Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Report
6. OPERATIONS STRATEGIES AND TREATMENTS TO
IMPROVE TRAVEL-TIME RELIABILITY
This chapter identifies a list of key strategies and their strengths, weaknesses, threats, and
opportunities for improving travel-time reliability under the baseline and three future scenarios
developed in Chapter 2. In order to identify the strategies and treatments that are most likely to
have the greatest impact, a literature review focused on previous and current work of other SHRP
2 Reliability and Capacity projects including L03, L06, L07, and C05 was conducted. In addition
to SHRP2 projects, information from the FHWA, state DOTs, universities, and other countries
were reviewed to ensure a broad assessment of strategies and treatments. Innovative technologies
that may impact travel-time reliability in the future were also reviewed and presented at the end of
this chapter.
SOURCES OF CONGESTION/UNRELIABILITY
Congestion occurs when the traffic volume on a roadway exceeds the available capacity. However,
roadway capacity is not a constant and is influenced by a variety of factors that reduce effective or
operational roadway capacity from the “baseline” capacity computed through Highway Capacity
Manual procedures. Previous research has identified seven sources of congestion. They are briefly
summarized as follows. (1)
1. Physical Bottlenecks: Bottlenecks are sources of congestion that occur on short
segments of roadway that exhibit lower capacity than upstream segments of roadway,
essentially resulting in unreliable travel. Bottlenecks commonly form either at changes in
roadway geometry (e.g., lane drops) or where significant traffic movements reduce
effective roadway capacity for a given number of roadway lanes (e.g., merge and weave
sections).
2. Traffic Incidents: Traffic incidents are events that disrupt the normal flow of traffic,
usually by physical impedance in the travel lanes. Events such as vehicular crashes,
breakdowns, and debris in travel lanes are the most common form of incidents.
3. Weather: Environmental conditions can lead to changes in driver behavior that affect
traffic flow. Weather events such as fog, snow and heavy rain can negatively impact travel
conditions, causing delays and congestion.
4. Work Zones: Construction activities on the roadway can result in physical changes to
the highway environment. These changes may include a reduction in the number or width
of travel lanes, lane “shifts,” lane diversions, reduction, or elimination of shoulders, and
even temporary roadway closures.
5. Traffic Control Device: Intermittent disruption of traffic flow by control devices such
as railroad grade crossings and poorly timed signals also contribute to congestion and
travel time variability.
6. Fluctuations in Normal Traffic: Variation in day-to-day demand leads to some days
with higher traffic volumes than others.
7. Special Events: Special events are a special case of demand fluctuations whereby traffic
flow in the vicinity of the event will be radically different from typical patterns. Special
events occasionally cause “surges” in traffic demand that overwhelm the system.
The FHWA website (2) provides national estimates of the amount of delay caused by each of the
sources of congestion noted above. SHRP 2 Project L03 (3) further quantifies these estimates by
OPERATIONS STRATEGIES AND TREATMENTS TO IMPROVE TRAVEL-TIME RELIABILITY Page 62