Researchers and health policy planners have theorized and demonstrated that restricting lethal means can help reduce suicide rates, as delaying action until depression passes. In general, strong evidence supports the effectiveness of means restriction in preventing suicides. There is also strong evidence that restricted access at so-called suicide hotspots, such as bridges and cliffs, reduces suicides, whereas other interventions such as placing signs or increasing surveillance at these sites appears less effective. One of the most famous historical examples, of means reduction, is that of coal gas in the United Kingdom. Until the 1950s, the most common means of suicide in the UK was poisoning by gas inhalation. In 1958, natural gas (virtually free of carbon monoxide) was introduced, and over the next decade, comprised over 50% of gas used. As carbon monoxide in gas decreased, suicides also decreased. The decrease was driven entirely by dramatic decreases in the number of suicides by carbon monoxide poisoning.