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Alcohol:

Alcohol Concern's 'The State of the Nation' report:
"One in seven people killed on the roads are involved in drink-drive accidents".
http://news.bbc.co.uk/1/hi/health/1656142.stm

Department of Transport's 1998 campaign against drink-driving:
"3,500 people are killed or seriously injured each year in drink drive accidents".
www.think.dft.gov.uk/drinkdrive/ddc98/leaflet.htm

"….road accidents where drink is a factor account for a further [cost to society of] £189m."
http://society.guardian.co.uk/drugsandalcohol/story/0,8150,410967,00.html

Cannabis:

The World Health Organisation's report 'Cannabis: a health perspective and research agenda':
"The epidemiological studies indicate that in its own right, cannabis makes at most a very small contribution to motor vehicle accidents, and so on the whole it may seem be a minor road safety problem by comparison with alcohol."
www.druglibrary.org/schaffer/hemp/general/who-magnitude.htm

The World Health Organisation's report 'Cannabis: a health perspective and research agenda':
"Blood levels of cannabinoids do not indicate whether a driver or pedestrian was intoxicated with cannabis at the time of an accident."
www.druglibrary.org/schaffer/hemp/general/who-probable.htm

Advisory Council on the Misuse of Drugs report 'The classification of cannabis under the Misuse of Drugs Act 1971':
"4.3.6 Cannabis differs from alcohol, however, in one major respect: it seems not to increase risk-taking behaviour. This may explain why it appears to play a smaller role than alcohol in road traffic accidents."
www.doh.gov.uk/drugs/acmd/cannabisreportmar02.pdf

UK Department of Transport's 'Influence of Cannabis on Driving':
"...under the influence of cannabis, users are acutely aware of their impairment."
"It is also interesting to note that, despite participants having smoked some form of cannabis before 42 of these examinations, on only 11 occasions did the FME consider the participant to be impaired. This finding could have implications for the number of cases that will be detected by the Field Impairment Testing recently launched in the UK by the police." [FME = Forensic medical examiner.]
www.roads.detr.gov.uk/roadsafety/research16/index.htm

UK Department of Transport's 'Cannabis & Driving' review:
www.roads.detr.gov.uk/roadsafety/cannabis/index.htm

New Scientist:
"The first thing the researchers noticed was that the subjects drove more slowly under the influence of dope, compensating for their intoxication by driving more cautiously. Tracking ability was the only test criterion that was adversely affected: the volunteers found it very difficult to follow a figure-of-eight loop of road when given a high dose. Reaction times to motorway hazards and performance on cognitive tests in the lab were not significantly affected. Trials previously completed under similar test conditions at the TRL have shown that alcohol and tiredness have a more adverse effect on driving ability, affecting higher cognitive processes. The results of the cannabis and driving study agree with similar research carried out in Australia, the US and Holland."
www.newscientist.com/news/news.jsp?id=ns99992063

New Scientist referring to Transport Research Laboratory (UK) research:
"A SINGLE glass of wine will impair your driving more than smoking a joint." "…drivers on cannabis tended to be aware of their intoxicated state, and drove more cautiously to compensate. Indeed, doped-up volunteers often rated themselves as being more impaired than police surgeons brought in to evaluate their sobriety. Surprisingly, drinking alcohol didn't offset this cautious behaviour, opening up the unproven possibility that a driver who is moderately drunk might be better off under some conditions if they had also smoked."
www.newscientist.com/news/news.jsp?id=ns99992063

Tobacco:

New Scientist:
SMOKERS would be wise to wear nicotine patches during exams, according to a study of their ability to concentrate under pressure.
Elliot Stein at the Medical College of Wisconsin, Milwaukee, and his colleagues scanned the brains of smokers with and without nicotine patches while they performed complex cognitive tasks. Those deprived of the drug for just two hours showed suppressed activity in brain regions associated with visual attention (Neuron, vol 36, p 539).
"Even if they appear to be functioning, these people are on the edge," says Stein. Smokers deprived of a fix also found the tasks more emotionally draining.
New Scientist, 'Concentration fix', vol 176, issue 2367 - 02 November 2002, page 25

Links:

Department of Transport's 'Cannabis and Driving: A Review of the Literature':
www.roads.dft.gov.uk/roadsafety/cannabis

Department of Transport's Drink Driving Campaign: www.think.dft.gov.uk/drinkdrive/index.htm