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TABLE 3 Support for the second recommendation
Second recommendation: MUTCD §2C.42 should be changed to read as follows:
An Advisory Speed (W13-1) plaque should be used to indicate the advisory speed for a change in
horizontal alignment when the advisory speed is X mph or more below the applicable speed limit.
Employer
Overall Guideline Questions Answer national
county road state highway committee
commission department members
(n=61) (n=66) (n=9)
Do you believe the change from yes 63.9* 77.3 55.6
may to should in the second
recommendation should be
adopted? no 36.1 22.7 44.4
(n=57) (n=61) (n=5)
0 1.8 3.3 0.0
5 21.1 34.4 50.0
If the second recommendation
is adopted, what value of X 10 45.6 45.9 37.5
should be specified?
>10 26.3 14.8 12.5
other 5.2 1.6 0.0
*
All entries are percentages of total respondents.
defined in the specific context of horizontal curves. This is in half thought that the list was adequate "as is" with the rest
contrast to the provisions in other sections of the MUTCD, desiring to modify the list in some way--primarily by adding
which are explicit. One example is §2B.13 of the 2003 factors.
MUTCD, which provides factors to consider when setting Table 6 is the distribution of responses to a follow-up
speed limits. To that end, the third recommendation was in- question regarding the factors that should be included in an
tended to increase uniformity in the use of an engineering engineering study to determine the appropriate TCDs for
study by defining the term, at least in the context of horizon- horizontal curves. As noted in Table 6, there is some dis-
tal curves. Basic results are shown in Table 4. agreement in terms of what an appropriate study might in-
Both state and local practitioners supported this recommen- clude. The factors listed are interesting from several perspec-
dation (just greater than 60% of the respondents) although not tives. While some such as consideration of the accident history
quite as strongly as they did the first two recommendations. A make sense, the relative number of respondents who single
majority of the respondents from the national committee did out 85th-percentile speeds to be disregarded or at least dis-
not support the third recommendation, although the sample counted is somewhat alarming, especially in conjunction with
size is small. Respondents were also asked to indicate which the number who believe that continued emphasis should be
factors should be added or eliminated, if any. placed on the ball-bank indicator. The latter was without any
While comments per se were not solicited on the basic reference to interpreting the readings. Several respondents
question, about 10% of the respondents made one anyway. also mentioned the need for research in using the ball-bank
The comments were generally negative: "too cumbersome, indicator.
confusing, and nebulous"; "engineers [already] know what to
consider"; "not appropriate for MUTCD but perhaps the Fourth Recommendation (Expert System)
TCD Handbook"; and "didn't like the list presented."
Table 5 is an overview of what the respondents thought The fourth (or "other") recommendation was to develop an
about the adequacy of the list of study factors. Just less than "expert system" that could be used to provide guidance on
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TABLE 4 Support for the third recommendation
Third recommendation: Add a section to the MUTCD (similar to §2B.11 for setting speed limits in
speed zones) to define the factors to be considered when conducting an engineering study to establish the
appropriate TCDs when there is a change in the horizontal alignment of the highway. The language
should read as follows:
Standard:
After an engineering study has been made in accordance with established traffic engineering
practice or where engineering judgment determines the need for horizontal alignment signs,
advisory speed plaques, and/or supplemental guidance, these TCDs shall be used.
Guidance:
The factors that should be considered in determining the system of TCDs to be displayed when
there is a change in the horizontal alignment of the highway include
· The difference in the posted speed limit and the 85th-percentile speed of free-flowing
traffic;
· The approach sight distance to the beginning of the curve;
· The visibility around the curve;
· Unexpected geometric features within the curve, such as an intersection or a change in
the curve radius; and
· The position of the most critical curve in a sequence of relatively closely spaced curves.
Employer
national
county road state highway committee
Overall Guideline Question Answer commission department members
(n=60) (n=65) (n=8)
Do you believe the third
recommendation defining yes 61.7* 61.5 37.5
elements to be included in an
engineering study for TCD
application at horizontal curves
should be adopted? no 38.3 38.5 62.5
*
All entries are percentages of total respondents.
TABLE 5 Adequacy of study factors listed in the third recommendation
If answered "yes" regarding adoption of a recommendation for an engineering study for TCD
applications at horizontal curves, is the list of factors sufficient?
Employer
Possible Responses national
county road state highway committee
commission department members
(n=37) (n=42) (n=4)
list in recommendation is sufficient 51.4 * 42.9 25.0
add factors 32.4 38.1 50.0
eliminate factors 5.4 7.1 25.0
both add and eliminate factors 10.8 11.9 0.0
*
All entries are percentages of total respondents answering "yes" or providing suggestions.
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TABLE 6 Factors suggested for inclusion in horizontal curve study
Number of Respondents Concurring
Factors
5 factors in add to 5 eliminate
Guideline 3 factors from 5 factors
sufficient
the difference in the posted speed limit and the
85th-percentile speed of free-flowing traffic 37* -- 2
85th-percentile (part of above, but listed
separately as several wanted to eliminate
-- -- 5
reference to this speed)
the approach sight distance to the beginning of
the curve 37 -- 3
the visibility around the curve
37 -- 3
unexpected geometric features within the curve,
such as an intersection or a change in the curve
37 -- 1
radius
the position of the most critical curve in a
sequence of relatively closely spaced curves 37 -- 2
shoulder width
-- 2 --
shoulder type
-- 1 --
obstruction close to pavement
-- 1 --
gradient
-- 2 --
ball-bank study
-- 5 --
ball-bank study using 10°
-- 1 --
electronic ball-bank study
-- 1 --
85th-percentile speed of vehicles in curve
-- 1 --
accident history
-- 7 --
superelevation
-- 1 --
roadway volume or average daily traffic
-- 3 --
signing practices in area
-- 1 --
degree of curvature -- 1 --
advisory speeds. As shown in Table 7, support for this rec- becomes too prescriptive or that the discretion encompassed
ommendation fell below a majority for all respondents. in the term "engineering judgment" is compromised. A typi-
There may be some clues for the lack of support in the (12) cal comment was that "engineers should determine sign instal-
comments that respondents offered. First, some respondents lations." Still others, who made comments such as "not as
are not aware of the capabilities of an expert system or how part of MUTCD," did not understand its relationship to the
one works. This is clear from comments such as "do not MUTCD. Obviously, if such a system is developed, it will
understand what an `expert system' would be or what it have to be accompanied by extensive "marketing" so that end
would achieve" and "ball-bank indicators are already avail- users know what it is and how to use it. However, the danger
able and have been used extensively for this purpose." Other- exists for such systems to be used as a "black box" by un-
wise, some respondents worried that use of such a system sophisticated users.
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TABLE 6 (Continued)
length of curve
-- 1 --
correlation to no-passing zone
-- 1 --
vertical alignment
-- 2 --
curve advisory speed
-- 1 --
overall alignment characteristics of segment
-- 1 --
surface conditions
-- 2 --
road/lane width
-- 2 --
guardrail
-- 1 --
curve widening
-- 1 --
roadside hazards
-- 1 --
number of respondents not supportive of the third
48
recommendation
*
Note that 77 respondents were in favor of Guideline 3. Of those, 37 responded that the list of factors was
sufficient; the rest suggested one or more additions and/or deletions from the list.
TABLE 7 Support for the fourth recommendation
Fourth Recommendation: NCHRP should consider funding a project to incorporate these factors in an
expert system similar to the U.S. Limits system being developed to provide guidance on the speed limit to
be posted in speed zones.
Employer
Overall Guideline Questions Answer national
county road state highway committee
commission department members
(n=58) (n=62) (n=9)
Do you believe the fourth yes 44.8 * 46.8 22.2
recommendation regarding the
development of an expert system
should be pursued? no 55.2 53.2 77.8
*
All entries are percentages of total respondents.
