benzodiazepines are prescribed to relieve anxiety and aid sleep; opioid analgesics are prescribed to relieve severe or chronic pain. both medications act as central nervous system depressants and can impair ones ability to drive safely. currently, most epidemiological research has focused on the association between these medications and traffic crashes. yet, the role of opioid analgesics and benzodiazepines on crash responsibility is still not properly understood. therefore, we examined the impact of short, intermediate, and long half-life benzodiazepines and opioid analgesics on crash responsibility by drug half-life and driver age, using a case-control design with drivers aged 20 and over involved in fatal crashes in the u.s.a. from 1993-2006. drivers (all with bac=0) were classified as having no benzodiazepines detected versus short, intermediate, or long half-life benzodiazepines or no opioid analgesics detected versus present. cases were drivers with at least one potentially unsafe driving action (uda) in relation to the crash (e.g., speeding), a proxy measure for crash responsibility; controls had no udas recorded. odds ratios (ors) of any uda by benzodiazepines half-life and opioid analgesic exposure were calculated, with adjustment for age, sex, other medication usage, and prior driving record. compared with drivers not using benzodiazepines, drivers taking intermediate or long half-life benzodiazepines demonstrated increased odds of an uda from ages 25 (intermediate or: 1.59; 95% 01=1.08, 2.33; long or: 1.68; 95% 01=1.34, 2.12) to 55 (intermediate or: 1.50; 95% 01=1.09, 2.06; long or: 1.33; 95% cl=1.12, 1.57). drivers taking short half-life benzodiazepines did not demonstrate increased odds of an uda compared to drivers not using benzodiazepines. compared with drivers not using opioid analgesics, odds of an uda were increased from 25 (or: 1.35; 95%: 1.05, 1.74) to 55 years of age (or: 1.30; 95% cl: 1.07; 1.58) for a male driver, and from 25 (or: 1.66; 95% cl: 1.32; 2.09) to 65 years of age (or: 1.39; 95% cl: 1.17;1.67) for a female driver. given the potential impact of these medications on driver safety, further experimental research is needed to better understand their effects on crash responsibility.