When law enforcement responds to an alcohol-related crash, officers often have multiple charging options available. Beyond impaired driving offenses, drivers may also receive moving violations or administrative citations. A new study from the University of Iowa examined whether those additional charges influence conviction outcomes and found that they do.
Researchers analyzed more than 8,200 alcohol-impaired drivers involved in Iowa motor vehicle crashes between 2016 and 2019, linking crash records with court outcomes to better understand how charge combinations affect convictions.
High Conviction Rates Once Charges Are Filed
The study found that 83% of alcohol-impaired drivers involved in crashes received at least one traffic-related charge, while nearly 76% received an alcohol-related charge. Among those who were charged, conviction rates were remarkably high: more than 96% were convicted of some traffic offense and nearly 88% received an alcohol-related conviction.
These findings suggest that when impaired driving cases make it into the court system, convictions are common. However, the researchers identified a notable concern: nearly 17% of alcohol-influenced drivers in the study did not receive any traffic charges at all.
The Impact of Multiple Charges
One of the study’s key findings was that drivers charged with a combination of alcohol offenses, moving violations, and administrative violations were significantly more likely to be convicted than drivers charged only with alcohol-related offenses. Drivers facing charges across all three categories had 2.6 times greater odds of receiving a conviction on any charge. Those charged with alcohol offenses and moving violations also had increased odds of conviction.
Researchers also found that conviction rates generally rose as the number of charges increased. Drivers with only one charge were convicted about 93% of the time, while drivers with three or four charges had conviction rates exceeding 97%.
The findings provide evidence that “charge stacking,” or filing multiple applicable charges stemming from a single incident, may strengthen overall conviction outcomes.
Alcohol Convictions Tell a Different Story
While additional charges increased the likelihood of receiving some type of conviction, they did not significantly improve the likelihood of an alcohol-specific conviction. Once an alcohol-related charge was filed, conviction rates for those charges remained consistently high regardless of what other violations were included.
The study also found that drivers charged with Operating While Intoxicated (OWI) offenses had substantially greater odds of receiving an alcohol-related conviction than drivers charged with other alcohol-related offenses.
Implications for Enforcement
The authors conclude that comprehensive charging practices may help strengthen impaired driving case outcomes and potentially increase deterrence. Their findings suggest that law enforcement officers and prosecutors should ensure that all applicable alcohol, moving, and administrative violations are documented and charged when appropriate.
At the same time, the study highlights the need to better understand why some alcohol-impaired drivers involved in crashes receive no charges at all and whether barriers in reporting, investigation, or prosecution contribute to those outcomes.
Looking Ahead
Effective impaired driving enforcement depends not only on detecting violations but also on achieving meaningful legal consequences. This research suggests that thorough charging practices may improve conviction rates while reinforcing the deterrent effect that impaired driving laws are designed to create. As traffic safety professionals continue working to reduce alcohol-related crashes, understanding how enforcement decisions influence case outcomes remains an important piece of the impaired driving prevention puzzle.
Source: Hamann, C. J., Jansson, S., Wendt, L., Reyes, M., Davis, J., Cavanaugh, J. E., & Peek-Asa, C. (2025). Charge combinations and conviction rates among alcohol-influenced drivers involved in motor vehicle crashes in Iowa. Journal of Safety Research, 92, 375-384. https://doi.org/10.1016/j.jsr.2024.12.008
