Techniques for making a physical area safe for a human to enter when mobile drive units (e.g., robots) are operating in the physical area are described. A management device may send an activity message to the mobile drive units instructing the mobile drive units to perform a set of activities. If a human is detected in the physical area, the management device may stop sending the activity command. If the mobile drive units do not receive the activity command, the mobile drive units may stop performing the set of activities. After the management device determines that the human has left the physical area, the management device may resume sending the activity message.