Traffic Control Systems Handbook: Chapter 4. Control and Management Concepts For Freeways
A freeway is a limited access highway with high speeds, and ramps to allow entry and exit. Freeways may or may not have tolls. Freeways were originally intended to provide free-flowing, high-speed traffic flow over long distances. Very little thought was put into managing congestion because freeways were never expected to be congested. However, as cities grew and commuters moved to suburbs, freeways began mixing long-distance traffic with commuter traffic and the congestion that previously affected surface streets now also affected freeways.
This chapter provides a brief overview of freeway management. For more information, refer to the Freeway Management and Operations Handbook (1).
The main objectives of a freeway management system are to:
- reduce the impacts and occurrence of recurring congestion on the freeway system.
- minimize the duration and effects of nonrecurring congestion on the freeway system.
- maximize the operational safety and efficiency of the traveling public while using the freeway system.
- provide facility users with information necessary to aid them in making effective use of the freeway facilities and to reduce their mental and physical stress.
- provide a means of aiding users who have encountered problems (crashes, breakdowns, confusion, etc.) while traveling on the freeway system.
Congestion occurs on a freeway when demand exceeds capacity. When this occurs on a freeway section, a bottleneck exists. A bottleneck occurs when:
- Demand increases to a level greater than capacity, or
- Capacity decreases to a level less than demand.
To understand what causes freeway congestion, one must understand the theory of traffic flow summarized below.
Important traffic flow parameters include:
- Flow (V) = Number of vehicles passing a certain point during a given time period, in vehicles per hour (veh / hr)
- Speed (S) = The rate at which vehicles travel (mph)
- Density (D) = Number of vehicles occupying a certain space. Given as veh / mi.
D = V / S
The fundamental diagram shown in Figure 4-2, relates flow and density. This diagram is highly idealized and actual traffic flow characteristics vary considerably. Examples of typical flow characteristics are provided in HCM 2000 (2).
Figure 4-2. Generalized Relationships Among Speed, Density, and Flow Rate on Uninterrupted-Flow Facilities.
Free flow speed (Sf) occurs during light traffic conditions. When density reaches the critical density (D0), the freeway reaches its maximum flow (Vm). Speed at that point is decreased to S0. When the density increases beyond the critical density, the flow actually decreases, until the density reaches the jam density (Dj), where the flow becomes zero and all traffic is stopped. When the density is below the critical density, the flow is said to be stable, or uncongested. When the density exceeds the critical density, the flow is said to be congested, or unstable, and the freeway capacity decreases. Because more vehicles are processed when the flow is stable, it is best for the density to be as close as possible to, but below the critical value so the freeway can operate at its full capacity.
Congestion has become a daily occurrence on many portions of urban freeway networks. Even casual observers can locate points of expected congestion. Congestion commonly expected at predictable locations during approximately predictable periods of time is termed recurrent congestion.
In contrast, other forms of congestion result from random or less predictable events. Such nonrecurrent congestion results most frequently from incidents. Congestion from special events (e.g., sporting events, maintenance and construction activities) may be considered nonrecurrent congestion.
To measure congestion, researchers at Texas Transportation Institute developed a Roadway Congestion Index (RCI), depicted as follows (3):
Congestion, both recurrent and nonrecurrent, is characterized by the following conditions that cause user dissatisfaction:
- Slow travel speeds
- Erratic speeds (stop-and-go movement)
- Increased and inconsistent travel times
- Increased accident potential
- Inefficient operation
If users expect a certain level of congestion during peak periods, they can plan trips accordingly. However, nonrecurrent congestion can severely impact an otherwise satisfactory trip during peak or off-peak periods. The inability to provide a reliable, although sometimes lower, level of service may prove to be a more serious problem.
4.3 Forms of Freeway Management
Freeway management includes the categories shown in Figure 4-3.
Figure 4-3. Freeway Management Categories.
These topics are outlined below and covered in detail in Reference 1.
- Surveillance and incident detection: Techniques for surveillance and incident detection include vehicle detectors, closed-circuit television (CCTV) cameras, and 911 calls.
- Mainline / lane use control: Lane use control makes the most efficient use of mainline capacity. Techniques include temporary shoulder utilization, reversible lanes and signs, variable speed limits, large truck restrictions and mainline metering.
- High occupancy vehicle (HOV) priority treatment: Giving priority to HOVs increases the number of people served by the facility, while possibly decreasing the number of vehicles. An example of HOV priority is having either a lane or an entire facility open only to buses or carpools. Another example is priority access, by having a special HOV lane on a metered on-ramp to allow buses or carpools to bypass the ramp queue.
- Ramp control: Ramp meters are special traffic signals on a freeway on-ramp that control the traffic entering the freeway, in order to keep the freeway uncongested. Ramps can also be closed under extreme circumstances.
- Information dissemination: Information dissemination gives real-time information to motorists, which is considered one of the most important functions of freeway management. Information can be given on incidents, adverse weather and driving conditions, speed and travel time information, construction and maintenance activities, planned special event destination, special lane and roadway control measures, and information on alternate routes. Methods used for information dissemination include Internet services, radio information, in-vehicle navigation devices, changeable message signs (CMS), and highway advisory radio (HAR). Figure 4-4 shows an example of a CMS on a freeway.
- Freeway management system: A freeway management system is the common interface for multiple agencies in a region. This can help coordinate different freeways and surface streets, as well as other modes of transportation.
- Incident management: The purpose of incident management is to detect and respond to incidents, in order to restore the freeway to full capacity as quickly as possible after an incident, as well as to provide aid to stranded or injured motorists. Incident management requires coordination among various agencies, and coordination among various human and technological resources.
- Road weather information systems (RWIS): RWIS consists of sensors embedded in the road and weather stations located near the road that monitor and report temperature and weather conditions. RWIS is especially useful in the winter. Fog detectors may be used in certain locations.
Figure 4-4. CMS on a Freeway.
4.4 Relationship Between Freeways and Surface Streets
In recent years, as both freeways and surface streets experience more congestion, it is becoming increasingly important to improve coordination between freeways and surface streets. With ramp metering, for example, some traffic may avoid the freeway, leading to increased traffic on a surface street. A few ways in which surface streets can be taken into account in freeway management include:
- CMS signs on a freeway can display information relating to congestion on surface streets, and display alternate routes to minimize congestion on both freeways and surface streets.
- Ramp metering may be reduced or suspended if congestion on surface streets is extreme, especially if the ramp meters are causing a spillback queue.
- If there is congestion or an incident on the freeway, traffic signal timing on surface streets could be modified to improve traffic flow and encourage diversion to surface streets.
- Improved signal timing and geometries at exit ramp intersections with surface streets may alleviate mainline freeway congestion at exit ramps.
1. Neudorff, L.G., J.E. Randall, R. Reiss, and R. Gordon. "Freeway Management and Operations Handbook." Federal Highway Administration Report No. FHWA-OP-04-003, Washington, DC, September 2003.
2. "Highway Capacity Manual." Transportation Research Board, Washington, D.C., 2000.
3. Schrank, D.L, S.M. Turner; and T.J. Lomax. "Estimates of Urban Roadway Congestion - 1990", Federal Highway Administration Report No. FHWA/TX-90/1131-5.