Iowa DOT High Crash Locations Ranking Procedure

An interest has been expressed in examining the current Iowa DOT high crash locations ranking procedure. The purposes of this investigation are to determine the validity of the current procedure and improve the selection criteria.

The Iowa DOT generates an annual list of top 100 high crash locations using a 5-year period statewide crash data (The latest ranking uses the 1994-1998 crash data). The crash locations consist of nodes and links. The following roadway facilities have been classified as nodes:

Links are the distances between adjacent nodes. Crashes assigned to a link do not include the crashes assigned to either of the two nodes.

To become an initial candidate location, a site needs to meet one of the following three criteria: 1 fatal crash, 4 injury crashes, or a total of 8 crashes. Once the candidate locations have been determined, a three-phased ranking scheme is used as the basis to determine the high crash locations (see data flow diagram below).


1. Crash frequency
The first ranking is based on the number of crashes, or frequency, occurring at each location. Each site is given a ranking based on the number of crashes. A site that has the highest frequency of crashes receives the number one ranking. In the case of a tie each location receives the same rank and the subsequent ranking is skipped.

2. Crash rate
Second, each site is ranked according to the crash rate. For nodes and links up to 0.6 miles, the crash rate is calculated using the following equation:


where:
MEV is million entering vehicles and
DEV is daily entering vehicles for nodes or average daily traffic (ADT) for links.
For links 0.6 miles or longer, the DEV is determined using the following equation:


The site that has the largest crash rate receives the top ranking. The same implication of a tie applies to this ranking as well. For locations where their traffic volumes are unknown the ranking of zero is assigned.

3. Crash loss
Finally, each site is ranked according to the financial loss from the crashes. This is determined by using values based on the injuries sustained in each crash type as seen in the table below. These values are then multiplied by the number of people that fall into each category. For example, if two fatalities, four major injuries, 12 minor injuries, and 15 possible injuries occur at a location, the value loss due to injuries is $X (2 ´ $X + 4 ´ $X + 12 ´ $X + 15 ´ $X).

Type

Dollar Value

Fatal

$X

Major Injury

$X

Minor Injury

$X

Possible Injury

$X

Property damage is incorporated as well. Officers report estimates on the crash report form. In some instances there is no estimate of property damage; when this occurs a default value of $X is used. All of these values are summed up and result in a ranking based on the value lost at each location.

To determine the top 100 high crash locations within the state, each of the three ranks are added together and a final ranking is performed with the lowest cumulative ranking receiving the highest ranking of a 1. Those falling within the top 100 ranking are deemed high crash locations within Iowa. In the table shown below, an example of this process is shown, using fictitious data, for the top 13 locations throughout the state. This process is performed for approximately 17,000 locations that meet the initial threshold.

Reference

Node

# of

Crashes

Rank

Crash Rate

Rank

Dollar

Loss

Rank

Total Rank

Statewide

Rank

11111111

47

5

2.63

23

2,327,237

15

5+23+15 = 43

1

33333333

29

31

Unknown

0

1,909,420

20

31+0+20 = 57

2

44444444

25

35

2.76

15

2,734,603

9

35+15+9 = 59

3

22222222

24

37

2.71

19

3,150,760

4

37+19+4 = 60

4

55555555

53

1

2.46

29

1,373,300

35

1+29+35 = 65

5

77777777

40

10

2.92

8

1,120,949

47

10+8+47 = 65

5

00000000

34

21

2.40

33

2,000,850

18

21+33+18= 72

7

10101010

49

2

2.65

21

1,117,965

50

2+21+50 = 73

8

32323232

28

32

2.41

32

2,684,259

10

32+32+10= 74

9

88888888

19

51

3.15

3

1,824,587

22

51+3+22 = 76

10

99999999

18

53

2.47

28

3,501,985

1

53+28+1 = 82

11

66666666

36

18

2.28

41

1,740,548

27

18+41+27= 86

12

21212121

32

24

1.98

61

1,357,951

39

24+61+39=124

13

As indicated, crash frequency, rate, and cost equally contribute in ranking of the top 100 high crash locations. It was also noted that in current ranking procedure roadway links are treated as nodes. A link length of less of 0.6 miles is not taken into the consideration in the crash rate calculation. It is only for the longer link (0.6 miles or greater) where the link length is employed in a form of a multiplier (Link Length /0.3), rather than the actual link length, in the crash rate equation.

The research team is interested in validating and enhancing the factors involved in the current ranking procedure. Except the sensibility analysis [e.g., changing crash parameters (i.e., frequency, rate, and loss) contributing factors] no other approaches are sought to date. Once updated, the ranking procedure will be applied to selected crash types (e.g., fixed-object and cross centerline crashes) to rank specific crash types.