4.0 Operational Objectives for Arterial Data in an ICMS
The overall operational objective for an ICMS is to keep all of the component networks operating optimally all the time. The ICMS allows for the integration of transportation-related data across the corridor. Each agency within the network will have the necessary data and/or control to assist in facilitating the optimal movement of people and goods through the corridor. To manage a corridor in an integrated fashion requires the corridor manager to develop and implement strategies in four areas:
- Demand management
- Load balancing
- Event response
- Capital improvement
These strategies must be supported by the ICMS within the time constraints of the decisions that must be made. The ICMS corridor has three major time horizons for operation. These time horizons are:
- Current (a.k.a. "real-time")
- Planned Event (including pre-planning for emergencies and disasters)
- Long-term Planning and Optimization
These time horizons correspond to two distinct operating modes for corridors: Normal mode and Event mode. Normal mode is the mode that constitutes all the actions it takes to ensure that day-to-day transportation needs are addressed. Event mode has two sub-modes: Planned Event mode and Unplanned Event mode.
Planned Event mode is the mode where, prior to its occurrence, it is known that an event affecting corridor capacity or travel demand will occur. Capacity may be reduced due to construction, anticipated weather conditions, or a special activity such as a parade. Travel demand may increase due to a large venue activity like a sporting event.
Unplanned Event mode is the mode where an event changes corridor capacity or demand with little or no prior warning. This could be a current event (an incident that reduces capacity) or an emergency situation corresponding to one or more emergency/disaster plans (e.g. an evacuation).
A corridor may shift between Normal mode and Event mode several times during a single day, or even shift from one Event mode to another. In some cases (e.g. during construction or long-term maintenance activities), a Planned Event mode may become the "normal" operation mode.
A corridor does not change modes automatically. Whatever the triggering event, the corridor manager has to assess the severity and impact on the entire corridor, and the expected duration of an event before deciding the operational response. If the severity of an event is low, there may be no need to change operational modes or adopt a new operational strategy.
4.1 Current Time Horizon
The current time horizon is the real-time activity within the corridor. Whether the corridor is in Normal mode or Planned Event mode makes little difference to the corridor operators. The transportation network operators respond to changing conditions by evaluating the surveillance and detection data, sharing event information, and implementing controls to mitigate the impact of the unplanned events on all parts of the corridor. The overall scale of the unplanned event will affect the data needed to meet the operational objectives and large incidents may require more data sharing and coordination than smaller incidents. However, the defining characteristic of the current time horizon remains the same: responses are constrained by the resources at hand and the current capacities of the corridor transportation systems.
Transportation network operators and/or decision support systems may be able to recognize similarity between the impacts of different incidents. Using experience and historical data, in addition to current data, they may be able to take a pre-planned response for another event and use it as a basis for the response to an incident.
Typically, in this operational horizon, operators can respond to unplanned events by making changes to signal timing and ramp metering. Demand on the affected corridor component can sometimes be reduced by notifying the public so that they will change routes, mode of travel, or travel schedules. Coordination with public safety officials may allow for the emergency re-routing of traffic. However, increasing the capacity of the routes is usually not an option.
4.2 Planned Event Time Horizon
The planned event time horizon involves an event within the corridor for which there was prior notification and time to plan the corridor optimization for the event. Pre-planning the response to an event allows the transportation network planners the opportunity to model different responses.
Modeling algorithms will use historical data to validate solutions. This type of modeling may be done with traditional corridor modeling tools or with decision support modeling. The pre-planning exercise allows different agencies within the corridor to work together to optimize the response. In a planned event mode, the ICMS should be capable of evaluating multiple strategies and identifying the likely impacts of each strategy with regard to the performance measures and capacity utilization on all transportation modes within the corridor. If this planning does not identify a strategy that will avoid capacity overloading of one or more of the corridor transportation assets, the corridor participants must understand, in advance, the likely impacts of the selected plan.
During the planning, it may be determined that additional capacity within the affected area is needed and some routes may be designated as one-way for the duration of the event. Transit agencies may respond by providing more high occupancy vehicles and lower cost parking in satellite locations. Public safety agencies and road maintenance agencies will assist during the event with the reconfiguration of roadways.
4.3 Long-Term Planning and Construction Horizon
This time horizon allows the agencies within the corridor to review the current corridor optimization and then determine if there are additional intersections, lanes, transit vehicles, or other ICMS infrastructure needed. Current usage is reviewed and historical data is used to model new configurations. Each configuration is optimized to determine the impact of the proposed modifications and determine which modification has the highest benefit/cost ratio. Long-term planning and construction allow for the building of new pavement, implementation of high occupancy vehicle incentives, and addition of mass transit options.
Long-term planning is usually thought of in terms of capacity planning. To the extent that current and short-term operational decisions are made on the basis of optimizing capacity utilization, long-term planning is an extension of ICM strategies.
4.4 Summary
Demand management, load balancing, event response, and capital improvement are all ways of getting the most "bang for the dollar" out of existing and future investments in corridor transportation capacity. Regardless of how the public measures satisfaction with transportation, transportation providers are investing based on demand for capacity and cost per incremental change in capacity. It is imperative then, that good corridor management depend on measures of capacity utilization, cost of capacity, and the optimization of existing capacity to meet current needs.
Researchers are evaluating performance measurements for traffic signal systems through a variety of approaches. While there is not a clear agreement on what performance measures should be used, there is a growing agreement that improvements in the analysis, modeling, and decision support capabilities for arterial systems will require some basic improvement to the data that is currently available from a controller, as well as changes to data collection methodologies which traditionally have been used primarily for intersection control.
- Data collected or calculated in the controller should be time-stamped and sent to the central system within a few minutes, not averaged or discarded.
- Firmware should be modified to collect phase, overlap, control mode, detector, and other event information at a significantly finer time resolution. (Controllers typically scan data inputs at 0.1s intervals and some researchers are suggesting sending 1.0s data updates to the central server.)
- Typical detector configurations may need to be modified to allow finer resolution of intersection volumes by approach and exit. Occupancy and volume need to be collected on a lane-by-lane basis.
- Additional sensors or modifications to existing sensor configurations may be needed to provide improved speed data (either in the form of travel time or actual measured vehicle speeds)
If it is impractical to implement these changes in existing controllers due to processor, memory, or firmware limitations, it may be feasible to implement separate data acquisition modules that share access to the primary detection devices.
