The Death and serious injury (DSi) equivalents are severity indices that are used to estimate the number of deaths and serious injuries likely to occur at an intersection or on a corridor based on the total number of injury crashes that have been reported. The use of the DSi equivalents approach gives a better indication of the level of risk on a corridor or at an intersection than fatal and serious crash data alone. This is because the fatal and serious crash data is not a good indicator of the underlying risk of high-risk crash types that are more likely to result in death and serious injuries at many intersections or along corridors.
DSi severity indices represents the average number of people killed and seriously injured for every reported injury crash. Severity indices have been calculated for different speed environments, intersection and midblock locations, different intersection controls, different road user groups and for different crash movement types.
The DSi severity indices were first developed using all nationwide reported injury crash data from the 2008-2012 crash period and the tables below have been updated using all nationwide reported injury crashes for the 2015 – 2019 period from Waka Kotahi Crash Analysis System.
Fatal and serious crashes are relatively rare and random events on the transport network, and because the safe system focuses us from crashes to causalities the use of DSi equivalents is considered best practice.
At that time the severity indices were developed they used the traditional urban and rural definitions based on the speed limit (eg Urban where the posted speed limit is 70 km/h and less and rural where the posted speed limit is 80 km/h and greater). For future updates we will review how to better define the severity indices to align with the One Network Framework urban and rural road and street categories.
The number of injury crashes that have occurred in the last 5 or 10-year interval is determined using relevant data from the Crash Analysis System (CAS).
By using all injury crashes instead of just fatal and serious crashes to predict the number of DSi likely to occur in future, the number of crashes analysed increases, and the impact of individual high severity or low severity crashes are normalised.
For assessments completed after 31 March each year, include crash data from the previous calendar year in the analysis.
Urban DSi severity indices are calculated at a generic level, intersection form and users by primary movement code on corridors and intersections where the posted speed limit is 70 km/h and less.
Rural DSi severity indices are calculated at a generic level, intersection form and users by primary movement code on corridors and intersections where the posted speed limit is 80 km/h and greater.
DSi severity indices are calculated at a generic level only and not disaggregated by primary movement code.
Speed scaling factors are calculated for all speed limits from 30 km/h through to 80 km/h, as well as 100 km/h. There are currently insufficient injury crash numbers in 90 km/h and 110 km/h speed limit areas to form robust conclusions about the speed scaling factors that should apply.
For calculation purposes we recommend adopting the average speed scaling factor of 80 km/h and 100 km/h for application in 90 km/h speed limit areas, and for the 100 km/h speed scaling factor to be used in 110 km/h speed limit areas.
Speed scaling factors are applied to DSi severity indices in a multiplicative manner.
For instance, the DSi equivalent value for a roundabout in a 60km/h area will be the sum of the crash movement code severity indices multiplied by 1.15 (the speed scaling factor for this context).
A simplified method may be used by multiplying each injury crash by the corresponding generic, midblock or intersection DSi severity indices within the Urban or Rural tables below.
Requires the use of the appropriate crash movement code either for midblock, intersection type or road user types of crashes because these give a more accurate measure of the DSi equivalents.
A spreadsheet has been developed to assist practitioners calculate collective and personal risk profiles for intersections and corridors.