Traffic Signals

Traffic signal systems are among the many tools for managing traffic congestion in urban areas. Well designed and operated signal systems provide motorists with recognizable improvements in travel time; however, improperly designed or poorly operated systems can frustrate drivers. Systems appropriately designed, properly operated and maintained provide: safety benefits, reduced travel times and vehicle operating cost, and lower fuel consumption and vehicle emissions. These systems also form one of the essential components of future Intelligent Transportation Systems (ITS) aimed at addressing growing urban traffic congestion.

Rudimentary traffic signal systems appeared on Long Island in the 1950s and have become ever more sophisticated. Most existing traffic signal systems are street systems with signal equipment along the roadway. Many of these systems use roadway installed sensors to change signal timing plans, adjusting to heavy traffic flows.
Computerized traffic signal systems continue to evolve, relying on increased vehicle detection to continually adjust timing plans. These systems have been termed real-time signal systems. Central computer systems manage the automatic adjustment of timing plans from one traffic operations center.

These systems have not been extensively deployed on Long Island, except for a centrally controlled system in Nassau County and one controlled by INFORM. Another type of signal system, the Closed Loop Signal System, monitors the typical street system and communicates with a central office informing operators of system failures. Engineers then examine system operation and make necessary adjustments.

The ability to monitor for failures and adjust operations from a central point allows for the correction of problems within a short timeframe. A traffic signal malfunction in an unmonitored system may cause thousands of hours of motorist delay before being reported.

The State plans to upgrade the traffic signal systems to real-time signal systems on heavily traveled roadways. On less traveled arterials, Closed Loop Signal Systems can be installed. Closed Loop Signal Systems only require standard type phone connections to the traffic signal controller on the street. The Closed Loop Signal System can be installed as an intermediate step on heavily traveled arterials until funding permits more sophisticated systems to be installed.

Much of the existing traffic signal control equipment can be easily expanded to operate in conjunction with these new systems; therefore upgrades are not necessary. While communication cable upgrade for the more sophisticated real-time signal systems will be necessary, the underground conduit systems are mostly already in place to support the existing street systems. Thus, Long Islanders can look forward to driving without the frustrating, frequent, unnecessary stops which often occur.

Detector Types

In past years, the inductive loop detector (ILD) was used to sense traffic in almost all United States traffic monitoring and control systems. In New York State, INFORM and the Van Wyck Expressway systems were designed with ILDs. In recent years, a number of manufacturers and suppliers have introduced traffic detectors based on new detection principles.

New detection principles include: video image processing system (VIPS), passive sonic, optical ranging, doppler microwave radar and microwave ranging radar.

The new detectors do not necessarily provide all the same information provided from an ILD, and differ in accuracies and performance characteristics with ILD and with each other.

On the Southern State Parkway the LI ITS will use a VIPS detector capable of providing basic traffic data. There is virtually no traffic disruption during installation and maintenance as this detector is not located in the roadway pavement.

The LI ITS is being designed using OPEN ARCHITECTURE principles, that permit acquisition of equipment from different sources, but also allow different technologies to perform the same function. This open architecture concept supports different technologies where deployment issues may limit use of particular detector types (e.g., where survivability precludes roadway invasive ILDs). It also permits incorporation into projects of emerging detector technologies as they become available and proven.

Open architecture emphasizes standard interfaces, e.g., T-1 and Synchronous Optical Network (SONET) and software protocols such as the National Transportation Communications for ITS Protocol (NTCIP). It also requires the establishment of data such as information formatting specifications to which the equipment is designed and supplied.

Each detector type does not need to perform all functions and provide all variables. However, a detector at one location, possibly in conjunction with the controller data processor should provide all required variables for that location; otherwise, it will require an additional detector type.

The key LI ITS detector functions include:

• Ramp metering
• Incident detection
• Data collection for development and updating of traffic management plans

The parameters or variables that are required include:

• Volume by lane
• Occupancy by lane
• Speed by lane
• Vehicle classification by lane
• Incident presence based in a single station
• Density in region of detection by lane
• Vehicle presence by lane
• Volume for roadway
• Occupancy for roadway

Ramp Metering

Ramp metering has demonstrated effectiveness in increasing mainline speed by approximately 20% and reducing travel time, congestion and accidents.

Long Island's major east-west roadway corridors exhibit congested traffic conditions in both directions during peak travel periods. Ramp metering aims to reduce this congestion by staggering (metering) the volume of traffic that can enter the highway mainline from on-ramps when the mainline is heavily congested. To be eligible for metering, peak period ramp volumes must satisfy the following criteria:

Minimum Volume Criteria

• 240 vehicles per hour for one metered lane
  
• 400 vehicles per hour for two metered lanes

Maximum Volume Criteria

• 900 vehicles per hour for one metered lane
  
• 1,500-1,800 vehicles per hour for two metered lanes for merge into single lane.

Fully Restrictive ramp metering is used when a ramp has ample storage space to accommodate queues for a metering rate established to control congestion systemwide. Metering continues until the queue is cleared. It is important to note that the queue should never exceed the ramp storage capacity. This type of metering proves most effective as it reduces the volume entering the congested roadway to the level required to prevent or forestall stop and go operations. Because of the queue length, a number of vehicles will divert and use alternatives, thus reducing demand on the facility. The analysis for the Southern State Parkway ramps estimated the storage requirements as 5% of the peak hour ramp volume (before metering).

Partially Restrictive ramp metering is used when a ramp has considerable ramp storage though insufficient for fully restrictive metering during the entire peak period.
This can be used at a metering rate identical to the desired fully restrictive rate. However, because of insufficient queue storage, it may become necessary to initiate a rate which will not lengthen the queue. A non-restrictive rate is then implemented. There is another alternative partially restrictive strategy. It establishes an initial metering rate somewhat higher than the rate desired for systemwide control. The queue builds at a sufficiently low rate so that available storage space is not exceeded. Partially restrictive metering is planned when available storage proves insufficient for fully restrictive metering.

Non-Restrictive ramp metering follows the ramp demand rate volume profile. Since ramp demand volume is not significantly restricted, benefits of non-restrictive metering result from breaking up groups of vehicles prior to merging into the mainline.

Metering will likely be used during both the AM and PM peak periods. Since ramp demand volumes differ for these periods, it is possible that a particular ramp may use partially restrictive metering during the heaviest peak period and fully restrictive metering during the lighter ramp demand period. Also, portable variable message signs may be installed at critical sites to manage ramp and mainline traffic through construction areas.