SPEED HUMPS - FOUR HILLS STUDY
The Four Hills Speed Hump Study
In 2007 Michael J. Cunneen published a study of the impact of speed humps in the Four Hills area of Albuquerque, New Mexico. The full report is available below. One of the most interesting aspects of this report is that it includes one of the largest studies of the accident figures before and after the installation of speed humps in 93 street sections in Albuquerque. Over 1,100 accidents had occurred in these streets in the two years before and after speed hump installation. From this large sample, it was found that the aggregate accident rate for all accidents and for injury accidents had declined between 6-7%. But as the report suggests that the overall trend accident rate reduction in New Mexico was 1.7% per year, the net benefit allowing for that "control group" is therefore 6.8% less the 3.4% (over 2 years) giving a net benefit of only 3.4%.
This is obviously much less than the claims made by many people for the effectiveness of speed humps at reducing accidents. In addition the report demonstrates that the fatal accidents saved by speed humps are likely to be less than the increase in deaths caused by delays to emergency service vehicles.
A summary of the report is given below..
Note there are a few points worth making on this report, as even the 3.4% improvement is questionable. The report data is based on 2 years before and after as that was all the data that was available but that is a relatively short period. In the UK it is normal to use 3 years before and after as the normal basis for comparison. The reason is because in the first year after making any changes to a road layout, the accidents tend to fall. When drivers, who may only use the road occasionally, notice it has changed, they take extra care and drive more slowly. This effect wears off after a few months, but it has a big impact on the first year post the change.
Another problem is that it is very common for other changes to be made when humps are installed - for example improved signage (even extra warning signs), repainted road markings, new kerb alignments, etc. So the alleged benefit of 3.4% could be due to other reasons - and signs alone can have that size of effect.
A third possible explanation is a "Hawthorne" effect known from industrial physchology, where if you experiment on humans and suggest there will be a benefit, then you will end up seeing one. Unfortunately human behaviour, and reporting of accidents, is subject to unconscious biases that distort the results.
Summary of Four Hills Speed Hump Study
This was an independent study commissioned by the City Council of Albuquerque, New Mexico in 2007 to investigate installed the efficacy of speed humps installed along two major streets and to recommend alternatives. The neighborhood containing the speed humps was Four Hills Village, an outlying suburban, residential district at the city’s southeast corner. The City Counselor for the neighborhood was Don Harris and the consultant retained was Michael J. Cunneen.
The study found little need for the speed humps as traffic accidents in the affected roadway sections had been declining and averaged only 0.5 accidents per year since 2001. Less than 20% of the traffic 1990-2005 accidents involved injuries, all of them minor while less than 20% involved either “excessive speed” or speed “too fast for conditions” as one of the highest contributing factors causing the accidents. The accidents which had occurred were predominantly concentrated in two curve sections.
The study also examined speed hump results generally in Albuquerque, focusing on 93 street sections on which over 1,100 accidents had occurred in two years before and after speed hump installation. From this large sample, it was found that the aggregate accident rate for all accidents and for injury accidents had declined between 6-7%. Overall, volumes on streets with speed humps declined 7.5% as drivers diverted to other routes while average speeds declined about 11.5%. The small sample of pedestrian accidents actually rose from 6 to 10. Accident rates only declined on a minority of streets; in most cases they remained the same or increased slightly. A high proportion of speed humps had been installed where average speeds were only 25 miles per hour or less. Another major finding, drawn from City data covering all speed hump installations, was made significant declines in 85th percentile speeds tended to occur only where “before” speeds were above 30 miles per hour. Below 27 miles per hour, City data showed these speeds actually had increased after humps were installed.
Even assuming a 10% decrease in accidents attributable to speed humps, the study found that over 50 years only 3.4 injuries and 0.024 deaths would likely be prevented by the humps. On the other hand, delays to emergency service vehicles, even at only four seconds per speed hump, were estimated to cause several additional fatalities due to cardiac arrest over 50 years, many times higher than the lives that might be saved by the speed humps. This is because cardiac arrest requires rapid treatment; the likelihood of survival diminishes rapidly after five minutes and a delay of even a fraction of a minute significantly decreases the chances of surviving.
The consultant recommended removing most of the speed humps, retaining a few in the two sections where most accidents had occurred. He further recommended the use of optical speed bars and the installation of median islands and bulb-outs (neckdowns or chokers) to slow traffic without impeding emergency vehicle movement. Re-striping and better delineation at curves were recommended as well. In examining City speed data, the consultant found that re-striping had caused a slight decline in speed even where traffic was already moving at about the speed limit. He also found a positive “shadow effect” that speed humps had up to 3,300 feet away, in slowing traffic. City speed surveys showed that after speed hump installation, speeds dropped 2-11% at sites located 1,500 to 3,500 feet away from the nearest speed hump. This suggested that spacing between humps need not be so close and that fewer humps were necessary. Guidelines were recommended that would ban speed humps where volumes are less than 500 vehicles a day or greater than 3,500 vehicles a day or on collector or primary emergency response routes or where 85th percentile speeds do not exceed 30 miles per hour.
Postscript: On the 18 September 2007 a bill was passed by Albuquerque City Council with a unanimous vote to implement the recommendations contained in the report mentioned above.