This report covers a study of SENTRE Impact Attenuation Devices, done to identify potential design or installation improvements, to make these improvements, and evaluate performance of the devices once the indicated modifications were made. During the study, a total of 21 SENTRE devices were installed on I-40 in Oklahoma City. Ten accidents involving SENTREs were documented. Analysis of these accidents led to modifications in amount of torque applied to anchor bolts , type of clips holding posts to base plates, and depth of topsoil covering the redirecting cable. The limited number of SENTRE installations, and the even more limited number of accidents precluded a statistical comparison of accidents involving SENTREs to those with exposed guardrail ends or other types of impact attenuation. In all of the accidents occurring after the modifications were made, the SENTRE units performed as the designers intended, suggesting that they were effective in reducing impact severity.