KiwiRAP

Measures of risk and what they mean

For the purposes of displaying the safety risk of the state highway network, we look at two different measures of risk, Collective Risk (or Crash Density) and Personal Risk. The focus of both is on crashes where people have been killed or seriously injured. The crash statistics used for the calculations are for the fiveyear period between 2002-2006.

Each of the KiwiRAP state highway links has been assigned a rating for both Collective and Personal Risk. The methodology used to do this is discussed in the following section.

The definitions of fatal and serious injuries are:

Fatal injuries- Injuries that result in death within 30 days of the crash.

Serious injuries- Fractures, concussion, internal injuries, crushings, severe cuts and lacerations, severe general shock necessitating medical treatment and other injury involving removal to and detention in hospital.

Collective Risk (or Crash Density)

Collective Risk is a measure of the total number of fatal and serious injury crashes per kilometre over a section of road, as described in the following equation. Collective Risk can also be described as the Crash Density.

Collective Risk 
(Fatal crashes + serious injury crashes) / number of years of data
                     Length of road section

Unlike Collective Risk, Personal Risk takes into account the traffic volumes on each section of state highway.

Because Collective Risk is measured in terms of the number of crashes per kilometre of state highway, you would generally expect that those with higher traffic volumes would have a higher Collective Risk. For example SH 1 from Pukerua Bay to MacKays Crossing, or SH 16 north of Auckland.

However, this is not always the case, as some road sections can have a high traffic volume but a relatively low Collective Risk. For example SH 1 between Albany and Orewa. Two road sections could have the same traffic volumes, but because of their engineering have quite different levels of safety - for example a road with a median barrier versus one without, or a roadside free of power poles and other roadside hazards versus a roadside with them.

Personal Risk

Personal Risk is a measure of the danger to each individual using the state highway being assessed.

Personal Risk 
(Fatal crashes + serious injury crashes) / number of years of data
           Distance Travelled/ Number of years of data

Unlike Collective Risk, Personal Risk takes into account the traffic volumes on each section of state highway.

What is the difference between Collective and Personal Risk?

Roads highlighted as being of higher risk than others are likely to have specific reasons why. The road, the car and the driver each contribute to risk.

Collective Risk highlights which road links have a high number of fatal and serious crashes on them - which can be used to help determine where the greatest road safety gains can be made from investment in engineering. Collective Risk is perhaps of most interest to the road controlling authority, Transit New Zealand.

However, as stated previously, risk cannot be eliminated through infrastructure improvements alone. The driver must always share responsibility for a safe road system. The Risk Maps strengthen the connection between infrastructure and personal responsibility by highlighting sections of road where safety improvements are warranted, but also where drivers may need to take extra care to minimise their risk until road improvements are made.

Personal Risk shows the likelihood of a driver, on average, being involved in a fatal or serious road accident on a particular stretch of road. Personal Risk is of most interest to the public, as it shows the risk to road users, as individuals. A risk aware driver will be better informed and more able to identify situations along their journey where they need to modify their behaviour to respond to the conditions. Personal Risk is typically higher in more difficult terrain where traffic volumes and road standards are often lower.