Combined Metric Analysis

Winter weather-related crash density, crash proportion (the proportion of winter-weather related crashes to all winter crashes), and personal-level injury severity (injuries on each

roadway segment by frequency and severity) were considered in the evaluation of one-mile roadway segments with respect to winter weather safety. Crash density was represented by crash

frequency on a particular road segment by summing up the total winter-weather related crashes occurring during the winter seasons of 2002 to 2009 and dividing the total by the length of the road segment and the number of years in the analysis period. A Crash proportion metric was computed by summing the total winter-weather related crashes and dividing by the total winter crashes over a road segment for the analysis period. A Severity metric was computed from a total score assigned to each road segment based on the total frequency of injury severities experienced in winter weather-related crashes. The following equations show the three different metrics used for deriving the combined metric.

𝐶𝑟𝑎𝑠ℎ 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = (𝑐𝑟𝑎𝑠ℎ 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑛 𝑎 𝑟𝑜𝑎𝑑 𝑠𝑒𝑔𝑚𝑒𝑛𝑡) ÷ (𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑜𝑎𝑑 𝑠𝑒𝑔𝑚𝑒𝑛𝑡 × 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑦𝑒𝑎𝑟𝑠)

𝐶𝑟𝑎𝑠ℎ 𝑝𝑟𝑜𝑝𝑜𝑟𝑡𝑖𝑜𝑛 = 𝑤𝑖𝑛𝑡𝑒𝑟 𝑤𝑒𝑎𝑡ℎ𝑒𝑟 𝑟𝑒𝑙𝑎𝑡𝑒𝑑 𝑐𝑟𝑎𝑠ℎ𝑒𝑠 ÷ 𝑡𝑜𝑡𝑎𝑙 𝑐𝑟𝑎𝑠ℎ𝑒𝑠 𝑑𝑢𝑟𝑖𝑛𝑔 𝑤𝑖𝑛𝑡𝑒𝑟 𝑠𝑒𝑎𝑠𝑜𝑛 𝐶𝑟𝑎𝑠ℎ 𝑠𝑒𝑣𝑒𝑟𝑖𝑡𝑦 = ∑ 𝐶𝑟𝑎𝑠ℎ𝑖× 𝑃𝑜𝑖𝑛𝑡𝑖

𝑖

where i = fatal, major, minor, and possible injury crash.

The combined metric was created by computing the total frequency of injuries by severity over an analysis period and assigning each injury severity a certain number of points, based on the standard Iowa DOT scale shown in Table 5.1. It is to be noted that all three metrics for a specific one-mile road segment were normalized in order to index the metrics against a maximum value of 1.0. In order to do that, the crash density and crash severity metrics were divided by the maximum value of the metrics for a common type of roadway and an analysis period from 2002 to 2009. The crash proportion metric was already normalized against a maximum value of 1.0 as it was expressed as a percentage of total winter crashes during the analysis period. The resulting values were aggregated into a combined score

Table 5.1 Standard Iowa DOT scale for assigning points by injury severity

for each roadway segment. The combined score for each road segment was computed by putting equal weight to the normalized score of each of the three metrics. In this process, a one-third (1/3) weighting was assigned to each of the three metric for computing the combined score. While equal weight was put on the three metrics in this research to consider all three metrics equally important, it may be preferred to weight the three metrics differently. Combined road segment scores based on alternate weighting can be recomputed in the future at the discretion of concerned officials from Iowa DOT. The following equation shows the calculation of the combined score by putting equal weight on the three metrics.

𝐶𝑜𝑚𝑏𝑖𝑛𝑒𝑑 𝑠𝑐 = (1₃) × 𝐶𝑟𝑎𝑠ℎ 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 + (1₃) × 𝐶𝑟𝑎𝑠ℎ𝑝𝑟𝑜𝑝𝑝𝑜𝑟𝑡𝑖𝑜𝑛 + (1₃) × 𝐶𝑟𝑎𝑠ℎ 𝑠𝑒𝑣𝑒𝑟𝑖𝑡𝑦 (5.1) This combined score was categorized based on their relative magnitude within the appropriate road type and analysis period. The total mileage of a common road type (i.e. interstate/freeway and tow-lane roads) was computed and categories were created based on the combined score being among a certain percentage of the system mileage. Following the United States Road Assessment Program (usRAP) risk-mapping protocol, the five categories included five derived ranges for the combined metric. Table 5.2 shows the categories with the subsequent percentage of system mileage. Category 1 indicates the values of the combined score within the lowest 40 percent of the total mileage of a road type while category 5 indicates the values within highest 5

Injury severity Points each occurrence

Fatality 200, with the first fatality at a site treated as a major injury

Major 100

Minor 10

percent of the total system mileage. Road segments which were assigned to category 5 indicated maximum safety problems in term of winter weather safety when crash density, crash proportion and crash severity are considered while road segments belonging to category 1 were considered to be least prone to winter weather crashes. Figure 5.1 and 5.2 shows the map of interstate and tow-lane roadway systems categories based on the combined score considering crash density, crash proportion, and crash severity.

Table 5.2 Mileage category ranges (for each road type) by relative magnitude Category

Metric value is among percentage of system mileage

1 Lowest 40 percent 2 Next 25 percent 3 Next 20 percent 4 Next 10 percent 5 Highest 5 percent

Figure 5.1 shows that Interstate 35 north has quite a few segments with higher categories compared to the Interstate 35 south. In fact, there is a continuous stretch of roadway belonging to categories equal or greater than 4. On the other hand, Interstate 80 has road segments with high category values scattered from east to west with the east portion having quite a few black and red colored segments. Interstate 80 has less clustered high category road segments unlike Interstate 35. The map for two-lane roadways also shows the segments or the cluster of segments to identify sites prone to winter weather crashes. Although it is difficult to derive any pattern from the two-lane roadway map, segments belonging to higher categories are more prominent in the upper region of the map compared to the lower region.

Figure 5.1 Combined metric categorization for Interstate/freeway road segments