Big drop in crashes at targeted sites - LTSA study

The latest data from a crash reduction programme set up in 1985 shows an overall drop in injury crashes of 29% at more than 2,400 sites across New Zealand, the Land Transport Safety Authority (LTSA) said today.

A report on crashes occurring up to June 30, 2000 estimates a reduction of over 7,300 injury crashes associated with low-cost road works and other measures implemented at 2,408 sites since the Crash Investigation Programme was established 16 years ago.

The programme, run jointly by the LTSA, Transit New Zealand and local authorities, recommends inexpensive engineering treatments aimed at reducing crashes at selected sites.

The LTSA has been collecting data to analyse the effects of the programme since 1989. In addition to "before and after" statistics from the sites where works were completed, underlying crash trends within each local area were taken into account. Each site was assigned a "comparison group" to measure crash trends at similar sites before and after the works were implemented.

Using this method, an injury crash reduction of 29% over the 16 year period was estimated at the sites where works have been completed, with an estimated reduction of $2.4 billion in social costs.

Regionally, the biggest drop was recorded in Waikato/Bay of Plenty, with a 46% reduction at 405 sites. The smallest reduction was recorded in Northland/Auckland, with a 27% drop at 747 sites. See the table and note below for more details.

When sites are selected on the basis of high crash counts, there is likely to be some reduction in crashes over time, even if no remedial works are carried out, due to a statistical phenomenon known as "regression to the mean".

The reductions reported in the study take into account underlying local area trends, but they have not been corrected for regression to the mean. While crash reductions are still expected to be attributed to the programme after regression to the mean is taken into account, they may be smaller than the figures reported in the latest data.

Overall injury crash reduction by LTSA region

LTSA Region Sites Injury crash reduction

Northland/Auckland 747 27%
Waikato/Bay of Plenty 405 46%
Gisborne/Hawke's Bay 185 27%
Taranaki/Manawatu/Wanganui 258 43%
Wellington/Nelson/Marlborough/Kaikoura 450 31%
Canterbury/West Coast 189 28%
Otago/Southland 172 37%

Note: Crash reductions in each region depend on the mix of site types available for treatment. For example, on a national basis higher crash reductions were recorded at intersection sites than on routes, and the overall reduction was higher at open road sites than at urban sites.

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