The authors of this paper, Bernard Laumon and colleagues (BMJ 2005;331:1371-4), have alerted us to some wrong values in two of the tables in the full version (see bmj.com). In table 1 the correct values for Friday and Saturday night respectively are: all drivers, 532 (5.4%) and 649 (6.6%); positive for cannabis, 67 (9.8%) and 92 (13.5%); and positive for alcohol, 259 (12.4%) and 355 (16.9%). In table 4 the correct values for Friday and Saturday night respectively are: cases, 410 (6.1%) and 523 (7.7%); controls, 122 (4.1%) and 126 (4.2%). The authors confirm that these changes do not affect the conclusions of the paper.