Ramp Metering. Index. Purpose. Description. Relevance for Large Scale Events. Options. Technologies. Impacts. Integration potential.

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1 Ramp Metering Index Purpose Description Relevance for Large Scale Events Options Technologies Impacts Integration potential Implementation Best Cases and Examples 1 of 7

2 Purpose Ramp metering is a technique that involves the use of traffic signals to control the entry of vehicles to a motorway or ring road. Its aim is to prevent traffic volume exceeding capacity and leading to a breakdown in flow and, as a result, a build-up of queues. The aim of the system is to smooth traffic flows, reduce stop and go behavior and also and reduce trip times. It also helps to increase the effective capacity of the motorway or ring road. Description The most congested part of motorways is often the section immediately following a junction where traffic joins it. When large numbers of vehicles try to enter a motorway which is already operating at near capacity, this causes the traffic to slow down. A "shock wave" can occur, as traffic further upstream slows, causing bunching, and even a complete halt to the flow. As a result the effective highway capacity is reduced. Ramp meeting is a means of preventing this happening. By reducing problems on the motorway, congestion can also be reduced on the neighbor network. As there are fewer disruptions on the highway, drivers will prefer to use the highway rather than the local network. Traffic signals are placed on the entry ramp to the motorway or ring road. When high traffic volumes are detected, the traffic signals are switched on in order to regulate the entry flow. Traffic is released after a given period, or when there are suitable gaps in the flow on the motorway. In the latter case an "override" allows the entry ramp traffic access to prevent very long waiting times or the buildup of queues which could block the local network. When the flow on the main carriageway falls below a given threshold, the signals are switched off again. Relevance for Large Scale Events Ramp metering can be an extremely useful tool for large events when exceptionally heavy traffic flows are likely to be generated. When such flows (e.g. coming from an airport or other cities towards the event city) attempt to enter the motorway system or a ring road, the use of traffic signals on ramps can help to avoid total paralysis of traffic. The identification of motorway/ring road entry points where such problems could occur is important. Options The signal control needs to be managed in relation to the density of traffic on the ring road or motorway. The approaches fall into two types: a) Upstream control systems (which base the on-ramp flow on the difference between the upstream flow and the capacity of the main highway 2 of 7

3 b) Downstream control systems (which regulate the on-ramp flow to keep occupancy on the main downstream carriageway below a pre-set optimal level). Technologies The system consists of a traffic adaptive UTC system (comprising traffic signals, local controllers and a vehicle detection system installed both on the motorway/ring road and on the entry ramps. In the case of an integrated system, the ramp metering must communicate directly with the urban traffic control system. The exact number and position of the detectors depends on the detailed design of the network. other system Central Server other system MFO Communication network MFO MFO Figure 1 Urban Traffic Control System Impacts The impact of the system can be measured in terms of trip time, average speed and road capacity. There are also indirect benefit environmental benefits to the reduction of "stop/start" conditions. In addition, as the maneuvers of joining a motorway carries its own accident risk, this may be reduced by ramp metering by heightening a driver's awareness of the maneuvers. CRITERION IMPACT COMMENTS MEASURED IMPACTS* TRAFFIC EFFICIENCY *** Travel speed increase 19-20% Travel time increase 20% (Source: EUCOR trial, Pairs and Netherlands) 3-5% increase in motorway capacity Significant reduction in shockwaves 3 of 7

4 (RWS, Netherlands) SAFETY ** Reduction in peak period accidents of around 30% (US trials) Integration potential The effectiveness of ramp metering can be increased through integration with variable speed control systems (which communicate to driver a recommended speed for each lane via VMS), although this makes the optimization of the system far more complex. In urban areas, ramp metering may be linked to the UTC system to ensure co-ordination between traffic signals on the ramps and those on the local network. This can prevent the entry-ramp queues from perturbing the local network conditions. Vehicle detectors on the motorway carriageways and entry-ramps provide data that may be processed and issued via a Traffic Information service. If a Supervisory Management system is in use, this could also include ramp-meeting sites. Implementation In the present section there is the description of the implementation of an operational Ramp-metering system called TIME (Traffic Introduction MEtering) installed, in a real life context in Stockholm. The overall system overview can be divided into two segments, the Central level and the local one. The central level consists of a central computer; the local one is the Omnivue system that can be used for monitor the system by the operators. The central system, called TIME system, handles the centralised control of when to allowgreen periods to the ramps, basing on all the traffic data. The local segments consist of the three ramps Nyboda, Hägersten, Gröndal and of the microwave measuring point at Tomteboda. The local and central systems are connected to each other using TCP/IP over the GCP network. 4 of 7

5 Figure 2- Overview of the system 5 of 7

6 Microwave detectors Essingeleden Microwave detectors Traffic Lights Cabinet Warning sign ITC-1 Spot Inductive Loops Ramp Omnivue access via GCP TIME central access Via GCP Figure 3 - schematic overview of the local control In the following paragraph examples of the system behavior are provided: Information from local system to central The ITC-controller processes the data information from the inductive loops on the ramps and the microwave detectors on the Essingeleden. The information is sent to the local SPOT unit. The local spot unit transmits the information to the Central TIME system. The information that is sent to the central level is both traffic data as well as alarms if something in the system is reporting an error. From the local system the status of the ITC is also sent from the ITC to the Omnivue system. The Omnivue system can then monitor the faults in the ITC. Information from central to local The central TIME system gathers information from the local SPOT units and, by using an optimization algorithm determines when and how much flow should be allowed in from each ramp. Once a minute the TIME centre decides the strategy and sends reference information to the Local SPOT units, in terms of desired traffic condition on the stretches of motorway from one ramp to the next. SPOT units then perform local optimisation to reach the condition suggested from the centre. The optimal strategy, in terms of allowed in vehicles, is actuated through green commands provided to the ITC. The ITC then controls the traffic lights and light up the signals. The Omnivue system can override the TIME system and can through the ITC put the Signals into a Dark mode. Omnivue also has the possibility to put the ITC controller on a fixed time plan. Allowing green periods solely based on time and not through the optimization algorithm provided through TIME. Information local to local 6 of 7

7 The SPOT units exchanges information about road stretches connecting their respective controlled areas. For example the flow leaving the Hägersten area is provided to the Gröndal SPOT unit which considers that as approaching flow. Examples High efficient Ramp-metering system or TIME is used in Stockholm and Italy (Mester ring road). The objective of ramp metering is to prevent traffic densities exceeding the critical threshold values beyond which the flow capacity breaks down and queues rapidly begin to build up. By maintaining traffic density at the optimal level, road capacity is fully exploited. TIME uses different kinds of sensor, including inductive loops and coupled infrared-radar detectors, to monitor traffic conditions on the motorway lanes and entry ramps. The strategy is to keep the traffic already on a motorway travelling smoothly at the optimal density, while ensuring that the queues waiting on the ramps are below the maximum acceptable threshold. Traffic is controlled by means of traffic lights located on the entry ramps. In Stockholm 3 ramps are controlled using data from inductive loops and microwave detectors. The 3 ramps are the ramps to enter the Essingeleden at Nyboda, Nybyhov (Hägerstensåsen) and at Gröndal. Within the project there is also a microwave measurement point at Tomteboda. The information from the ramps and the measurement point is sent from the local SPOT (RSU intelligence) units to the TIME server and then a optimization of the global strategy is made at the TIME server. The operator also have the possibility to check the system for faults using the Omnivue system and also the Omnivue system is delivered in order to override the TIME system by setting the traffic signals into a Dark mode. Omnivue can also have the possibility to put the ITC controller on a fixed time plan. Allowing green periods solely based on time and not through the optimization algorithm provided through TIME. Mestre Ring Road, Italy The TIME ramp metering system (developed by MIZAR Automazione) is deployed on the Mestre Ring Road, which carries heavy traffic flows at peak times. It is operated in conjunction with a dynamic speed adaptation system. The control actions are calculated on the basis of a two-level strategy which takes into account the data received from the local computing units (MFO - multifunctional out stations). At the central level, the overall control strategy is computed every 60 seconds, so as to be able to optimize traffic distribution and prevent a build up of queues on any single ramp. The central server can exchange data with other traffic management systems, including variable message signs, video surveillance, traffic monitoring and enforcement systems. TIME can also take into account traffic predictions received from other systems. 7 of 7