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Publication Number: FHWA-HRT-11-024
Date: April 2011

Safety Evaluation of the Safety Edge Treatment

Appendix B. Pavement-Edge Drop-Off Data Collection Methodology

This appendix presents the methodology used to collect field measurements for pavement-edge drop-offs.

Selection of Data Collection Locations

Several data collection locations were selected within each resurfacing project site to obtain field measurements of pavement-edge drop-offs. Data collection locations were generally 2-4 mi apart. There were typically three to four data collection locations within each site, depending on the overall site length.

Each data collection location was predefined as being a specified distance, in whole miles, from the start of the site. Then, to remove bias from the data collection process, a random offset was added to the predefined distance. This random offset, selected separately for each data location, was 0.1-0.9 mi, in increments of 0.1 mi. The location defined by the predefined distance plus the offset was used as the starting point for data collection. Field data collection personnel were given discretion to move the starting point if the measurement location was clearly not representative of the roadway as a whole or if sight distance was too limited for measurements to be made safely. Data were not collected at a selected location if recent maintenance had occurred or if weather did not permit data to be collected safely or accurately.

Field Measurements

Roadway characteristics were recorded at the selected starting point, and pavement-edge drop-off height was measured every 52 ft on both sides of the roadway over a 0.1-mi interval beginning at the starting point. A field data collection form is illustrated in figure 8. The data collection intervals are illustrated in figure 9. The measurements illustrated in the figure were repeated at intervals of 2-4 mi along the roadway, as previously described.

The roadway characteristics recorded at each data collection starting point included the following:

  • Speed limit.
  • Pavement type.
  • Shoulder type.
  • Shoulder grade.
  • Shoulder width.
  • Lane cross-slope.
  • Lane width.
  • Pavement edge drop-off shape.
  • Grade.

This figure shows the form used in the field to record data on the safety edge treatment. The data recorded include county and State, date, site, weather condition, name of main street or road, beginning cross street, ending cross street, speed limit, orientation, pavement type, shoulder type, pavement edge slope, random start point, grade, width, horizontal curve characteristics, and drop-off heights.

Figure 8. Illustration. Sample data collection form.

This figure shows the intervals at which drop-off heights were measured in the field. A group of ten measurements were made at regular intervals over a length of 0.1 mi, followed by a gap of several miles, followed by another group of ten measurements at regular intervals over a length of 0.1 mi.

Figure 9. Illustration. Data collection intervals.

Shoulder Type and Width

Shoulder types were generally recorded as paved, gravel, or earth. When a mixture of shoulder types was found (i.e., a composite shoulder), the width of paved shoulder beyond the edge of the traveled way was recorded and the presence of the other shoulder type was noted.

Drop-Off Shape

Drop-off shapes are shown in the data collection form in figure 8. Shapes A, B, and C are defined in other literature.(2) Most shapes correspond to A, B, or C. Shape A typically corresponds to the edge of concrete pavement. The likely cause of such drop-offs is settling of the concrete pavement. Shape A may also occur when asphalt pavement breaks. Shape B is the most common shape for drop-offs at the edge of an asphalt pavement. It is the shape that occurs from a typical overlay. Shape C corresponds to the safety wedge. It is recorded when the edge shape is angled at approximately 45 degrees and appears to be intentionally shaped at that angle. Other drop-off shapes were also recorded when present.

Lane Width and Pavement Width

Both pavement widths (i.e., traveled way width) and lane widths were measured. Lane widths were measured from the edge of the lane to the painted centerline of the roadway. Where no centerline was present, the lane width was calculated as half of the total pavement width. Where pavement extended less than 4 inches beyond the pavement edge line, it was included in the lane width. Where pavement extended more than 4 inches beyond the pavement edge line, it was treated as a paved shoulder.

Drop-Off Height

Drop-off height was measured to the nearest 0.125 inch since most measuring tools measure in 0.125-inch increments. Additionally, measurement tools marked with 0.125-inch increments are easier to read consistently than those marked with 0.1-inch increments. It is assumed that a tire could catch on just a few inches of drop-off, even if shoulder material is at grade beyond that distance. Therefore, drop-off height was measured approximately 4 inches from the edge of the pavement for shape A and 4 inches from the base of the pavement for shapes B and C (see figure 10).

This figure shows that the drop-off height was measured in the field as the vertical distance from the ground surface to the pavement surface.

Figure 10. Illustration. Measurement of drop-off perpendicular to pavement surface.

Drop-off height was measured by placing a level across the top of the pavement surface so that it hung over the shoulder. A ruler was then used to measure the vertical distance between the shoulder and the level at the appropriate location. Drop-off height was measured from the ground to the base of the level, as shown in figure 11.

This photograph shows the use of a straight edge and a ruler to implement the measurement of pavement edge drop-off height.

Figure 11. Photo. Measurement of pavement-edge drop-off height.

Pavement-edge drop-off height was not measured at driveways or minor intersections if they coincided with a planned data collection point. If a driveway or intersection was located at a data collection point along a segment, data collectors recorded that information and moved to the next data collection point.

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