A key aspect of the modern power grid is demand-side management, a tool that can help grid operators to balance supply and demand in real-time more efficiently and cleanly and reduce energy costs. The California Independent System Operator (CAISO) has implemented a statewide, voluntary demand response tool, called the Flex Alert program, that calls for reduced energy usage during peak hours. The alerts are typically issued during hot summer afternoons when air conditioning usage is high and surges in electricity demand threaten to exceed available energy resources. Significant drops in demand during typical ramping hours have been observed on Flex Alert days, suggesting the alerts were overall successful. However, most studies have analyzed statewide demand, ignoring how participation in the demand response event may have varied across sectors, regions, or individual customers. Understanding how different electricity customers respond to Flex Alerts is essential to achieving optimal energy reductions and load shifting, but studies have been limited by lack of statistically representative electricity data with high spatiotemporal resolution. In this study, we use five years of hourly smart meter electricity data from ~200,000 homes in Southern California, provided by Southern California Edison, to analyze the energy demand of residential customers on Flex Alert days. The following research questions are addressed: 1) Are Flex Alerts effective at reducing ramping and demand during peak hours in the residential sector?, 2) Have certain Flex Alerts been more successful in shifting residential energy demand and are there factors that contributed to the success?, and 3) Which types of customers are more likely to change their energy behavior in response to Flex Alerts? The results of this study will provide insight into the potential of voluntary demand response programs and how they can be tailored to better engage and motivate different groups of residential customers.