Entry and exit counts usually require a short-term count (24 to 48 hours) taken near the entry point, supplemented by visual observation of traffic flow at the existing checkpoint. Most commonly used short-term traffic count equipment is more accurate when vehicle speeds are at least 10 miles per hour. Thus, the best location for short-term counting equipment is in the interior of the facility or installation. However, the equipment must be located prior to cross-streets or major driveways to accurately reflect the number of entering and exiting vehicles. Visual observations include the number of occupants per vehicle, the time required for vehicle and occupant identification checks, and the classification of the vehicle.
With the field information collected, a report can be generated presenting the information gathered. Multiple-day counts are generally averaged to reflect average conditions. For traffic-flow purposes, the peak-15-minute and peak-hour traffic volumes generally present the most important data. For entry/exit points, the rise and fall in traffic volumes will affect personnel and equipment needs.
Figure 1 shows a typical traffic-flow chart for an average day. In many cases, there is so little traffic in the early morning hours that they can be ignored, as was done in the example chart. As would be expected, there is a clear directional basis to the traffic flow: predominantly inbound during the morning rush hour and outbound at close of business. The typical difference in time-dependent flow rate is also clear. The inbound rate is slower but spread over a longer period of time, while the afternoon peak is more compressed. Since vehicular screening is normally only applied to inbound traffic, the morning peak (0600 to 0900) is the design basis period in this example.
Traffic-flow data such as this allows definitive analysis of existing conditions and the impact of proposed security upgrades. This allows more accurate design of necessary infrastructure changes, budget preparation for security personnel and equipment, and scheduling of screening personnel.
Randall Nason, PE, is a corporate vice president and manager or the Security Consulting Group at C.H. Guernsey and Co. His experience spans a broad spectrum of the security profession including threat assessment, vulnerability analysis and master plan development through complete system design and construction management.