Primer Caso

Dynamic traffic management systems are cost-effective systems that improve traffic flow on expressways and arterials. Dynamic traffic management systems use simulation models combined with real-time traffic information to predict the effects of various management strategies.152 _{Route time, travel time and departure time are collected from sources of real-}

times on routes, given the combination of management strategies used on those routes, including incident management, ramp metering, signal control and traveler information. Based on these predictions, the system selects optimal strategies and suggests travel time predictions and route recommendations to travelers. These programs have been

successfully deployed in Europe and Japan resulting in capacity improvements on major expressway corridors of up to 30% as well as significant increases in trip predictability and safety.153

Intelligent transportation systems (ITS) are the most popular subset of dynamic traffic management systems. U.S. engineers have been implementing ITS for over two decades and have installed vehicle sensors and message signs, as well as backbone communications systems, on many major urban expressway corridors and selected arterial highways.154 These sensors gather data about traffic conditions on a 24/7 basis, and this information is collected, compiled and distributed to the motoring public in near real-time through a variety of public and private information channels.

Many metro areas use several ITS systems that operate in static mode. However, ITS systems would be even more effective if they operated in a dynamic mode. In “static mode” expressway incident management and service patrols quickly observe, respond to and clear accidents from travel lanes. But in “dynamic mode” ITS systems seek to prevent accidents by reducing speed limits and warning of congestion. Traffic signs that report congestion ahead provide valuable information to motorists. However traffic signs that detail congestion and suggest alternate routes and where to exit the highway to avoid congestion, a.k.a. “dynamic mode,” are even more useful.

The California Department of Transportation (Caltrans) operates a partially dynamic traffic monitoring system called “Quickmap” that provides updated information on changeable travel signs.155 _{The website provides traffic cameras and information on travel alerts,}

weather, road conditions, speeds, roadwork, detours and information to truckers. Caltrans also has a mobile application for smart phones, but the agency could improve its service by providing real time updates of incidents and severe congestion events. The state also uses other dynamic systems, such as converting a shoulder to a direct exit lane. However, the state needs to complete its transition from a static plan to a dynamic operations plan. The following paragraphs detail several leading dynamic ITS technologies.156

Ramp metering uses a traffic control device, typically a red and green traffic light, and a signal controller that regulates the flow of traffic entering expressways at current traffic conditions.157 Ramp metering restricts the total flow of vehicles entering roadways by temporarily storing it on an on-ramp. Ramp metering decreases congestion by reducing demand and eliminating platoons of cars jamming up the right-most expressway lane. Most major metro areas use static ramp metering. Metro areas need to adopt active ramp

two-lane highway on-ramp that at 7:00 AM has 20 cars in the left lane and five in the right and at 8:00 AM has seven cars in the left lane and zero cars in the right. With a static ramp meter one car from each lane of an on-ramp would enter a highway per green signal for the entire morning rush hour. With an active ramp meter, at 7:00 AM four cars would enter from the left lane per green signal for every car that entered from the right lane per green signal. At 8:00, with a different traffic pattern, the ramp meter would turn green for the left lane but stay red for the right lane since there is no traffic in the lane. Active ramp

metering does much more to reduce congestion.

Static queue warnings are electronic signs that detail travel speeds and travel times that may change due to congestion, traffic construction or an accident. They can be used for traffic control on congested facilities or to enhance safety during major incidents. Dynamic queue warnings offer the same features, but they also suggest alternate routes and provide detailed guidance on when the congestion starts. Dynamic signs are often placed at expressway entrances so drivers can choose an alternate route before they enter the highway.

Speed harmonization uses variable speed limits to smooth traffic flow and improve safety.158

Hard shoulder running involves upgrading shoulder pavement quality and opening shoulders to traffic during peak periods.159 Some states have converted shoulders to general purpose lanes and allow traffic to use them 24 hours a day, seven days a week. Shoulder lanes are used to increase capacity on constrained highways.

Junction control uses signs, typically red and green electronic signs, to open and close lanes based on conditions.160 For example, if there is an accident in the middle lane of three lanes, road operators may place a red X in the box over the middle lane to indicate that it is closed and drivers should move to the right or left.

The most effective active transportation systems use multiple technologies together. For example, queue warnings are used in conjunction with speed harmonization to slow speeds and warn drivers of congestion ahead.

Enforcement of these dynamic roadway systems is important. While traditional

enforcement—a police officer sitting in a patrol car—is still used, automated enforcement is much cheaper and safer. Many states use automated traffic cameras to ensure drivers obey dynamically imposed operating signs. If drivers do not obey the signs, the

California’s systems based on the data available. But the urban area is encouraged to upgrade to more dynamic ITS systems wherever possible.