Transverse rumble strips are implemented on roadways across the country for various purposes, including reducing vehicle speeds and increasing driver alertness. Understanding the characteristics of rumble strips—particularly the extent of their speed-reduction influence zone—is essential for evaluating and selecting this measure; however, this aspect has received limited attention in the domestic scientific literature. The present study aims to determine the effective influence zone of rumble strips on speed reduction by collecting field speed data and conducting statistical analyses on the Qaleh-Shur roadway in Isfahan Province. Data analysis indicates that driver behavior upstream of the rumble strip is natural and unaffected by any intervention; however, a sudden and statistically significant reduction in speed occurs starting 50 meters before the rumble strip. At the location of the rumble strip, the mean traffic speed reaches its minimum value, representing a 13% reduction. Speed recovery begins after vehicles pass the rumble strip, and at a distance of 25 meters downstream, traffic speed increases significantly and returns to its pre-influence level. Statistical tests reveal no significant speed variations within the three zones (upstream, influence zone, and downstream), except for a single significant decrease before and a significant increase after the rumble strip influence zone. Based on commonly used speed–crash models, the observed speed reduction within the rumble strip influence zone can lead to an approximate 24% to 43% reduction in crash rates. The findings of this study can be utilized in evaluating rumble strip performance and in related policy-making processes.