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Road lane reduction with the addition of roadway countermeasures (also known as a 'road diet') is a transportation planning treatment to achieve systemic traffic improvements. Although lane reductions are expected to increase traffic safety overall, the impacts may vary by different roadway characteristics among sites and by crash types. The main objective of the study reported in this paper was to identify the effectiveness of lane reductions considering various factors including roadway characteristics through estimating crash modification factors (CMFs) for different crash types. The study first evaluated the CMFs for lane-reduction cases to check the overall safety effects using the observational before and after comparison group method. Second, the variation of CMFs due to different roadway characteristic factors was identified. Ultimately, the CMFs were developed for different crash types and severities. The results of the estimated CMFs indicated that lane reductions decreased crash frequency in general. From the CMFs with different traffic volume ranges, it was found that lane reductions have higher safety impacts to reduce total and injury crashes when there are fewer vehicles on roadways. [ABSTRACT FROM AUTHOR]