The law increases the certainty of punishment for DWI offenders by establishing the illegal per se section, i.e., the assumption of guilt when the blood alcohol content exceeds 0.10 percent, and by introducing a provision for mandatory jail sentences of 1 day for all first-time DWI offenders and longer terms for repeat offenders. The intent of the law was deterrence of future DWI occurrences. Impact on jail operations in Washington was anticipated. Study data were gathered from seven counties for the first half of 1980. Findings indicate that for the most part, police procedures involving DWI arrests did not change with the initiation of the new law. Early indications, as measured by the percentage of DWI accidents and the percentage of DWI injury and fatal accidents per the number of reported DWI arrests, are that the new law is not having the desired deterrent effect. The illegal per se section of the law led to a reduction in the percentage of cases where the DWI charge was reduced to a lesser charge. All jails show a large increase in the number of short-term sentenced DWI offenders confined. Increased costs of operating the criminal justice system were absorbed by the agencies with a resulting decrease in the quality and efficiency of other services. Eight tables, nine charts, and four appendixes are included. (Author abstract modified)