Roadways: Horizontal Alignment

Horizontal Alignment

Primary Guidance Primary Guidance Horizontal Alignments should be: Designed using the anticipated posted speed as the design speed Coordinated with vertical alignments while being developed Designed based on the functional classification and the context of the roadway Superelevation should be provide on all high speed rural and urban State roadways Horizontal Alignments for 3R projects should be evaluated to determine if there is crash or maintenance history to suggest the need for modification. If no crash or maintenance history suggests a condition warranting a change to the existing alignment, the existing alignment should remain.

Discussion Discussion The operational characteristics of a roadway are directly influenced by the horizontal and vertical alignments. The designer should coordinate both horizontal and vertical alignments while under development. There are many different factors to consider when developing the alignments. Some of the most important are design speed, superelevation rate, existing topography, and context of the road. For long corridor alignments that may transition from rural to urban zones and vise-versa, consider the changing road context rather than designing the corridor to one context or one design speed. This will result in an alignment that may transition if there is a need to reduce speeds through urban sections. In the areas of transition, pick an appropriate design speed along the transition zone alignment to help reduce prevailing speeds as the driver approaches the urban zone. This approach will allow the designer to use more flexible design techniques and not overdesign sections not meeting the facility’s context. When designing for low speed conditions, consider the proposed and existing cross sectional elements that may encourage lower prevailing speeds such as minimum building set-backs and vertical features such as street trees. In addition to the cross sectional elements, geometric treatments such as minimum curvature, shorter tangent sections, and lower superelevation rates, may be used to encourage lower prevailing speeds. Lower design speeds allow for more flexibility to adjust the horizontal alignment to fit within the existing environmental and right-of-way constraints. See Also Roadways: Vertical Alignment Practical Design Implementation Guidance