Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 7
Speed Considerations 7
approach on level grade with unlimited sight distance has an unlimited design speed regardless of
the posted speed approaching the intersection). A yield condition may operate as a stop control dur-
ing some time periods, depending on traffic flows. In other cases, a yield condition may be virtually
free flow and the operating speeds for this movement may be dictated by the roadway approach
geometry (i.e., a separate turn lane, turning roadway width, and turning roadway radius).
2.3 Designing for Appropriate Speeds
In general terms, good roadway geometric design provides a sufficient level of mobility and
land-use access for motorists, bicyclists, pedestrians, and transit while maintaining an appropri-
ate degree of safety. Higher-speed roadways are typically provided in locations where travel time
and mobility are priority needs. Speed is often used as a performance measure to evaluate the
effectiveness of highway and street designs. High speeds generally are associated with long trips,
and low speeds generally are associated with short trips or with facilities that have more frequent
access. Posted speeds frequently correlate with these intended uses. High-speed facilities serve key
network needs--it is not always appropriate to expect reduced speeds at intersections.
Environmental and operational indicators should be in place to provide drivers with a con-
sistent message about the potential for conflict so they are best able to select an appropriate
speed. The goal is to provide geometric street designs that look and feel like the roadway's
intended purpose. Because drivers choose their speed based on what they see on the roadway
ahead, calling a driver's attention to roadway features that present a potential risk provides
increased opportunities to avoid conflicts. Drivers who perceive potential risk can better adapt
their driving behavior to roadway conditions.
A facility's design speed is a fundamental design criterion that affects three-dimensional road-
way design parameters (plan, profile, and cross section). Aside from roundabouts, where entry
speeds of about 25 mph are specifically attained through geometric design, there is no common
intersection design speed. Intersection speed typically is assumed to be that of the roadway segment.
Although designers generally seek speed and operational consistency, intersection operations
(e.g., queuing, deceleration, turning vehicles) and/or geometry may create localized conditions
that require reduced speeds. Drivers must perceive and comprehend a greater variety of situa-
tions at an intersection than while driving through the high-speed roadway segment that pre-
cedes the intersection.
Intersection conditions should be considered independently from the adjacent roadway seg-
ments. For new facilities, this means ensuring that an intersection's operational and geometric
elements are appropriately configured. The existing geometric and operational elements of an
intersection should be assessed when considering appropriate actions for retrofit projects.
Such a design philosophy and approach can produce geometric conditions that are more likely
to result in operating speeds consistent with driver expectations and commensurate with the
roadway's function. This, of course, does not account for impaired or overly aggressive motorists
or conditions such as adverse weather. Ideally, drivers should operate their vehicles in a manner
appropriate for the conditions for as long as those conditions prevail.
2.4 Factors Affected by Speed
High speeds serve key network functions. However, operating speeds inconsistent with the
prevailing conditions may adversely affect environmental quality and safety, or require a larger-
than-desirable facility size.
OCR for page 8
8 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections
The effects of speed are most pronounced at intersections where friction between competing
movements is concentrated. This friction may manifest itself as decelerating or accelerating
vehicles, queues, crossing traffic, or traffic yielding to crossing pedestrians. Any of these kinds of
friction create the potential for conflicts.
The ways in which speeds through intersections affect intersection operations, environmental
quality, and safety are discussed below.
2.4.1 Facility Size
Drivers' perceptionreaction times are essentially constant; therefore, higher speeds require
drivers to understand their driving tasks farther in advance of intersections, compared to slower-
speed environments. This means that the distance between driver decision points must increase
as speeds increase.
Higher-speed facilities require larger clear zones and flatter horizontal and vertical curves. In
addition, fundamental dimensions for stopping-sight distance increase with speed, leading to
flatter and more open roadways. Attaining appropriate sight distances affects horizontal and
vertical alignments as well as such cross-sectional features as cuts, fills, and landscaping. For
example, using values from Exhibit 9-58 from AASHTO's Policy on Geometric Design of High-
ways and Streets, known as the "Green Book," (AASHTO, 2004) for the case of a right turn from
a stop or a crossing maneuver, the intersection sight-distance value for 45 mph is 430 ft. This
value increases to 575 ft when speeds are 60 mph.
In urban environments, sight-distance needs can affect building setbacks, on-street parking
locations, and other design elements such as the locations of street furniture and landscaping.
The transition zones within which drivers are required to slow down as they approach an inter-
section need to be longer when a greater change in speed is required. This affects the length of
alignment tapers, bay tapers, and the deceleration components of turn-lane designs.
2.4.2 Quality and Comfort of the Roadway Environment
The function of roadways and intersections must be balanced with the needs of adjacent land
uses to both maintain environmental quality and to provide necessary mobility. High-speed
intersections can create a barrier to the mobility of non-auto users crossing the facility. The
increased noise levels and intense environments that result from proximity to motor vehicles can
create discomfort for pedestrians and bicyclists who travel parallel to the facility. Adjacent neigh-
borhoods and businesses may also experience adverse effects from high-speed traffic, such as tire
and engine noise, and may benefit from buffer treatments.
Intersections near schools, hospitals, or other concentrations of pedestrians--particularly
people who are elderly, young, or have disabilities--may be particularly sensitive to high speeds.
2.4.3 Safety
The relationship between speed and intersection safety is a critical concern for transportation
professionals. Road safety is often divided into three constituent elements: exposure (increases
with the number of conflicting movements), risk (increases with traffic volume), and conse-
quence (increases with speed). Reducing speed at intersections has the potential to improve con-
sequence, although it has little relationship to exposure or risk. There is a decisive relationship
between speed and crash severity, but the relationships between speed and crash frequency are
less clear.
OCR for page 9
Speed Considerations 9
The physical relationship between mass and energy explains that higher speeds and larger speed
differentials create the potential for higher-severity crashes. As speeds increase, the energy from the
mass of the vehicles increases. Studies of modern multilane roundabouts illustrate the relationship
between speed and crash severity: total crash rates and frequency may stay the same after an inter-
section is converted to a roundabout, but the slower speeds help reduce the severity of crashes.
A variety of intersection traffic conditions create large speed differentials, increasing the
potential for severe crashes. For example
· An unbalanced distribution of traffic for a given number and arrangement of lanes could
create high differentials in speeds between vehicles traveling in the same direction. Consider
that a channelized right-turn lane may operate under near free-flow conditions while the adja-
cent travel lanes experience queuing.
· An intersection or approach with extensive queuing, where the back of the queue is a signifi-
cant distance from the intersection proper, could cause speed differentials. In these condi-
tions, vehicle queues in turn lanes may exceed the storage length provided, requiring vehicles
to decelerate in the through travel lanes.
· Uniform traffic congestion at an approach could lead to queues that extend outside an intersec-
tion approach's typical available sight distance. In these conditions, vehicle queues extending
beyond the intersection's geometric influence area may require that drivers decelerate in advance
of visual cues of the impending intersection.
No research was found that identified a relationship between crash frequency and mean or 85th-
percentile speed. However, many studies have found that the likelihood of being involved in a crash
increases with deviation from the mean speed of traffic on the facility. (Taylor and Foody, 1965;
Hauer, 1971)
If the facility and intersection design provide adequate sight distance and appropriate user
expectancy of potential conflicts, there should be adequate space and time to react to, and avoid,
crashes. If, however, speeds are excessive for the facility and intersection design, there will be
insufficient space and time to avoid crashes, and a high crash frequency may result.
High-speed travel may affect crash avoidance because faster moving vehicles travel farther
than slow moving vehicles during the typical reaction time needed for a driver to avoid a poten-
tial hazard. In addition, the greater a vehicle's speed, the less time there is for other motorists,
bicyclists, or pedestrians to react to, and avoid, that vehicle.
Thus, although there is no research to support the common assumption that reducing speeds
will reduce crash frequency, reducing speed variation may achieve this. The relationships
between intersection speed and safety are complex, and it cannot be assumed that reduced speeds
will result in a safety improvement.
2.4.4 Traffic Operations
Traffic operations refers to a roadway's performance and is typically measured in terms of
capacity, travel time, delay, number of stops, and queuing. Although vehicle speed directly cor-
relates to the motorists' perceived level of service along an arterial or highway segment, it does
not have a significant effect on the traffic operational performance of an individual intersection.
Furthermore, vehicle speed through the influence area of an intersection has little effect on over-
all travel time.
2.4.4.1 Capacity
Vehicle speed is not a primary determinant of intersection capacity--the number of vehicles
that the intersection can process in a given time period. For the case of a minor street left-turn
movement from a two-way, stop-controlled intersection, research performed as part of NCHRP
