Measures of risk and what they mean

For the purposes of displaying the safety risk of the state highway network, KiwiRAP looks at two different measures of risk: Collective Risk and Personal Risk.

The focus of both is on crashes where people have been killed or seriously injured.

The roads highlighted as being of higher risk than others are likely to have specific reasons why. The road, the vehicle, the speed and the driver/rider each contribute to risk.

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 any 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 equation below. (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 (excl urban sections)

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 authorities as this highlights where infrastructure improvements are most likely to be cost effective. It is also of interest to NZ Police from an enforcement perspective.

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. However, all risk cannot be eliminated through infrastructure improvements alone. The driver or rider 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 and riders may need to take extra care to minimise their risk.

Personal Risk (or Crash Rate)

Personal Risk is a measure of the danger to each individual using the state highway being assessed, as described in the equation below:

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. Personal Risk shows the likelihood of a driver or rider, on average, being involved in a fatal or serious road crash 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 or rider will be better informed and more able 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. In many cases infrastructure improvements on these roads are unlikely to be cost effective and other Safe System interventions such as safer road use and safe speeds need to be explored.