La evaluación diagnóstica: particularidad, diversidad y la complejidad de las

9.5.1 Ramp Meter Operating Philosophies

Ramp metering can be operated in a number of ways depending on the type and level of operation.

9.5.1.1 Local versus System-Based Control

In local metering, the operation at a ramp is independent of adjacent meters. An expressway may have many consecutive ramps operating in local modes. In system-based metering, a group of ramps are operated in a coordinated manner.

System-based control may be central or non-central.

9.5.1.2 Pre-Timed versus Traffic Responsive Control

In pre-timed control, ramp metering operation begins and ends at preprogrammed times. In traffic responsive control, meter operation times and metering rates are based on detected traffic conditions. For instance, a meter may be programmed to commence operation if expressway detector occupancy exceeds a certain threshold value or expressway speeds are lower than a corresponding threshold. Traffic responsive operation may also adjust metering rates by either using preprogrammed values or by using a feedback control loop to calculate metering rates in real-time.

9.5.1.3 Restrictive versus Non-Restrictive Control

In restrictive mode, metering rates are set based on expressway conditions. In non-restrictive mode, metering rates are adjusted based on queuing conditions on the ramp. An extreme form of non-restrictive mode shuts-off metering operation when the ramp queue reaches the maximum allowed length. Metering operation resumes when the queue clears.

9.5.1.4 Central versus Non-central Control

In central control, a computer located in the TMC controls ramp meters. Central control is typically used with system-based ramp metering at a group of ramps along a section of the expressway. Non-central control implements ramp metering using only in-field devices, and can be local or system-based.

9.5.2 Types of Ramp Metering

When the merge area of the expressway is not a bottleneck, an uncontrolled single-lane expressway entrance ramp may have a throughput capacity of 1800 to 2200 vph. The same ramp will have lower capacity when metered. The maximum theoretical metering capacity depends on the type of strategy used. There are three ramp-metering strategies. These strategies are described below.

9.5.2.1 Single-Lane One Car per Green

This strategy allows one car to enter the expressway during each signal cycle. To maintain consistency with normal traffic signals, ramp meters use three aspect signal heads with green, yellow, and red signal indications. The length of green plus yellow indications is set to ensure sufficient time for one vehicle to cross the stop line. The length of red interval should be sufficient to ensure that the following vehicle completely stops before proceeding. From a practical point of view, the smallest possible cycle is 4 seconds with 1 second green, 1 second yellow, and 2 seconds red. This produces a meter capacity of 900 vph. However, field observations have shown that a 4-second cycle may be too short to achieve the requirement that each vehicle must stop before proceeding. Also, any hesitation on the part of a passenger-car driver may cause the consumption of two cycles per vehicle. A more reasonable cycle is around 4.5 seconds, obtained by increasing the red time to 2.5 seconds. This increase in red would result in a lower meter capacity of 800 vph.

9.5.2.2 Single-Lane Multiple Cars per Green

Platoon metering, also known as bulk metering, allows for two or more vehicles to enter the expressway during each green indication. The most common form of this strategy is to allow two cars per green. Three or more cars can be allowed;

however, this will sacrifice the third objective (breaking up large platoons).

Furthermore, contrary to what one might think, bulk metering does not produce a drastic increase in capacity over a single-lane one car per green operation. This is because this strategy requires longer green and yellow times as ramp speed increases, resulting in a longer cycle length. Consequently, there are fewer cycles in one hour. For instance, the two-cars-per-green strategy requires cycle lengths between 6 and 6.5 seconds and results in a metering capacity of 1100 to 1200 vph.

This analysis illustrates that bulk metering does not double capacity and this finding should be noted.

9.5.2.3 Dual-Lane Metering

Dual-lane metering requires two lanes be provided on the ramp in the vicinity of the meter which merges to one lane. In this strategy, the controller displays an independent green-yellow-red cycle for each lane. Signal cycles in the two lanes can be synchronized such that the green indications never occur simultaneously in both lanes. Dual-lane metering can provide a metering capacity of 1600 to 1700 vph. In addition, dual-lane ramps provide more storage space for queued vehicles.

9.5.3 Geometric Design Considerations

Installation of a ramp meter to achieve the desired objectives requires sufficient room at the entrance ramp. The determination of minimum ramp length to provide safe, efficient, and desirable operation requires careful consideration of the elements described below:

 Sufficient room must be provided for a stopped vehicle at the meter to accelerate and attain safe merge speeds.

 Sufficient space must be provided to store the resulting cyclic queue of vehicles without blocking an upstream signalized intersection.

 Sufficient room must be provided for vehicles discharged from any upstream signal to safely stop behind the queue of vehicles being metered.

Figure 9-8 illustrates the distance requirements at the ramp meter location. In this figure, the dotted line shows the ramp length. Under strict metering, the primary queue detector controls the maximum queue length in real-time. Thus, the distance between the meter and the queue detector defines the storage space. For dual-lane ramps, the ramp storage area as shown in Figure 9-8 should also consider the transition from one lane to two lanes and dual-lane storage space. The transition zone should be at least 23m long, and the length of dual-lane storage should be sufficient to store a minimum of four cars per lane (approximately 31m). In addition, the signal poles should be placed where they cannot be reached by vehicle occupants.

The gore-to-gore length of a ramp depends on two geometric factors: outer separation and ramp angle. Outer separation is the distance from the outside edge of the right-most expressway lane to the inside edge of the frontage road. The top part of Figure 9-9 provides a cross section view of the expressway, a single-lane ramp, and the frontage road. In this figure, thick lines represent travel lanes and thin lines represent shoulders. As shown, the offset/setback to the signal pole should be a minimum of 1.1m from the shoulder or, in case of a curb, from the edge of the travel lane. The bottom part of Figure 9-9 illustrates the desired and minimum dimensions for ramps. These clearance requirements restrict the farthest downstream location of a signal on the ramp.

The placement of signal poles controls the distribution of available ramp length to storage and acceleration distances. If needed, additional acceleration distance may be provided by constructing an auxiliary lane parallel to expressway lanes. Similarly, additional storage may be provided on the frontage road upstream of ramp entrance.

Figure 9-8: Design Issues Related to Ramp Meters Single-Lane Meter

Storage Space

Safe Stopping Distance

Ramp Length

Acceleration Distance

Transition

2-Lane Storage

Dual-Lane Meter Primary Queue Detector

9.5.3.1 Minimum Stopping Distance to the Back of Queue

When a ramp meter is in operation, motorists arriving at the ramp will likely encounter the rear end of a queue. Adequate maneuvering and stopping distances should be provided for vehicles from a location where they are expected to first see the back of the queue. The following equation AASHTO (or equivalent BSEN) stopping sight distance equation can be used to calculate this distance:



X = Stopping sight distance, meters;

v = Speed of traffic approaching the on-ramp, km/h;

T = Perception-reaction time (2.5 sec), seconds;

a = Deceleration rate, m/s²; and

Figure 9-9: Clearances for Placement of Ramp Signal Posts