For national (or regional) road safety evaluations and comparison we have been using two safety performance indicators (SPIs):
- direct (crashes, fatalities, injuries...)
- indirect (speed, speeding, seat belt use, daytime running lights use, cell phone use...)
Now I am trying to find some correlations between them. And I ask, for example: Should percent of speeding correlate with crash counts? or with fatalities? or injuries?
All is done on the level of country (or regions) - so direct SPIs are from national/regional statistics. Indirect SPIs are from surveys, typically 7 locations per region, once per year.
And lots of other questions, for example: when surveys are done in weekdays, spring/autumn, dry weather, should crashes to correlation also come only from the same conditions? (or possibly even only in the days/hours of survey?)
I will welcome any ideas on how to do these analyses, references, etc. Thank you in advance.
go to monash university accident research centre website, Victorian injury surveillance unit and search
When discussing correlations you have to keep in mind that correlations do not necessarily imply causal relations. Often, they are a consequence of a common influence by an ignored third factor such as properties of the infrastructure (many or few freeways), regional structure (predominantly urban or countryside), cultural properties, (liberal or strict) legislation, (sloppy or stringent) control/enforcement. For example, in countries with liberal speed regulations (e.g., 75 or even 85 mph allowed on some roads) and sloppy enforcement (hardly any speed limit monitoring), few speed tickets are issued, i.e., the statistical data show little speeding. Nevertheless, in an objective sense, there may be many drivers driving too fast for the given situation resulting in comparatively many accidents. This results in a paradoxical correlation with an unexpected sign between speeding and accidents. This is a variant of the classical Simpson's paradox: The more firefighters are sent to the scene, the more damage the fire will cause.
I suggest the following web site; it is a masters thesis that I supervised. It includes excellent literature review that can be of great benefit for your search.
http://scholar.najah.edu/content/modeling-relationship-between-geometric-design-consistency-and-road-safety-two-lane-rural
Remember not to forget about low speed vehicle run-over injuries. These are extremely common in Australia and New Zealand where the constitute the 2nd highest trauma related fatality rate in children (after drowning).
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