Estimating benefits

Benefits for a public works project are generally comprised of three elements: safety benefits, operational benefits, and environmental benefits. Each benefit is typically quantified on an annualized basis and so is readily usable in a benefit-cost analysis. The following sections discuss these in more detail.

Safety benefits

Safety benefits are defined as the assumed savings to the public due to a reduction in crashes within the project area. The general procedure for determining safety benefits is as follows:

• Quantify the existing safety history in the study area in terms of a crash rate for each level of severity (fatal, injury, property damage). This rate, expressed in terms of crashes per million entering vehicles, is computed by dividing the number of crashes of a given severity that occurred during the “before” period by the number of vehicles that entered the intersection during the same period. This results in a “before” crash rate for each level of severity.
• Estimate the change in crashes of each level of severity that can be reasonably expected due to the proposed improvements. As documented elsewhere in this guide, roundabouts tend to have proportionately greater reductions in fatal and injury crashes than property damage crashes.
• Determine a new expected crash rate (an “after” crash rate) by multiplying the “before” crash rates by the expected reductions. It is best to use local data to determine appropriate crash reduction factors due to geometric or traffic control changes, as well as the assumed costs of various severity levels of crashes.
• Estimate the number of “after” crashes of each level of severity for the life of the project by multiplying the “after” crash rate by the expected number of entering vehicles over the life of the project.
• Estimate a safety benefit by multiplying the expected number of “after” crashes of each level of severity by the average cost of each crash and then annualizing the result. The values in Exhibit 3-19 can provide a starting point, although local data should be used where available.

Operational benefits

The operational benefits of a project may be quantified in terms of the overall reduction in person-hours of delay to the public. Delay has a cost to the public in terms of lost productivity, and thus a value of time can typically be assigned to changes in estimated delay to quantify benefits associated with delay reduction.

The calculation of annual person-hours of delay can be performed with varying levels of detail, depending on the availability of data. For example, the vehicle-hours of delay may be computed as follows. The results should be converted to person-hours of delay using appropriate vehicle-occupancy factors (including transit), then adding pedestrian delay if significant.

• Estimate the delay per vehicle for each hour of the day. If turning movements are available for multiple hours, this estimate can be computed directly. If only the peak hour is available, the delay for an off-peak hour can be approximated by proportioning the peak hour turning movements by total entering vehicles.
• Determine the daily vehicle-hours of delay by multiplying the estimated delay per vehicle for a given hour by the total entering vehicles during that hour and then aggregating the results over the entire day. If data is available, these calculations can be separated by day of week or by weekday, Saturday, and Sunday.
• Determine annual vehicle-hours of delay by multiplying the daily vehicle-hours of delay by 365. If separate values have been calculated by day of week, first determine the weekday vehicle-hours of delay and then multiply by 52.1 (365 divided by 7). It may be appropriate to use fewer than 365 days per year because the operational benefits will not usually apply equally on all days.

Environmental benefits

The environmental benefits of a project are most readily quantified in terms of reduced fuel consumption and improved air quality. Of these, reductions in fuel consumption and the benefits associated with those reductions are typically the simplest to determine.

One way to determine fuel consumption is to use the same procedure for estimating delay, as described previously. Fuel consumption is an output of several of the models in use today, although the user is cautioned to ensure that the model is appropriately calibrated for current U.S. conditions. Alternatively, one can estimate fuel consumption by using the estimate of annual vehicle-hours of delay and then multiplying that by an assumed fuel consumption rate during idling, expressed as liters per hour (gallons per hour) of idling. The resulting estimate can then be converted to a cost by assuming an average cost of fuel, expressed in dollars per liter (dollars per gallon).