Definitions

Active bottleneck – When traffic flow through the bottleneck is not (further) affected by downstream restrictions.

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. Traffic engineers usually speak in terms of "free flow" capacity.

Congestion – The FHWA "Traffic Congestion Reliability" reports define 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 (The seven root causes are bottlenecks (a.k.a. "capacity constraints"), 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 subelement 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) often interacting with one another. 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.

Downstream traffic – Traffic that is beyond (past) the subject point on a highway.
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, work zone, or other atypical event.

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 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.

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.