The last two years have seen a focused effort to raise the overall security posture of private and federal facilities alike. A key factor in securing sites against high-level threats is controlling vehicular access. Cars and trucks can be and have been used to transport large amounts of illicit material such as explosives to within a radius of damage of the protected asset. Reducing the risk of vehicle-borne threats requires screening both the vehicles and their drivers. This invariably slows the progress of traffic into a site, resulting in delays and traffic backup and the safety issues caused by those factors.
Traffic engineers have long dealt with issues of efficient and safe vehicular flow. New security-related vehicle screening procedures are disrupting engineered traffic flow at federal installations, so the basic engineering data must be revisited.
The initial engineering involved in traffic engineering is the traffic-flow study. The basic goal of this effort is to develop a time-dependent model of vehicular movement at one or more locations. This data allows traffic and security engineers to deal with the following issues.
Traffic backup. One of the key issues is how much traffic will be slowed and stopped as a result of security screening procedures. The delays are an inconvenience for the drivers, but more important, the stopped or slowed cars present a safety hazard. Knowing the time-dependent nature of the flow allows calculations of the queueing time.
Checkpoint redesign. If the impact on the traffic flow is unacceptable for any reason, checkpoint redesign may be necessary. Accurate traffic flow data allows analytical testing of the various design options. The proposed checkpoint location may present a sufficiently high number of constraints that it may need to be abandoned in favor of a new site.
Signalization changes. A reduction in the traffic flow at the security checkpoint may require changes in the signalization scheme at surrounding intersections.
Staffing. Once a checkpoint model is formulated that satisfies the traffic flow and safety objectives, a security staffing model can be applied. This results in a determination on a time-dependent basis of the number of personnel required to administer the security screening process and use the necessary equipment.
A traffic-flow study should answer a number of questions concerning the specific traffic features of the area. For example:
? Is the normal workweek defined as Monday through Friday, or is there some other type of schedule that will affect the traffic-flow characteristics?
? What are the predominant shift hours, and how many shifts represent a substantial number of employees?
? Are there nearby schools that will affect traffic counts on particular days or at particular times?
? What is the vacation and holiday schedule for large employers and area schools?
There are a number of counting tools available encompassing a wide range of technology.
Hard tally. In some circumstances in which the traffic volume is relatively low and stable, hard counters can be used to obtain reasonably accurate results with minimal capital expenditure. Hard tallies are useful in verifying the accuracy of automated counting methods. The staff associated with hard tallies can also be relied upon to provide a good characterization of vehicle types such as cars, two-axle trucks or semis that comprise the traffic flow.
Pneumatic counters. Pneumatic counters, the most common counting tools, use tubes stretched across a section of roadway. The tables are recorded by associated electronics that can be uploaded to a PC for analysis.
CCTV systems. Properly placed closed-circuit television cameras and specialized software are routinely used for traffic management. A subset of this function is traffic counts. This type of approach is especially applicable for establishment of long-term traffic-flow patterns and high-volume corridors.