This study performed before-and-after analyses (comparisons of collisions before and after the construction of auxiliary lanes) on collision rate at nine study sites in California in order to achieve two objectives: (i) to estimate the freeway Crash/Collision Reduction Factor (CRF) for auxiliary lanes, and (ii) to develop design guidelines for the construction of auxiliary lanes. Findings indicate that on average, collision rates decreased by 17.3 percent at nine study sites. The study also found that after construction of auxiliary lanes at two study sites, collision rates increased when there were high ramp flows, or when a lane-drop bottleneck was formed near the downstream off-ramp location. As a result of the increase in collision rates at these sites, the construction of auxiliary lanes may not be an appropriate measure if the candidate site has high ramp flows, or if the construction is likely to form a lane-drop bottleneck. When these guidelines are followed, auxiliary lanes reduce collisions 34.0 percent within study sites.