Traffic Studies
The past few years have seen a focused effort to raise the overall security posture of private and federal facilities alike. A key factor in security sites against high-level threats is controlling vehicular access. By their very size and construction, cars and trucks can 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 not only in delays but, more importantly, traffic backup and the resulting safety issues.
Traffic engineers have long dealt with issues of efficient and safe vehicular flow. As the new security-related vehicle screening procedures disrupt engineered traffic flow at federal installations, the basic engineering data must be revisited.
The initial engineering 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. Knowing this data allows both 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; more importantly, the stopped or slowed cars present a safety hazard. Knowing the time dependent nature of the flow allows calculations of the cueing 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 option alternatives. 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 staff the security screening process and the necessary equipment.
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The conduct of the traffic flow study needs to consider 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 their 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?
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There are a number of counting tools available encompassing a wide range of technology.
- Hard Tally: In some circumstances where 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 the varying vehicle types such as cars, two-axle trucks, or semis that comprise the traffic flow.
- Pneumatic Counters: The most common, the counters 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 (CCTV) 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.
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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 much 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 then 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 are generally 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 this 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) in this example, is the design basis period.
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.
Randy Nason, PE, VP
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