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curb and building when a screened entrance is in use, and the engineered barriers at the sidewalk delineate the transition to the first defense layer. Figure 6-8: A well-designed zero-setback protection. The engineered bollards define the transition between the first and second layers of defense and the street trees soften the intrusion of bollards.Figure 6-7: Unsatisfactory example of temporary protection for a high-risk zero-setback building. If the Jersey barriers are not embedded, they can be pushed aside by a vehicle.
SECURITY FOR CENTRAL BUSINESS DISTRICTS6-96.2.2 ALLEYSThe most extreme forms of the zero-setback building are found in alleys: a typical alley roadway has a width of about 20 feet, with a sidewalk per-haps as little as 6 feet wide. Sometimes there is a sidewalk on only one side of the alley (Figure 6-9).Figure 6-9: Alleys. Note single sidewalk (right).The protective measures described above for zero-setback buildings apply to buildings serviced by alleys.In alleys and typical urban streets, adequate stand-off distance is often an impossibility without street closure, but permanent closure is often not feasible because of service entry needs. In this instance, street closure that also allows service access can be achieved by use of active barriers, such as retractable bollards or other devices, together with security personnel and well-planned screening and inspection facilities.Well-planned and well-designed street closures can enhance the quality of a street, even in a high-risk area. It is critical that a permanent street closure be planned, not only as part of an organized traffic study that re-spects existing traffic patterns, but also tries to find an opportunity to improve them and enhance the neighborhood. Control of vehicular speed is also important for security. This is discussed in Section 5.4 but some of the methods noted in that section (such as traffic circles) may not apply in the urban environment because of lack of space.
SECURITY FOR CENTRAL BUSINESS DISTRICTS6-10Security measures can be both effective and attractive if design attention is focused on the required performance, and imagination is used in ma-terials and forms. Good design requires site-specific, context sensitive solutions. The function of the public realm and the site’s context must be carefully considered when designing and placing hardened streetscape el-ements, and placement of these elements must be carefully evaluated to avoid visually and physically cluttering the streetscape. Solutions should not be universally applied. In some cases, in important historic areas of cities or in relation to important historic buildings, security elements in public space should be discouraged altogether.Case Study 6 provides an example of a well-researched neighborhood protection plan that uses street closure to provide stand-off and also en-hances the urban values, vitality, and function of the protected area.