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Recent Roadway Geometric Design Research
foR Improved Safety and Operations
Summary Since the publication of NCHRP Synthesis 299 in 2001, a considerable amount of research
related to operational and safety effects of highway and street geometrics has been com-
pleted. Application of this knowledge is sometimes limited because of the sheer volume of
information that exists and the rapid pace in which it is produced and published. Geometric
design research results are scattered across a variety of different tools and publications that
are not easily accessible to highway and street geometric designers and geometric design
policy makers. This synthesis identifies and summarizes roadway geometric design literature
completed and published from 2000 through early 2011, particularly research that identi-
fied impacts on safety and operations. Findings within the synthesis are presented in groups
similar to key chapters and sections within AASHTO's A Policy on Geometric Design of
Highways and Streets (commonly referred to as the Green Book). Some key findings are
provided here.
It is important to note that the recommendations included in the list of findings from the
literature shown here and throughout the report are those of the authors cited. Before any
revisions to AASHTO's Green Book were to be made on the basis of these recommenda-
tions, they would need to be considered on the basis of the rigor of the research and logic that
underlie them. No endorsement of these recommendations is implied by their inclusion in the
listing of findings from the literature.
· Review of existing long-standing guidelines was a common theme, evaluating whether
changes in vehicle performance or driver behavior necessitated changes in design prac-
tices. In many cases, vehicle performance did not affect the perceived appropriateness
of guidelines, although changes in headlamp performance did prompt a recommended
change in sag curve design. Driver behavior, however, was the source of several sug-
gested changes, including perceptionreaction time for stopping sight distance and con-
sideration of older drivers for intersection sight distance.
· During this period, finding ways to make intersections more efficient was also a frequent
topic of research. The use of modern roundabouts in the United States has grown tremen-
dously, leading to two comprehensive FHWA Informational Guides, which are sum-
marized in the body of the report. Innovative intersection designs that seek to improve
capacity by adjusting left-turn movements were also often investigated. These designs
were often shown to have increased capacity under certain conditions, but they typically
require additional right-of-way and increased construction costs to install.
· Many of the research topics found in the assembled body of knowledge were not directly
investigating the characteristics of a particular geometric design element; rather, common
topics were traffic control devices, access management techniques, or other treatments
that had a relationship with one or more design elements, and the research investigated
what effects, if any, the design had on the treatment, or vice-versa.
· A growing trend is research that attempts to quantify the safety effects of geometric
design elements. Crash modification factors and similar metrics have been developed
in an attempt to directly relate safety to design; the first edition of AASHTO's Highway
Safety Manual is a comprehensive source of such measures on a wide variety of treat-
ments and countermeasures, including those that are geometric in nature.
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This report is a synthesis of research, not of current or implementable practice. Therefore,
the study did not employ a survey or questionnaire on current practices, as is typical for
NCHRP synthesis projects. The study used two approaches to identify information: (1) a
review of the literature contained in national databases, and (2) a request to state design and
traffic engineers to supply additional information on studies conducted within their juris-
dictions. The national literature review represented the vast majority of the effort for this
synthesis study. TRB's Transportation Research Information System (TRIS), the Transport
online database, and the TRB online publications catalog were all used to identify potential
sources from papers and reports published during the previous decade.
Findings from research conducted during the decade addressed a variety of issues related
to geometric design. A selection of key findings included:
· Dimensions of commonly used trucks have changed in recent years, prompting recommen-
dations to revise the dimensions of those vehicles in the Green Book (Harwood et al. 2003a).
· Along with changes in dimensions have come changes in performance; however, design
guidelines are sufficient to accommodate their performance for many design elements
(Harwood et al. 2003a).
· Posted speed limit and anticipated operating speed were frequently associated with the
selection of design speed (Fitzpatrick and Carlson 2002).
· Observation of driving behavior revealed that the strongest indicator of operating speed
was posted speed limit. "Design speed appeared to have minimal impact on operating speeds
unless a tight horizontal radius or a low K-value was present" (Fitzpatrick et al. 2003a).
· New values for stopping sight distance and new design controls for vertical curves were
recommended, based on a perceptionreaction time of 2.5 s, a 10th percentile decelera-
tion rate of 11.2 ft/s2, a 10th percentile driver eye height of 3.5 ft, and a 10th percentile
object height of 2.0 ft (Fambro et al. 2000).
· Increased consistency between AASHTO design standards for passing sight distance
and Manual on Uniform Traffic Control Devices (MUTCD) pavement marking practices
was recommended, specifically accomplished by using the MUTCD criteria for marking
passing/no-passing zones on two-lane roads in the Green Book's passing sight distance
(PSD) design process. In addition to providing the desired consistency between PSD
design and marking practices, two-lane highways could be designed to operate safely
with the MUTCD criteria (Harwood et al. 2008).
· Lane widths of 11 or 12 ft provide optimal safety benefit for common values of total
paved width on rural two-lane roads. Although 12-ft lanes appear to be the optimal
design for 26- to 32-ft total paved widths, 11-ft lanes perform equally well or better than
12-ft lanes for 34- to 36-ft total paved widths (Gross et al. 2009).
· Crash data on roads treated with centerline rumble strips or shoulder rumble strips
revealed noticeable crash reductions on all classes of roads (rural and urban two-lane
roads and freeways). Shoulder rumble strips placed as close to the edgeline as possible
maximize safety benefits. The safety benefits of centerline rumble strips for roadways
on horizontal curves and on tangent sections are for practical purposes the same (Torbic
et al. 2009).
· A minimum skew angle of 15 degrees can accommodate age-related performance defi-
cits at intersections where right-of-way is restricted (Staplin et al. 2002).
· Adding "left-turn lanes is effective in improving safety at signalized and unsignalized
intersections," reducing crashes between 10% and 44%. Positive results can also be
expected for right-turn lanes, with reductions in total intersection accidents between 4%
and 14% (Harwood et al. 2002).
· A series of projects during the decade led to the publication of two FHWA Informational
Guides containing recommendations and guidelines for all aspects of roundabout design.
· A number of innovative intersection designs were considered, many of which showed
benefits in capacity and/or delay, but the additional right-of-way needed to construct
each of these innovative designs is a potential drawback.
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· "ADA requires that new and altered facilities constructed by, on behalf of, or for the use
of state and local government entities be designed and constructed to be readily accessible
to and usable by individuals with disabilities" (Rodegerdts et al. 2004).
· Spacing assessments indicate that ramp spacing of less than 900 ft is likely not geometri-
cally feasible. That spacing value increases up to 1,600 ft for entranceexit ramp pairs
(Ray et al. 2011).
Results from the research synthesized in this document recommended a number of
changes to the AASHTO Green Book, the MUTCD, and other guidance documents. The
research also produced two FHWA Informational Guides on roundabouts and contributed to
other guides on access management, pedestrian and bicycle accommodation, designing for
older drivers, the Highway Capacity Manual, and the Highway Safety Manual. Discussion of
the relationship between this synthesis report and other documents, along with relevant cross
references, is also provided.
