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21st Century Operations Using 21st Century Technologies

Recurring Traffic Bottlenecks: A Primer
Focus on Low-Cost Operational Improvements (Fourth Edition)

Appendix D. Definitions and Traffic Bottleneck Typologies

Auxiliary lanes—Typically, any lane whose primary function is not simply to carry through traffic. This can range from turn lanes, ramps, and other single purpose lanes, or it can be broadened to imply that a traffic bearing shoulder can be opened in peak periods to help alleviate a bottleneck, and then "shut back off" when the peak is over.

Bottleneck—There can be many definitions. Here are a few that are typically used.

1) A critical point of traffic congestion evidenced by queues upstream and free flowing traffic downstream; 2) A location on a highway where there is loss of physical capacity, surges in demand (traffic volumes), or both; 3) A point where traffic demand exceeds the normal capacity; and 4) A location where demand for usage of a highway section periodically exceeds the section's physical ability to handle it, and is independent of traffic-disrupting events that can occur on the roadway.

Capacity—The maximum amount of traffic capable of being handled by a given highway section in a given time. It is usually recorded in terms of vehicles per lane (VPL) or vehicles per hour (VPH) or passenger cars per hour per lane (pcphpl). Traffic engineers usually speak in terms of "free flow" capacity as the maximum which can occur under unimpeded conditions.

Congestion (specifically, traffic congestion)—The FHWA's Traffic Congestion and Reliability Report defines congestion as "an excess of vehicles on a portion of roadway at a particular time resulting in speeds that are slower—sometimes much slower—than normal or free flow speeds; (congestion is) stop-and-go traffic. Previous work has shown that congestion is the result of seven root causes often interacting with one another." (The seven root causes are physical bottlenecks (a.k.a. "capacity constraints"), traffic incidents, work zones, weather, poorly timed signals et al., special events, and over-capacity demand (i.e., daily and seasonal peaks superimposed on a system with a fixed capacity). Some sources cite only six root causes because they see over-demand as an inherent sub-element necessary for any of the other causes to exist in the first place. Put another way, absent over-demand, there would just be "volume," but not necessarily "congested" volume.) Since a bottleneck is a cause of congestion, congestion cannot be solely analogous to a bottleneck. Congestion is more. For example, a "congested" corridor may harbor multiple bottlenecks or any combination of the seven root causes of congestion.

Downstream traffic—Traffic that is beyond (past) the subject point on a highway.

Free-flow speed—In short, it is unimpeded speed. An expanded definition may be the optimum speed that can be safely achieved on a road or highway which allows the motorist an undelayed trip. Typically this is achieved without having to react to or adjust (much) to external factors like lane width, lateral clearance, geometric design, weather, visibility, vehicle limitations, alignment, traffic control devices (signals, meters, or road appurtenances) or of course, volume.

Hidden bottleneck—A highway location where some type of physical restriction is present, but traffic flow into this area is metered by an upstream bottleneck so the location does not appear as a bottleneck under prevailing conditions. Removal of the upstream bottleneck will cause the hidden one to emerge as a new bottleneck.

Nonrecurring events—As it pertains to traffic, a delay caused by an unforeseen event; usually a traffic incident, the weather, a vehicle breakdown, a work zone, or other atypical event. Even if planned in many cases, like work zones and special events, they are irregular and not predictably habitual in location and duration.

"Phantom" traffic jam—A slowdown that occurs for seemingly no apparent reason (e.g., no crash, no lane drop, no merge, no event, etc.) that is purely the result of one or more cars at the forefront braking to cause the car following too closely behind to brake, and so on, until a snake-like ripple takes effect that can result in cars further back coming to complete or near-complete stops. The larger the gaps between cars, the less probability of a phantom traffic jam. However, many drivers are "profiteers" that will speed up or lane-change into sufficiently large gaps, thereby perpetuating a new phantom jam.

Ramp metering—The practice of managing access to a highway via use of control devices such as traffic signals, signing, and gates to regulate the number of vehicles entering or leaving the freeway, in order to achieve operational objectives. The intent of ramp metering is to smooth the rate at which entering vehicles will compete with through vehicles. Done properly, ramp metering will calm the "mix" that occurs at these junctions.

Recurring event—As it pertains to traffic, a recurring event is a traffic condition (i.e., a bottleneck or backup) that one can presume to occur in the same location and at the same time daily, albeit for weekday or weekend conditions. Examples would be peak- hour slowdowns at junction points, intersections, and ramps. One can "plan" for these events because one knows by routine that such events will occur time and again in the same manner and place.

Systemic (traffic) congestion—Systemic congestion is pervasive throughout the system. There is no one simple source other than to surmise that traffic congestion is "everywhere"; it is infused in just about every element of a city or region and is due to the overall number (crush) of vehicles, the concurrent demand on just about all the roads (or at least the major ones) and exists for practically all hours, or least the daylight hours, when demand is greatest.

Traffic microsimulation tools—Complex microsimulation tools that rely on input of traffic data, intersection "nodes," facility "links," and the associated parameters of each input, in order to output simulated conditions. By changing the inputs, engineers can test different sizes, characteristics, and out-year scenarios of traffic demand.

Upstream traffic—Traffic that has not yet arrived at the subject point on a highway.

Zippering—also known as the late merge or zipper merge is a convention for merging traffic into a reduced number of lanes. Drivers in merging lanes are expected to use both lanes to advance to the lane reduction point and merge at that location, alternating turns.

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