Crash & Safety Analysis

Crash Analysis Requirements & Description

The purpose of collecting and analyzing historic traffic crash data for a project during consecutive periods is to identify possible crash patterns and to determine the probable causes of those crashes. The crash analysis includes patterns related to roadway conditions; time of day; weather conditions; type of crash; locations, i.e.: roadway, intersection, etc.; crash severity and driver characteristics.

Utilizing crash data also assists with determining expected values of a specific type of crash and ultimately identifying benefit costs and estimated Rate of Return (ROR) for improving roadway segments or intersection locations with the study boundary. These "estimated" ROR values should not be construed as "True" values, but more as approximated for planning purposes.

Crash Data

Crash data for 2015, 2016 and 2017 was requested from the NMDOT Traffic Safety Bureau. The crash data can be found in Appendix E – Crash Analysis

Crash Analysis and Improvement Cost Rate of Return (RoR)

In order to create a comparison between crashes from one location to the other, crash rates are used. These rates are based on data such as traffic volume, length of road sections considered and period of time in years. Typical crash rate equations for intersections are rates per million of entering vehicles (RMEV) and for roadway segments are rates per 100 million vehicle miles (RMVM).

RMEV = C x 1,000,000 / n x 365 x v

where:

R = Roadway Crash Rate per million entering vehicles (mev)

C = Total Crashes in an n-year period

n = year period of study (minimum 3 years)

v = total entering volume in vehicles per day

RMVM = C x 100,000,000 / n x 365 x l x v

where:

R = Roadway Crash Rate per 100,000,000 veh-mi

C = Total Crashes in an n-year period

n = year period of study (minimum 3 years)

l = length of roadway in miles

v = Average Daily Traffic (ADT) in vehicles per day

It should be noted that the New Mexico Department of Transportation (NMDOT) crash information presented yearly is based on a modified formula of the crash rate/100-million vehicle miles. Their reports show a crash rate (CR) = total crashes/100 MVM. It could be assumed that the state crash rate calculation is a derivation of a methodology typically used by the medical profession, where infection and mortality rates for various diseases are expressed in relation to population. This assumption can be made due to the inclusion of licensed drivers and population data in the crash summary report. Since the state crash rate is significantly higher than typical crash rate calculations, it can be concluded that comparing the state, county or city crash rates to the crash rates developed within this analysis will result in inconsistent comparisons due to the discriminating factors. Also noted, is that state crash rates are only calculated for roadway crashes (RMVM) not intersection crashes. An intersection crash may be included in a roadway crash but are not provided in a separate calculation (RMEV). This analysis also includes a Critical Rate calculation for segment locations (CRI). The critical rate analysis helps to identify locations or spots with observed crash rates higher than would be expected due to normal variation. The critical rate is calculated as follows:

CRI = RAR + k x sq rt(RAI/m) + 1/(2 x m)

where:

RAR = Average Roadway Crash Rate

k = statistical confidence level (typically 1.645 for a = 0.05)

m = travel on a particular section in million vehicle miles

If RAR is greater than CRI, then the location should be investigated further for problems associated with geometric or environmental factors.

Crash Analysis – Highway Safety Manual (HSM) Criteria

The Highway Safety Manual (HSM) has methodology to measure the safety of roadway corridors and intersections. This methodology includes:

Crash rate analysis (already completed): this takes into account the number of crashes in a given segment or at a specific intersection, daily traffic volumes and number of years of available crash data.
Cost factors based on crash severity (property damage only, injury, and fatal crashes): The HSM identifies a societal cost for crashes based on their severity:

Fatal crashes: $6,800,000.00
Injury crashes:$390,000.00
Property Damage Only (PDO) crashes:$12,000.00
Equivalent Property Damage Only (EPDO) Average Crash Frequency: this method uses the ratios between costs of each type of crash and assigns “weights”, or point value, to each type of crash by its severity, and can be used for roadway segments and intersections:
Fatal crashes: 0
Injury crashes: 5
PDO crashes: 3

The above values indicate that a fatal crash has a “weight” of 567. In other words, the societal cost of a fatal crash is 567 times the societal cost of a PDO crash. Therefore, an injury crash has a weight of 33, since the societal cost of an injury crash is 33 times that of a PDO crash. This allows for a location to have added emphasis if there is a higher rate of severity (fatal or injury) in the reported crash history of a roadway or intersection.

The EPDO formula is as follows:

EDPO score = (Fw + Fn) + (Iw +In) + (Pw + Pn)

where:

Fw = Fatal Crash weight (567)

Fn = number of reported fatal crashes

Iw = Injury Crash weight (33)

In = number of reported injury crashes

Pw = PDO crash weight (1)

Pn = number of reported PDO crashes

Using the EPDO method, Tables V.2 shows the EPDO values for the US 60 roadway segments (no intersection data was available):

The following figures and tables describe the results of the crash analysis. The largest single types of crashes observed were animal and vehicle sideswipe along the same side. There was a total of 2 each for animal and vehicle sideswipes for the years analyzed.