Wallis and Ford describe CPTED as "an attempt to reduce crime and fear in a target setting by reducing criminal opportunity and fostering positive social interaction among the legitimate users of that setting .... A principal means by which CPTED attempts to achieve this goal is by modifying the physical features of the target setting." The target setting is not considered only the physical environment, but also the social and managerial environment, including the businesses, organizations, and individuals who use the facility. Public sector applications include schools. Some CPTED measures used in schools are reduction in the number of exterior doors to limit access points; designing the facility in a quadrangle that includes a courtyard, so as to enhance surveillance; and the use of steep-sloped roofs to limit rooftop access. There are many potential secondary impacts that can result from the use of CPTED. The author notes secondary impacts in residential, school, and transportation applications. A discussion of the costs of CPTED advises that for a new project the cost of CPTED measures is insignificant compared to the overall cost of the project. Overall, the research suggests that it is the aggregate of the physical environment in combination with the social environment that provides positive results in the area of crime prevention. Appended examples of CPTED strategies and an outline of secondary CPTED impacts