Safety Evaluation of Corner Clearance at Signalized Intersections

CHAPTER 8. ECONOMIC ANALYSIS

The research team conducted an economic analysis to estimate the cost-effectiveness of changing corner clearance at mainline access points near signalized intersections. The economic analysis examined the effect on total crashes from removing mainline access points on the receiving corners of four-leg, signalized intersections within a corner clearance distance of 50 ft. Due to the cross-sectional nature of this study and the uncertainty around the results—which is discussed further in chapter 10—the research team does not advocate adding access points on approaches as a crash-reduction measure at this time. However, the research team expects no safety disbenefits in total crashes from keeping access points with limited corner clearance (less than 50 ft) on the mainline approach corner for an average intersection. The research team used the total CMF rather than considering separate effects of fatal and injury and property-damage-only (PDO) crashes because the CMFs by severity are relatively consistent with total crashes (i.e., within 10-percent difference).

For this analysis, the research team assumed increasing corner clearance involved the removal of driveways with corner clearance of 50 ft or less by installing concrete curbing and a sidewalk in place of the mainline access for a commercial property. The intent was to shift traffic to an existing access on the cross street or further downstream (corner clearance more than 50 ft) on the mainline. The cost did not include the construction of a new access point, which, if necessary, would drastically increase the estimated cost of the treatment. The research team assumed that another mainline or cross-street access could continue to provide access to the property. Based on cost information for concrete sidewalks with curb and gutter from NCHRP Report 500: Volume 10: A Guide for Reducing Collisions Involving Pedestrians, curbing costs an average of $15 per linear foot, and walkways cost $11 per square foot.⁽¹²⁾ Assuming a sidewalk width of 6 ft, the average installation cost is $81 per linear foot of curb and sidewalk. Although most access points are narrower, the analysis used a conservative assumption of 100 ft of curb and sidewalk to connect walkways on either side of an existing driveway. Given these assumptions, the construction cost for removing access points was approximately $8,100 per access point per corner. The research team assumed that the construction cost per corner was the same regardless of the number of corners treated.

The research team assumed that the service life of the treatment was 10 years. Although the corner clearance will not deteriorate, the research team used a conservative service life of 10 years as a period in which significant maintenance and operations costs are unlikely. As such, this study assumes annual maintenance and operations costs to be negligible.

The FHWA Office of Safety Research and Development suggested using the Office of Management and Budget Circular A-4 as a resource for the real discount rate of 7percent to calculate the present value benefits and costs of the treatment over the service life.⁽¹³⁾ With this information, the analysis used a capital recovery factor of 7.02.

The research team used FHWA mean comprehensive crash costs by crash geometry as a basis for the benefit calculations.⁽¹⁴⁾ The mean comprehensive crash cost for a fatal and injury crash was $158,177 in 2001 U.S. dollars (USD). The cost for a PDO crash was $7,428 in 2001 USD. The research team weighted these values using the distribution of crash severities across study sites (i.e., approximately 43 percent fatal and injury crashes) to determine the mean comprehensive cost of a total crash as $71,553 in 2001 USD. At the time of analysis, the research team updated this value to 2016 USD by applying the ratio of the United States Department of Transportation (USDOT) 2016 value of a statistical life of $9.6 million to the 2001 value of $3.8 million, yielding an aggregate 2016 cost of $176,998 for a total crash.^(14,15)

To determine the safety benefits of increasing corner clearance, the research team analyzed the safety effects of removing access points with less than 50 ft of corner clearance on one or both mainline receiving corners of an average signalized intersection that had two receiving corners with limited clearance. Table 2 gives an average crash frequency of 4.36 crashes per site per year at four-leg, signalized intersections with no limited corner clearance on receiving corners. The research team multiplied this average crash frequency by the total CMFs of 1.33 and 1.76 from table 11 to estimate the crash frequency at sites with limited clearance on one (5.80 crashes per site per year) and two receiving corners (7.67 crashes per site per year). The research team used the differences in crash frequency between sites with two and one limited clearance corners (1.87 crashes per site per year) and two and zero limited clearance corners (3.31 crashes per site per year) as the average reduction of total crashes in each scenario.

The research team calculated the annual economic benefits by multiplying the total crash reduction per site per year by the average cost of a total crash, and then annualizing the result over the service life. USDOT recommended conducting a sensitivity analysis by assuming values of a statistical life of 0.55 and 1.38 times the 2016 value as lower and upper bounds.⁽¹⁵⁾ Researchers can apply these factors directly to the estimated B/C ratios. Table 18 presents the results.

Table 18. BC ratios for removing receiving corner access points from a site with limited clearance on two receiving corners.

These results suggest that removing access on mainline receiving corners to improve corner clearance—with reasonable assumptions on cost, service life, and the value of a statistical life—can be cost effective for reducing crashes at signalized intersections.

It is important to note that these results represented the change in total crashes under average conditions with several cost assumptions. The research team recommends conducting an economic analysis to determine if improving corner clearance is likely to be cost effective for specific sites where proposed projects are considered. Table 11 through table 17 list the CMFs for other crash types and severities that analysts should use when considering the safety effects of corner clearance.