Chapter 4 - Elements of Design

4.4. Combined Horizontal and Vertical Alignments

4.4.3. Divided Highways

A well designed roadway will incorporate a litany of considerations including safety, economy, and aesthetics, etc. When terrain is hilly, mountainous or undulating, the profile of the roadway should generally follow the contours of the land (barring overriding considerations). On divided highways and rural interstates, the Designer should recognize where terrain dictates, separate horizontal alignments and vertical profiles can be utilized for opposing traffic.

Independent Profiles and Increasing Median Width

On state and federal divided highways, an increase in the width of the median and the use of independent alignments to derive the design and operational advantages of one-way roadways should be considered. Where right of way is available, a superior design, without significant additional costs, can result from the use of independent alignments and profiles. Bifurcated medians are especially effective where the general fall of the terrain is significant and perpendicular to the roadway.
   

Increasing the width of the median and/or bifurcating the roadway should be considered in the following situations:

• Where right of way is available and where the general fall of the terrain is significant and perpendicular to the roadway

• In isolated areas on rural reconstruction projects where the height of vertical reconstruction is significant. This will facilitate efficiency and ease conflicts during an intermediate stage of construction. As a general rule of thumb, standard 44-ft. median width can be maintained with independent profiles until the difference in elevations in opposing PGL’s is approximately 5-ft. Consideration should be given to increasing the median width (beyond 44-ft.) a minimum of 2-ft. for every 1-ft. of vertical profile reconstruction greater than 5-ft. Obviously, increasing the median width will result in greater right of way impacts. However, in many instances, minor right of way impacts - especially in rural areas where it is plentiful – are ultimately less costly than significant vertical reconstructions that require the contractor to utilize earth stabilization techniques or sheet pile to construct.

• At intersections to eliminate breakovers.