Major traffic incidents on the freeway typically cause significant traffic delays, extensive queues and roadway capacity reduction due to partial or complete freeway closures.
The results are excessive traffic congestion and travel-time delays on the freeway and increased traffic demands on local streets because of traffic randomly detouring from the freeway in search of faster routes.
In 2007, the City/County Association of Governments (C/CAG) of San Mateo County, Calif., led by Executive Director Richard Napier, embarked on a unique Incident Management Program that is strategically developing and deploying ITS elements along state and local streets adjacent to the 20-mile, eight-lane U.S. 101 freeway, which 20 jurisdictions in San Mateo County. The result will be a smart corridor that will enable local public works agencies, local public safety and Caltrans to use technology to partner across jurisdictions to manage incident traffic that diverts onto local streets and designated alternative routes. Equipment that will be available to the stakeholders on the alternate routes include dynamic route-guidance signs, video surveillance cameras, traffic-signal flush plans and integration with Caltrans and the California Highway Patrol (CHP) operations to reduce the effects of incident congestion on local operations and freeway queuing.
This article presents some of the high-level program concepts along with detailed design and implementation challenges encountered during program development.
80-strong alternates
The initial development of the alternate routes originated with input from the Incident Management Committee stakeholders. Each member identified local routes that were acceptable in their jurisdiction to function as an alternate route. This effort resulted in roughly 80 alternate routes in the northbound and southbound directions of U.S. 101. Additional routes were defined for other freeways in San Mateo County, including I-280, I-380 and State Rte. 92, representing future phases of the smart corridor program.
The planning document, which became known as the Alternate Routes for Traffic Incidents (ARTI) Guide, detailed specific locations for ITS elements along each defined route. The elements included the following:
- Electronic route-guidance signs to provide route guidance along the local streets;
- Closed-circuit television (CCTV) cameras to monitor traffic flow along the alternate routes;
- Traffic-signal timing modifications to implement “flush plans” that move large volumes of traffic on local streets;
- Personnel deployment to help manage traffic at critical local intersections;
- Freeway changeable message signs (CMS) to alert traffic of a freeway incident; and
- Freeway entrance ramp meters to manage the flow of traffic entering the freeway before and after an incident.
One key challenge that arose was how to prioritize which alternate routes should be considered primary alternate routes during incidents. There are many options in San Mateo County. First, there is another freeway (I-280) that runs parallel to U.S. 101 about six miles to the west. This is a low-volume, six-lane (in both directions) facility that will serve as the primary alternate for longer trips between San Francisco and San Jose. Next, El Camino Real is a state-owned local facility that is primarily a four- and six-lane arterial running parallel to U.S. 101 about one mile to the west and will serve as a secondary alternate route through the corridor. Finally, the local alternate routes defined by the stakeholders would serve as tertiary alternates primarily for shorter, local trips. These local routes do not have the available capacity to accommodate a large volume of traffic but are generally two- and four-lane facilities.
Another key challenge is how to implement the designated alternates. Most stakeholders agreed that Caltrans would not use the CMS to direct traffic off the freeway onto a specific local street. The alternate routes were to be activated for traffic that “chose” to exit the freeway to seek an alternate. The traffic would be guided from the exit ramp to an entrance ramp downstream from the incident.
But what if the incident congestion extends beyond the limits of a single defined alternate? Rather than defining an infinite number of possible alternate routes for all scenarios, a “stitching” concept was defined in the ARTI Guide that provides guidance on connecting multiple consecutive alternate routes into a single larger route. This allows for flexibility to Caltrans and the local agencies when determining which alternate(s) to implement during an incident.
Determining work loads
As the project continued to develop, the next recurring issue was, “Will the alternate routes defined by the stakeholders really work? The answer is clearly, “That depends on what you mean by ‘work.’” At the beginning of the project, there was an acknowledgement that none of the local streets had sufficient capacity to accommodate freeway-level traffic. That is obvious. So the challenge became how to evaluate how much traffic each route could accommodate, since there were many factors to consider including available capacity, volume-to-capacity ratio, truck traffic, the ability to accommodate trucks, signal timing and geometry.
It became quickly apparent that large-scale macroscopic modeling was unnecessary. Instead, throughput and queuing analyses were conducted based on Highway Capacity Manual and traffic-flow fundamentals. This identified how much freeway traffic would be accommodated by each route and what reduction in queue length, delay and time to recover could be expected on the freeway. Later, a Synchro analysis was conducted on a portion of the corridor to substantiate the throughput and queuing analyses.
Another consideration was to identify any geometric or operational constraints that would impede the traffic flow along the alternate route. This included single left-turn lanes along the route; on-street parking; lane widths; stop signs; bus stops; schools; and cross-street demand. The presence of these constraints influenced the decision to eliminate certain routes as desirable alternates.
Caltrans can
During project development there was significant discussion about field-element placement and how the alternate routes would be used during an actual incident, since the corridor crosses multiple jurisdictions, including Caltrans. These issues affect system development and include:
- Environmental impacts: San Mateo County is a very culturally and biologically sensitive region. With over 150 field elements and an extensive communications infrastructure, there is a high potential for considerable ground disturbance, and many field-element locations were limited by historical and biological conditions. Evaluation of historical sites, protected vegetation, sensitive soil and animal habitats required some of the locations to be adjusted or placed on existing equipment in order to avoid impacts.
- How much control will Caltrans have of the equipment owned by local agencies? Although there was considerable concern over allowing Caltrans to have access to control local agency equipment, particularly traffic signals, most were acceptable of a concept that would allow Caltrans to download predetermined signal-timing plans to the local controllers during an incident. This cooperation would allow local agencies to agree on the modifications before they happened.
- Who controls the field devices? Most of the local agencies participating in this program have a very limited traffic-engineering and maintenance staff. None has a dedicated traffic operations center or staff with the exception of Caltrans, which has an extensive traffic management center (TMC) colocated with the CHP. Since Caltrans operates the TMC 24 hours a day, most agencies agreed that Caltrans should be responsible for activating the equipment along the alternate routes.
- Integration of the ARTI Guide into incident-management activities: The ARTI Guide is a great plan for how the agencies can best coordinate traffic-management activities during incidents. The challenge is to mainstream those activities with other emergency-response activities. It was clear early on that CHP and local police and fire departments would not entertain any changes to their standard operating procedures for emergency response. Frankly, that is an appropriate position to take, since a tremendous amount of training and protocol would need to occur to make a procedural change possible. Public safety representatives did concur that the ARTI Guide will supplement the standard procedures for traffic management and will not replace any existing emergency-response activities. The ARTI Guide is made available in hard copy and electronic formats, so field personnel also have access to the information.
Leeway for freeways
It is unclear whether the question “Will it work?” has ever been conclusively answered on this project. Again, it depends on the definition of “work.” The analyses that were conducted provided an increased level of confidence that the defined alternate routes would accommodate at least some level of freeway traffic. The local routes cannot accommodate more than about 15% of the freeway demand, and the local network would be operating at a failing level of service. But the freeway queue in advance of the incident might be up to a mile shorter (or more), resulting in a reduction in the time the freeway takes to recover from the incident. That probably means that alternate routes would work.
Also, defined alternate routes would result in less traffic randomly filtering through the network finding their way around an incident. That probably means that alternate routes would work. This, of course, depends on multiple factors including time of day, incident severity, incident duration, ability to use primary and secondary routes and the ability to coordinate across jurisdictional boundaries. All that being said, it is clear that alternate routes will reduce freeway congestion during incidents. The new question might be, “What is the minimum acceptable amount of improved congestion?”
