, at least at selected locations on a project. The effects of these various factors on the selection of lane width and shoulder width on urban freeways are assessed.