* excessive [[Short-term effects of alcohol|alcohol]], with simple [[Blood alcohol content|blood-alcohol]] limits enforced through drink-driving laws (although some level of impairment may occur below the legal limit). See also: [[alcohol-related traffic crashes in the United States]];
* drug use - including some [[prescription drug]]s, [[over the counter drug]]s (notably [[antihistamine]]s, [[opioid]]s and [[muscarinic antagonist]]s), and [[illegal drug]]s.
Several conditions can work together to create a much worse situation, for example:
* combining low doses of alcohol and [[cannabis]] has a more severe effect on driving performance than either cannabis or alcohol in isolation,<ref name = trl1>{{cite web | url = http://www.dft.gov.uk/pgr/roadsafety/research/newsletters/roadsafetypart1alcoholdrugsa4708#TopOfPage
| title = Road Safety Part 1: Alcohol, drugs, ageing & fatigue | date = Spring 2003 | edition = Research summary, TRL Report 543 | publisher = UK [[Department for Transport]] | accessdate = 2008-01-01 }} </ref> or
* taking recommended doses of several drugs together, which individually will not cause impairment, may combine to bring on drowsiness or other impairment. This could be more pronounced in an elderly person whose renal function is less efficient than a younger person's.<ref name = trl2>{{cite web | url = http://www.dft.gov.uk/pgr/roadsafety/research/newsletters/roadsafetypart1alcoholdrugsa4708#TopOfPage
| title = Road Safety Part 1: Alcohol, drugs, ageing & fatigue | date = Spring 2003 | edition = Research summary, [[Transport Research Laboratory]] Road Safety Report No. 24 | publisher = UK [[Department for Transport]] | accessdate = 2008-01-01 }} </ref>
