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 Introduction

Risks change over time, and up to date trends should be monitored to ensure safety programmes are addressing the biggest risks 

There are three methods of estimating road safety risk:

  • collective safety risk (risk density measured as the number of fatal and serious casualties over a distance eg deaths and serious injuries (DSI) per kilometre or within a set distance of an intersection)
  • personal safety risk (risk to the individual of fatal or serious casualties per million vehicle kilometres travelled), and
  • infrastructure risk rating (IRR) score, a proactive measure of risk that aligns with personal risk but does not rely on (and is less sensitive to) crash history.

 

 

About calculating safety risk

The Transport Agency’s Safer Journeys Risk Assessment Tool(external link) or the KiwiRap Road Safety Risk(external link) website can be used as starting points for screening those corridors, intersections or routes that may satisfy the safety risk criteria.

It should be noted that the risk ratings presented on these websites may not correlate exactly with the criteria, and may be based on a different crash period.

 

  • Determining the number of injury crashes

    Determining the number of injury crashes

    The number of injury crashes that have occurred in the last 5-year 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 considered in analysis increases, and the impact of individual high severity or low severity crash events are normalised.

    Assessments completed after 31 March each year should include crash data from the previous calendar year in the analysis. Approved methods for calculating estimated DSI casualty equivalents are presented further below in "Calculating DSI casualty equivalents".

    Note:

    • Crashes can take up to three months to appear in CAS, therefore assessments completed after 31 March each year should include the previous calendar year of information in the five-year analysis period.
    • Assessments prepared earlier in the calendar year may use an older five-year analysis period.
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  • Converting safety risk to results alignment

    Converting safety risk to results alignment

    If a corridor or intersection has:

    • High or Medium-High collective safety risk, it becomes High overall safety risk for the purposes of results alignment.
    • Medium or Low-Medium collective safety risk, then a calculation of personal safety risk using either injury crash data or infrastructure risk rating(external link) (IRR) can be made:
    • The personal safety risk calculation requires information on corridor or intersection traffic volumes. If these calculations determine that the corridor or intersection has High or Medium-High personal safety risk, then it becomes High overall safety risk for the purposes of results alignment.
    • The infrastructure risk rating (IRR) is shown on the NZ Transport Agency’s Safer Journeys Risk Assessment Tool(external link). To calculate IRR, eight additional variables are needed. The information required to populate these eight variables is available through Google Street View and aerial imagery or can be manually calculated from site inspections and measurements. Once the IRR is determined, a calculation of High, Medium or Low overall safety risk can be undertaken (as set out below).

    Please use the information below to calculate safety risk and to determine whether the overall safety risk is High, Medium or Low for the purposes of results alignment.

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Identifying safety risk for results alignment

 There are two methods for identifying safety risk to determine the results alignment of corridor and intersection improvement proposals. 

 

Safety risk definitions

 The methods of calculating safety risk set out below involve using death and serious injury (DSI) casualty equivalents (see "Calculating DSI casualty equivalents" further below).  

 

  • High safety risk - corridors

    High safety risk

    An corridor can be assessed as High overall safety risk using one or more of the following methods.

    High overall safety risk for corridors is where:

    1. the estimated DSI casualty equivalents, based on the latest 5-year period, results in a High or Medium–High collective risk, determined using information from figure 1 below for urban (speed limit ≤ 70km/h) or figure 2 below for rural (speed limit ≥ 80km/h) speed environments respectively, and has a minimum of 3 injury crashes in 5 years.
    2. the estimated DSI casualty equivalents per 100 million vehicle kilometres travelled (based on the latest 5-year period) is ≥ 12 (Medium-High personal risk and above) and has a minimum of 3 injury crashes in 5 years.
    3. the estimated DSI casualty equivalents per 100 million vehicle kilometres travelled (based on the latest 5-year period) is ≥ 5 (Medium personal risk and above) and has a Collective Risk of Medium and has a minimum of 3 injury crashes in 5 years.
    4. the infrastructure risk rating(external link) (IRR) is High or Medium-High and the estimated DSI casualty equivalents value (based on the latest 5-year period) results in a Medium Collective Risk or above, based on figure 1 below for urban (speed limit ≤ 70km/h)and figure 2 below for rural (speed limit ≥ 80km/h) speed environments respectively and has a minimum of 3 injury crashes in 5 years.

    Note: For road sections with an AADT of less than 1,500vpd, 10 years of crash data may be used instead of 5 to increase the size of the crash sample. Where 10 years of crash data is used, the minimum number of injury crashes increases from 3 to 5.

     

    Figure 1:  Urban (≤ 70km/h) corridor estimated DSI casualty equivalents collective risk thresholds based on estimated DSI casualty equivalents per kilometre per year

     

     

    Figure 2: Rural (≥ 80km/h) corridor estimated DSI casualty equivalents collective risk thresholds based on estimated DSI casualty equivalents per kilometre per year

     

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  • High safety risk - intersections

    High safety risk - intersections

    High overall safety risk for intersections is where:

    1. Where collective risk is ≥ 1.1, estimated using DSi casualty equivalents based on the latest 5-year period, this is equivalent to the High or Medium-High collective risk definition in the High-risk intersections guide.
    2. Where personal risk is ≥ 32, estimated using DSI casualty equivalents per 100 million vehicle kilometres travelled based on the latest 5 year period, this meets the High personal risk definition in the High-risk intersections guide.
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  • Low safety risk

    Low safety risk

    Any intersection, corridor or route that does not satisfy any of the High or Medium safety risk definitions is classified as Low safety risk.

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Calculating DSI casualty equivalents

DSI casualty equivalents are an estimation of the number of deaths and serious injuries that are likely to occur at an intersection or on a corridor based on the total number of injury crashes that have occurred.

There are four approved methods for calculating estimated DSI casualty equivalents for the purpose of safety risk assessment (see below).

     Note:

    • Method 1 is for use in a strategic cases or programme business case or for a quick initial assessment for determining whether to proceed with a potential project or not.
    • Method 1 can result in an inaccurate estimation of DSI casualty equivalents where crash types with much higher or lower severity outcomes (such as Type B or F) are overrepresented within a corridor’s crash record.
    • A single-stage or detailed business case requires the use of methods 2, 3 or 4, because these give more accurate measures of DSI casualty equivalents.

    A spreadsheet has been developed to assist practitioners calculate collective and personal risk profiles for intersections (Method 1) and corridors (Method 2). The spreadsheet includes an infrastructure risk rating (IRR) calculation model.

    Download the ‘Determining safety risk practitioners  spreadsheet’ [XLSX, 30 KB]

     

    • Method 1: Strategic cases and programme business cases

      Method 1: Strategic cases and programme business cases

      A simplified method may be used by multiplying each injury crash by the corresponding DSi severity factor in Table A. 

      Table A: Generic corridor DSI severity indices

      Speed limit

      DSI severity factor

      Motorised road users

      Pedestrians and cyclists

      ≤ 50 km/h

      0.16

      0.24

      60–70 km/h

      0.23

      0.35

      80–100 km/h

      0.34

      0.51

       

       

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    • Method 2: Corridors

      Method 2: Corridors

      Use the Urban KiwiRAP(external link) method of classifying crashes at mid-blocks, and multiplying each injury crash by the corresponding DSi severity index (Table B) which varies as a function of primary crash movement type, speed environment and midblock form,

      Table B: Corridor severity indices (source: Urban KiwiRAP(external link))

      Primary crash type

      Corridor type

      70km/h
      2-lane

      70km/h multilane

      80km/h
      2-lane

      80km/h
      multilane

      A

      0.30

      0.24

      0.48

      0.18

      B

      0.37

      0.49

      0.82

      0.98

      C

      0.26

      0.30

      0.29

      0.17

      D

      0.28

      0.23

      0.33

      0.26

      E

      0.15

      0.17

      0.26

      0.18

      F

      0.05

      0.06

      0.15

      0.05

      G

      0.11

      0.13

      0.30

      0.28

      H

      0.19

      0.10

      0.43

      0.43

      J

      0.13

      0.06

      0.38

      0.38

      K

      0.18

      0.02

      0.22

      0.18

      L

      0.21

      0.14

      0.28

      0.29

      M

      0.15

      0.14

      0.32

      0.69

      N

      0.27

      0.31

      0.62

      0.62

      P

      0.27

      0.34

      0.66

      0.69

      Q

      0.49

      0.48

      0.43

      0.17

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    • Method 3: Intersections

      Method 3: Intersections

      Use the method described in the High-risk intersections guide. Severity indices for different intersection forms and controls are presented in Table C and D (below) for urban (speed limit ≤ 70km/h) and rural (speed limit ≥ 80km/h) speed environments respectively.

      Table C: Urban (≤ 70km/h) intersection severity indices (source: High-risk intersections guide)

      Primary crash type

      Intersection type

      Roundabout

      Traffic signals crossroads

      Traffic signals T-intersection

      Priority controlled crossroads

      Priority controlled T-intersection

      A

      0.10

      0.11

      0.11

      0.25

      0.25

      B

      0.16

      0.12

      0.12

      0.25

      0.21

      C

      0.27

      0.18

      0.18

      0.19

      0.25

      D

      0.20

      0.17

      0.17

      0.21

      0.24

      E

      0.11

      0.13

      0.11

      0.11

      0.10

      F

      0.05

      0.06

      0.06

      0.08

      0.07

      G

      0.13

      0.10

      0.07

      0.20

      0.11

      H

      0.15

      0.19

      0.10

      0.17

      0.18

      J

      0.15

      0.10

      0.10

      0.16

      0.15

      K

      0.10

      0.15

      0.10

      0.13

      0.13

      L

      0.15

      0.15

      0.18

      0.18

      0.18

      M

      0.09

      0.19

      0.19

      0.19

      0.14

      N

      0.23

      0.23

      0.24

      0.22

      0.24

      P

      0.22

      0.31

      0.31

      0.31

      0.31

      Q

      0.25

      0.25

      0.25

      0.25

      0.25

       

      Table D: Rural (≥ 80km/h) intersection severity indices (source: High-risk intersections guide


      Primary crash type

      Intersection type

      Roundabout

      Traffic signals crossroads

      Traffic signals T-intersection

      Priority controlled crossroads

      Priority controlled T-intersection

      A

      0.10

      0.22

      0.22

      0.40

      0.38

      B

      0.16

      0.40

      0.40

      0.70

      0.61

      C

      0.27

      0.30

      0.30

      0.30

      0.36

      D

      0.25

      0.30

      0.26

      0.30

      0.34

      E

      0.11

      0.19

      0.15

      0.33

      0.33

      F

      0.06

      0.09

      0.08

      0.10

      0.10

      G

      0.13

      0.14

      0.11

      0.25

      0.41

      H

      0.16

      0.27

      0.11

      0.50

      0.37

      J

      0.16

      0.20

      0.13

      0.36

      0.37

      K

      0.11

      0.23

      0.11

      0.25

      0.32

      L

      0.19

      0.18

      0.11

      0.35

      0.40

      M

      0.11

      0.23

      0.27

      0.30

      0.30

      N

      0.30

      0.60

      0.60

      0.60

      0.60

      P

      0.30

      0.60

      0.60

      0.60

      0.60

      Q

      0.25

      0.50

      0.50

      0.50

      0.50

       

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