September 30, 2022Measuring Lane Widths

On occasion, the question of lane widths becomes a subject of a lawsuit. The best example of this is when a roadway construction project on an existing highway requires the contractor to maintain a minimum lane width for temporary operations. When a lane requires a certain width, one must determine how to measure that lane width.

Choosing a Measuring Method

In a roadway construction zone, assume that a temporary two-lane, two-way operation exists, the contract requires a minimum width of 11 feet, within a no-passing zone, and edgelines are provided. Further, assume that measurements start from the center of the double yellow centerlines to the outside of the edgeline and that dimension is 11 feet. If someone measures the same location and uses the inside of the nearest solid yellow centerline (less than 2 inches from half the gap between the centerlines and less than 4 inches from the centerline width) as one lane edge and the inside edge of the edgeline (less than 4 inches from the edgeline width), the result is a lane width of 10 feet, 2 inches.

When choosing a measuring method, which method is correct? Is there another method that is better? The American Association of State Highway and Transportation Officials (AASHTO) and the Manual on Uniform Traffic Control Devices define an edgeline as “a line that indicates the edge of the traveled way.” AASHTO defines traveled way as “the portion of roadway for the movement of vehicles, exclusive of shoulders.” AASHTO defines shoulder as “the portion of the roadway contiguous with the traveled way primarily for accommodation of stopped vehicles for emergency use and for lateral support of base and surface course.” Clearly, shoulders are located adjacent to the travel way and do not share common areas. There are many questions that need answering when it comes to edgelines. The edgeline indicates the edge of the travel way, but how does it indicate the edge? Is the entire width of the edgeline in the travel way? Are the edgelines part of the shoulder? Is some part of the edgeline in the travel way and some part in the shoulder? Unfortunately, the answer to questions like this is inconclusive because it depends on many different factors.

How to Measure

If a contractor builds a highway designed to consist of two, 12-foot-wide travel lanes constructed of rigid concrete and 10-foot-wide gravel shoulders on both sides, the construction plans would call for a 24-foot-wide concrete section. When a highway has gravel shoulders, a yellow line marks the center of the 24-foot-wide concrete section and edgelines mark the edges of the concrete section. With this example, it is logical to conclude that the lane width measures from the center of the centerline to the outside edge of the edgeline. The entire edgeline would be part of the designed travel lane.

Consider a second highway designed with the same lane and shoulder widths but with a 44-foot-wide flexible pavement cross-section. Once the highway is complete, the contractor likely would paint the centerline of the highway in the center of the 44-foot-wide cross-section and then measure 12 feet from the centerline to both sides and center the edgelines at those locations. Hence, if the lane width measures from the center of the center line to the outside edge of the edgeline, then the lane width would be 12 feet, 2 inches.

In July 2017, the Federal Highway Administration (FHWA) published Model Inventory of Roadway Elements, referred to as MIRE 2.0. The publication intends to “…provide a structure for roadway inventory data that will allow State and local transportation agencies to use analysis tools with their own data rather than relying on default values that may not reflect local conditions.” The publication states that “lane width is measured from center of edgeline to center of centerline or to the center of the lane line (if multilane). If edgeline striping is placed inside the edge of the pavement (within approximately one foot) to keep traffic from breaking the pavement edge, ignore the striping and measure from the pavement edge to the center of a single (or double) centerline, estimate a reasonable split between the actual width used by traffic and the shoulder or parking lane based on State/local design guides.”

Real World Application

In the real world, the small variance in lane width (like two inches) is not an operational issue or something that is disconcerting. On the other hand, it may make a difference when a lawsuit addresses this issue. There is little doubt that the FHWA—assuming that the MIRE 2.0 publication reflects its policy—suggests measuring lane widths from the center of centerlines (or from the gaps between two solid yellow centerlines) to centers of edgelines (or to centers of lane lines if multilanes exist). If the edgeline is placed a short distance from the pavement edge to protect against the deterioration of the pavement edge, then the lane width will include the edgeline and a portion of the pavement between the edgeline and the pavement edge.

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