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Addressing Intersections in Curves

by Airton Kohls (Source: Low-Cost Treatments for Horizontal Curve Safety 2016 - FHWA Safety Program)

The AASHTO Policy on Geometric Design of Highways and Streets recommends that when an intersecting roadway is located within a curve “the alignment should be as straight and the gradient as flat as practical” to allow for easy recognition of the potential conflicts. It further states that “an intersection on a sharp curve should be avoided or designed to compensate for potential adverse grade and reduced sight distance.”

However, many agencies have existing intersections with less than ideal design. The demands on drivers approaching and navigating horizontal curves include visual demands, vehicle control demands, and speed selection. The closer the driver is to the curve, the harder it is for the driver to effectively assimilate information relating to anything other than navigating the curve. The geometry often limits the available sight distance for safe maneuvering and the physical constraints of the intersecting roadway often limit the application of signing and other delineation. The following discusses a few treatments unique to the combination of intersections and curves.

Adjusting Signs and Markings for the Intersecting Roadway
Where an intersecting roadway is within the curve, the traditional means of delineating the roadway alignment is often interrupted. Center line and edge line markings are typically not continued through the intersection. The edge line marking is of particular concern if the intersecting roadway has a wide throat. The MUTCD allows dotted edge line extensions consisting of 2-foot line segments and 2- to 6-foot gaps through intersections along the mainline. In fact, the MUTCD guidance recommends this treatment to help guide motorists through the intersection. Similarly, where chevrons or delineators would typically be used to provide delineation, the discontinuance through the intersection may leave a significant portion of the curve lacking delineation. Adjusting the location of the remaining chevrons or delineators may be appropriate to delineate the maximum curve length. Providing a visible stop line on the minor road approach may also be helpful, especially where the stop line can be seen from a significant distance from the intersection or where crashes indicate stop sign violations.

Pavement markings narrowing the travel lane as driver approaches the intersection.

Smooth Lane Narrowing
The “smooth lane narrowing” treatment narrows the lane width approaching the intersection with a combination of markings and rumble strips. The narrowing is accomplished by gradually tapering out from the center. The rumble strips are milled in along both the left and right sides of each direction of travel, with longitudinal center and edge line markings added. The combination of rumble strips and markings to narrow the lanes reduces operating speeds on the intersection approach. When a curve is present, the preferred design is to narrow the lanes on the approach to the curve. The paved width is not changed in this countermeasure, but the narrower lane width continues throughout the entire length of the curve. The rumble strips and markings are discontinued at the intersection.

Limited sight distance due to the intersection being inside the curve.

Intersection Sight Triangles
In the typical rural curve with an intersection, the minor road will be stop controlled.

Assuming the intersecting roadway is aligned perpendicular to the curve of the main roadway and is at or near the center of the curve, the sight distance issues on the outside of curve are similar or perhaps even better than for a tangent roadway section. Providing appropriate sight triangles will often be adequate. The intersection on the inside of the curve, however, is restricted by the geometry and requires the driver to have more mobility to see over-the-shoulder to view oncoming traffic. If the intersection is not near the center of the curve, sight triangles may cut across the curve and require significantly more clearing. If the terrain is not flat, it may be necessary to cut into slopes to provide the adequate minimum intersection sight distance. The use of and location of guardrails on grades should also be considered as it could interrupt the sight lines for intersections in and near curves. Where providing the appropriate intersection sight distance is not feasible, the intersection may need to be re-configured. In certain limited cases, an “All-Way” stop-controlled intersection may be appropriate.

Paving the intersection approach would avoid hazardous condition of having gravel on the main road.

Additional considerations includes:

“Addressing Intersection in Curves” is part of a comprehensive material provided by FHWA on the Low-Cost Treatments for Horizontal Curve Safety 2016 Guidelines (http://safety.fhwa.dot.gov/roadway_dept/horicurves/fhwasa15084/fhwasa15084.pdf) focused on reducing fatalities caused by roadway departure crashes.


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