Safety in numbers
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Safety in numbers is the hypothesis that, by being part of a large physical group or mass, an individual is proportionally less likely to be the victim of a mishap, accident, attack, or other bad event. Some related theories also argue (and can show statistically) that mass behaviour (by becoming more predictable and "known" to other people) can reduce accident risks, such as in traffic safety - in this case, the safety effect creates an actual reduction of danger, rather than just a redistribution over a larger group.
Evidence often advanced for this position includes the flocking of birds and schooling of fish. In both of these instances, by being part of a large group, individuals face less risk of falling victim to predators than they would if traveling alone.
 In road traffic safety
Safety in numbers is also used to describe the evidence that a motorist is less likely to collide with a pedestrian or cyclist as the numbers of pedestrians or bicyclists increase. This non-linear relationship was first shown at intersections. A Public Health Consultant showed this non-linear relationship also occurs at in cities and countries using ecologic data from cities in California and Denmark, and European countries, and time-series data for the United Kingdom and the Netherlands. He termed this relationship "safety in numbers." 
Substantial independent evidence shows that the number of pedestrians or bicyclists injured increases at a slower rate than would be expected based on their numbers. That is, the risk to the individual pedestrian or bicyclist decreases where there is more people walking or bicycling. 
A 2002 study into whether pedestrian risk decreased with pedestrian flow, using 1983-86 data from signalized intersections in a town in Canada, found that in some circumstances the risk per pedestrian decreased with increased pedestrian flow.
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Another author has written that, while such data shows a degree of correlation, conclusions of causality may very well be based on a statistically spurious relationship. In other words, the relationship of an increase in bicycle use and a decrease in cyclist injuries may have no actual causal connection due to a certain third, unseen factor (referred to in statistics as a "confounding factor" or "lurking variable"). A spurious relationship gives an impression of a worthy link between two groups that is invalid when objectively examined.
 Helmet debate
Safety-in-numbers is important in the debate over the efficacy of bicycle helmets, because compulsory helmet laws have been shown to discourage bicycling.  Charles Komanoff has concluded that in "diluting the effect of 'safety in numbers,' compulsory helmet laws could have the perverse effect of increasing serious injury rates among those who continue to cycle." 
 Examples of substantial increases in cycling associated with reduction in danger
After cycling was promoted in Finland, the number of trips increased by 72% and there was a 75% drop in cyclists deaths.
 London, UK
In the United Kingdom, Transport for London, the body responsible for the London Congestion Charge, stated in their April 2005 review of that scheme, that motor vehicle traffic had decreased by 16%, bicycle use had increased by 28% and cyclist injuries had decreased by 20% in the first year of operation of the scheme.
In January 2008, a London newspaper reported that the number of cyclists in London being treated in hospitals for serious injuries had increased by 100% in six years. Over the same time, they report, the number of cyclists had increased by 84%.
 Berlin, Germany
Between 1975 and 2001, the total number of bicycle trips almost quadrupled. Between 1990 and 2007, the share of trips made by bicycle increased from 5% to 10%. Between 1992 and 2006, the number of serious bicycle injuries declined by 38%. 
 Portland, Oregon, US
Between 1990 and 2000, the percentage of workers who commuted to work by bicycle rose from 1.1% to 1.8%. By 2008, the proportion has risen to 6.0%; while the number of workers increased by only 36% between 1990 and 2008, the number of workers commuting by bicycle increased 608%.
Between 1992 and 2008, the number of bicyclists crossing four bridges into downtown was measured to have increased 369% between 1992 and 2008. During that same period, the number of reported crashes increased by only 14%. 
 Copenhagen, Denmark
Between 1995 and 2006, cycling increased by 44% and the percent of people cycling to work increased from 31% to 36%. During the same period, the number of cyclists killed or seriously injured fell by 60% 
 York, UK
Comparing the periods 1991-93 and 1996-98, the share of trips made by bicycle rose from 15% to 18%. The number of bicyclists killed and seriously injured fell by 59%. 
Between 1980 and 2005, cycling increased by 45%, and cyclist fatalities decreased by 58%. 
 Western Australia, Australia
During 7 years of the 1980s, cycling in Western Australia increased by 82% and admissions to hospital of cyclists declined by 5%. 
Between 1975 and 1998, the percent of trips made by bicycle rose from 8% to 12%, and cyclist fatalities fell by 66%. 
 New York, New York, US
 See also
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