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A couple of general recommendations emerge from the lit- began, the 2003 edition of the MUTCD was not in wide use
erature review. The first is the need to define more explicitly and, indeed, some practitioners weren't even aware of the
what constitutes an "engineering study" in the context of millennium edition. It still had not been officially adopted or
horizontal curves. Items that should likely be included in used by all jurisdictions in mid-2005. So, somewhat arbitrar-
such a study for TCD selection and placement at horizontal ily, the millennium edition was used as the default "standard"
curves include consideration of the following: throughout the project--e.g., as a reference point for practi-
tioner survey instruments. Differences between the millen-
· How other potentially limiting factors such as weather nium and 2003 edition are addressed in later chapters.
conditions would affect curve perception and vehicle
performance;
· Natural features of the road and roadside environment PRACTITIONER FOCUS GROUPS
that provide delineation;
· Alignment that drivers may find difficult to perceive or The practitioner focus group exercises were held in three
interpret; states: Michigan, Indiana, and North Carolina. During the
· Driver expectations; exercises, the practitioners who were responsible for specify-
· Application of both the preview sight distance concept ing and implementing TCD treatments for horizontal curves--
and 85th-percentile speed to longitudinal warning sign including both engineers and technicians--were brought
placement guidelines; together to discuss those treatments. The focus of the dis-
· Degree of curvature and other parameters associated cussion was on local, typically county-level, two-way, two-
with high-risk alignments; lane, rural roads. Topics included the adequacy of existing
· Guidelines for when redundant warning or warning along guidelines for the specification and placement of various
with delineation is recommended; and TCDs, the adequacy of currently used TCDs, the identifica-
· Guidelines for when curve warning is not recommended tion of "problem" curves, the use of engineering judgment
based on curve or volume parameters apart from crash and studies, the appropriateness of advisory speeds and meth-
history. odologies for setting them, and the reasons for perceived
differences in treatments among jurisdictions and general
The second need is to recommend advisory speeds based inconsistency in the use of curve-related TCDs. In all, there
upon driver comfort or an assessment of how fast drivers are were four formal focus-group exercises (the number of prac-
already driving. Associating current operational speeds under titioners attending is shown in parentheses): two in Michigan
ideal conditions with curve properties and ball-bank indicator (15, 9) and one each in Indiana (5) and North Carolina (10).
ratings would suggest a more realistic and, perhaps, a more In addition to the formal focus groups, there were also two
respected advisory speed recommendation. separate interview sessions with practitioners in North Car-
Implicit in the above is the need for consistent messages olina and one in Tennessee. The interview sessions included
to be given to drivers. While drivers may not respond as well only one to three participants and therefore lacked the inter-
to different speed advisories as engineers would like insofar active nature of the focus groups. Nonetheless, some useful
as drivers don't slow down to the advisory level, it seems information was gleaned from the sessions.
clear from prior studies that there is some response. The four focus-group exercises provided valuable infor-
Given the findings from the literature review, it was neces- mation regarding the state of the practice for using curve-
sary to ascertain the current state of the practice relative to the related TCDs. While all groups were familiar with and regu-
specification and use of curve-related TCDs as well as deter- larly used the MUTCD or a state-level variant as a guide for
mining how drivers respond to them. For example, when and their curve treatments, some agencies supplemented them
how are curve-related TCDs used and is that use consistent with additional materials that were not necessarily written.
across different jurisdictions? Likewise, do drivers use the For example, there was a guideline in some Michigan coun-
information from TCDs as intended and are the TCDs per- ties that indicated that chevrons should be used on any curve
ceived to be used consistently across jurisdictions? requiring a speed advisory. As a general statement, the
The next sections document the tasks that were undertaken MUTCD was perceived to provide sufficient guidance but
to determine the perceptions of practitioners and drivers with leave room for judgment on the part of the practitioner. In
respect to the TCDs used for horizontal curves: are they ade- this context, the standard curve signs (W1-1 through W1-8)
quate, are they used consistently, and should guidelines for were considered sufficient in most situations. The winding
use be changed? Practitioner input was captured through the road (W1-5) and reverse curve or turn signs (W1-3, W1-4)
use of focus groups, discussions in several states, and a cause some confusion in application, especially when advi-
nationwide survey. sory speed plates are to be used. The confusion results from
It should be noted from the outset that there were some the determination of which curve in a sequence should be the
problems throughout the project with specifying exactly basis for the advisory--the first or the most severe? Beyond
which edition of the MUTCD is relevant. When this project the standard curve-related signs, RPMs, chevrons, and large
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target arrows appeared to be widely used as supplemental comed. At the same time, there are problems with such sim-
devices. ple guidelines unless the words "should" or "shall" are used.
Engineering judgment, as defined by the practitioners, is One Michigan group mentioned that such guidelines might
decisionmaking based on engineering or other technical train- also help to prevent over-signing of curves.
ing, experience, and/or common sense although the latter is Virtually all practitioners noted that horizontal curves are
defined by the practitioners. While there was an implicit under- most dangerous to drivers in combination with other risk fac-
standing of engineering judgment and engineering studies, tors such as driveways, vertical curves, geometric or design
it was clear that not many studies were done to support the problems such as inappropriate superelevation, weather, and
TCD-related judgments that were routinely rendered. In many anything that leads to a violation of driver expectancy.
instances, it appears that the day-to-day specification of TCDs As stated earlier, one of the objectives of this project was
for curves is left to technical support staff and not necessarily to identify the need for an improved methodology to deter-
done by engineers except in atypical cases. mine what information about horizontal curves is needed and
To assess appropriate advisory speeds, most of the practi- how best to communicate it to drivers in a consistent and
tioners used a ball-bank indicator in some way. Based on credible manner. From the practitioners in the focus groups
more informal discussion with some of the practitioners, it is and interviews, it is clear that inconsistency in use and de-
suspected that some practitioners still often use a "tried and ployment of curve-related TCDs is perceived to exist. How-
true" method of simply driving the curve to assess the need ever, the reasons for this inconsistency were thought to be
for an advisory plate. Notwithstanding the assertion that ball- related to budgets, personnel turnover, the exercise of engi-
bank readings are typically used to assess appropriate advisory neering judgment, and differences in topography between
speeds, practitioners generally agreed that almost all curves areas rather than differences in interpreting manuals or other
signed with advisory speed plates can easily and safely be rules or methodologies. Overall, there was a disconnect re-
traversed at "+10" mph over the posted advisory speed with garding the need or desire for well-defined guidelines or
the possible exception of roads in mountainous areas. The rules--while it was allowed that more refined or explicit
practitioners also believe that drivers perceive the advisory guidelines would lead to increased consistency, many, if not
speeds for curves in the same way: as a general guideline that most, practitioners still valued flexibility and discretion
says that one should slow down. It was not clear, however, if based on "judgment," and some objected to guidelines that
any of the practitioners had ever actually done a study at a could lead to liability issues if they were not followed.
curve to see whether deployed TCDs were effective in getting The practitioners in the focus groups believe that the worst
drivers to slow down or "comply" with the suggested advisory kinds of curves are those where there is a combination of the
speed. At the same time, there were no advocates for changing curve with other risk factors, a condition that does not lend
the perception that posted advisory speeds were "too low"-- itself to a straightforward or necessarily uniform solution.
in fact, the opposite view was typically expressed. These concerns are not easily converted into new or improved
All groups of practitioners agreed that there is some vari- guidelines or, at least, not into very explicit ones.
ance in curve-related TCD selection and placement in their With regard to advisory speeds, most practitioners ap-
state although not within their own jurisdiction or other geo- peared to use the ball-bank indicator. While the "rules" that
graphic area for which they are responsible. Most typically, they used varied, there was consistency in the perception that
practitioners thought their own practice and/or practice within advisory speeds were typically lower than they need to be.
their jurisdiction was consistent but that "other jurisdictions" However, in the discussions, no alternative method for assign-
introduced inconsistency. The Michigan group indicated that ing the advisory speed received endorsement although some
this is partially due to the flexibility afforded by "engineer- were opposed--for example, some practitioners thought that
ing judgment" but mostly because of the differences in bud- drivers already go too fast on curves and thus opposed using
gets allocated to TCD deployments. The latter resulted in average or 85th-percentile speeds as being "biased" toward
practitioners in some jurisdictions being "comprehensive" the high side. As a general rule, all practitioners indicated that
when it came to signing and marking horizontal curves. in their view, drivers expect advisory speeds to be low and
Others tried to get by with a bare minimum of TCDs. It was often exceed them. (This latter point was clearly not some-
the general feeling that additional guidance in placing TCDs thing that the practitioners had either done studies on or col-
is needed, but it was fairly clear that the guidance that the lected any data to support their view.) Changing the method
MUTCD does offer was not necessarily being followed. There of determining the numeric value of the advisory speed to be
was also a concern noted that more clearly expressed guide- more consistent may not be necessary since different methods
lines might lead to more litigation. This was interesting in yield the same results. Changing practice to be more credible
Michigan given that there is currently a high degree of legal would imply raising the advisory speed in most cases. This
protection for TCD use by practitioners and jurisdictions. would actually lead to larger inconsistencies in the short term,
Given that in many county departments signing curves is a and the risk associated with such a wholesale change in prac-
small part of a typical engineer's job, charts or tables that tice and driver education may not be worth taking. Practi-
would make the job easier and more consistent would be wel- tioners should be consulted once a standard methodology is
