Ramp Metering (Part - 1) - Civil Engineering (CE) PDF Download

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Introduction

Ramp metering can be defined as a method by which traffic seeking to gain access to a busy highway is controlled at the access point via traffic signals. This control aims at maximize the capacity of the highway and prevent traffic flow breakdown and the onset of congestion. Ramp metering is the use of traffic signals to control the flow of traffic entering a freeway facility. Ramp metering, when properly applied, is a valuable tool for efficient traffic management on freeways and freeway networks. 

Objectives 

The objectives of ramp metering includes:

1. Controlling the number of vehicles that are allowed to enter the freeway,

2. Reducing freeway demand, and

3. Breaking up of the platoon of vehicles released from an upstream traffic signal.

Figure 25:1 given below is a typical example of ramp metering. The signal placed at the ramp, controls the traffic flow which can enter the freeway through merge lane. Vehicle detectors are also shown at the downstream end of the freeway.

 Benefits Ramp 

metering has many positive benefits in freeway management with in measurable parameters such as reduced delay, reduced travel time, reduced accident risk and increased operating speed. The typical advantages are:

1. Improved System Operation: Ramp metering essentially aims to control the access to a freeway to reduce congestion, freeway delay and ultimately overall delay. Although several ramp metering strategies are available with individual pros and cons, overall, ramp metering helps to break up platoons of vehicles from entering a freeway and causing turbulence, reduces delay due to random access and defers if not eliminates the onset of congestion.

2. Improved Safety: Ramp areas are accident prone areas due to unmanaged merging and diverging. Ramp metering makes merging and diverging operation to a freeway smooth and controlled, reducing the risk of accidents arising out of sudden driver decisions. Random entry of platoons is also prevented which decreases the risk of accidents at merge or diverge areas.

3. Reduced vehicle operating expense and emission: Ramp metering essentially reduces the number of stops and delays for the freeway as well as the ramps. This in turn reduces the fuel consumption and emission for a vehicle.

Metering strategies

Metering strategies can be defined as the approach used to control the traffic the flow on the ramps. Three Ramp metering strategies are available to control the flow on the ramps which can enter the busy freeway. Capacity of an uncontrolled single-lane freeway entrance ramp is 1800 to 2200 vehicles per hour (VPH). Since Ramp metering is a traffic flow controlling approach it decreases the capacity of the ramps. Three ramp-metering strategies are as follows:

Single-lane one car per green 

Single-lane one car per green ramp metering strategy allows only one car to enter the freeway during each signal cycle. The salient features of this strategy are:

1. 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.

2. A typical cycle length is taken as, 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.

3. 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.

 Single-lane 

multiple cars per Green Single-Lane Multiple Cars per Green is also known as Platoon metering, or bulk metering. This approach allows two or more vehicles to enter the freeway during each green indication. The most common form of this strategy is to allow two cars per green. The salient features of this type of ramp metering are:

1. Three or more cars can be allowed; however, this will sacrifice the third objective(breaking up large platoons).

2. 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.

3. Two cars per green strategy requires cycle lengths between 6 and 6.5 seconds and results in metering capacity of 1100 to 1200 VPH. This analysis illustrates that bulk metering does not double capacity and this finding should be noted.

 Dual-lane metering

In dual lane metering two lanes are required to be provided on the ramp in the vicinity of the meter which necks down to one lane at the merge. The salient features of this type of ramp metering are:

1. In this strategy, the controller displays the green-yellow-red cycle for each lane.

2. Synchronized cycles are used such that the green indications never occur simultaneously in both lanes.

Figure 25:2: Comparison of metering quality of different approaches with Ramp demand volume

3. The green indications are timed to allow a constant headway between vehicles from both lanes. Dual-lane metering can provide metering capacity of 1600 to 1700 VPH.

4. In addition, dual-lane ramps provide more storage space for queued vehicles.

Quality of metering 

The quality of ramp metering essentially implies the efficiency of handling the flow and reducing unnecessary delays through metering strategies. For a ramp meter to produce the desired benefits, the engineer should select a metering strategy appropriate for the current or projected ramp demand. The ramp width will depend on this selection. The following fig. 25:2 shows the metering availability (percent of time the signal is metering) of the three metering strategies for a range of ramp demand volumes. In Figure 25:2, if the flow on a single lane ramp which has Single-Lane One Car per Green approach is 1000 vph, then the metering availability is only 80 percent since the metering approach installed has the capacity of 800 vph. Therefore metering availability decreases as the traffic flow increases. If the flow is around 1600 vph then Dual-Lane Metering gives 100 percent metering availability. Thus it is imperative to select the metering strategy based on the flow and accordingly select the required ramp width.

 Design of ramp metering 

There are some considerations to be taken into account before designing and installing a ramp meter. 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 several elements described below:

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

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

3. Sufficient room must be provided for vehicles discharged from the upstream signal to safely stop behind the queue of vehicles being metered. Provision for the distances mentioned is an integral part of ramp design. Figure 25:3 illustrates the requirements for the different types of distances explained above.

Minimum stopping distance to the back of queue 

Sufficient stopping distance is required to be provided prior to entry to the ramp. Motorists leaving an upstream signalized interchange will likely encounter the rear end of a queue as they proceed toward the meter. Adequate maneuvering and stopping distances should be provided for both turning and frontage road traffic. This stopping distance calculated similar to the stopping sight distance which is a combination of the brake distance and lag distance travelled by a vehicle before stopping. The equation to calculate the minimum stopping distance is given below:

where, X is the stopping distance in meters, v is the velocity of the vehicle in m/sec, t is the time in seconds, g is the gravity coefficient in m/sec2 , f is the friction coefficient. This is the minimum distance to be provided from the back of the queue for safe stopping of vehicles approaching the ramp. Figure 25:3 shows Safe stopping distance, storage distance and acceleration distance which are respective three criteria for ramp design.

Storage distance 

The storage distance is required to store the vehicles in queue to a ramp meter. The queue detector controls the maximum queue length in real-time. Thus, the distance between the meter and the queue detector defines the storage space. The following generalized spacing model can be used to determine the single-lane storage distance:

L = aV − bV ² ∀ V ≤ 1600 vph                                                      (25.2)

Figure 25:4: Variation of distance to meter with Ramp demand volume for different strategies of Ramp metering

In this equation, L (in meters) is the required single-lane storage distance on the ramp when the expected peak-hour ramp demand volume is V vph and a, b are constants. This figure shows the requirements for three metering strategies:

1. Single-lane with single vehicle release per cycle.

2. Single-lane with bulk metering (three vehicles per green).

3. Dual-lane metering assuming single-line storage.

In the Figure 25:4 the curve is shown for the variation of storage distance i.e. distance to meter with ramp demand volume for different strategy used for Ramp metering.

 Distance from meter to merge 

The distance from meter to merge is provided so that vehicles can attain a suitable merging speed after being discharged from the ramp meter. AASHTO provides speed-distance profiles

Figure 25:5: Acceleration length v/s merge speed for different strategies of Ramp metering

for various classes of vehicles as they accelerate from a stop to speed for various ramp grades. Figure 25:5, given below provides similar acceleration distances needed to attain various freeway merging speeds based on AASHTO design criteria. Table 25:1 provides the acceleration length for different merge speed and with ramps of different grade. The desired distances to merge increases with increasing freeway merge speed and the same ramp grade.

 Ramp design methodology 

To model the ramp influence area, a length of 450 m just upstream (for off ramp) and downstream (for on ramp) is considered to be affected. The input data required is the geometric data of the freeway and the ramp and the demand flow. The three steps of design are:

Figure 25:6: Schematic view of a typical merging area

1. The flow entering lanes 1 and 2 of the freeway upstream of merge area or diverge area is first determined.

2. The capacity of the freeway, ramp and merge and diverge areas are determined and checked with limiting values to determine the chance of occurrence of congestion.

3. The density in the ramp influence area is then found out and depending on the value f this variable, the level of service is determined.

From design point of view analysis of merge area and diverge area are treated separately but follows the same basic principle already explained.