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of it costing a considerable amount of money to re-check all obligation for a jurisdiction doing studies or periodic evalu-
existing curves within a jurisdiction and in terms of increased ations but rather provides a list of the factors to be considered.
liability, at least during a transition period. Some jurisdictions However, based on the results from the final practitioner sur-
may also selectively not adopt this definition, resulting in vey, the list of factors initially proposed has been modified in
long-term differences between jurisdictions--that is, less the final proposed statement given below.
consistency, not more.
Final Proposed Statement
THIRD RECOMMENDATION
Changes from the initial statement are shown in italic:
Initial Proposed Statement
Standard:
Add a section to the MUTCD (similar to section §2B.11
for setting speed limits in speed zones) to define the factors After an engineering study has been made in accordance with
to be considered when conducting an engineering study to established traffic engineering practice or where engineering
establish the appropriate TCDs when there is a change in the judgment determines the need for horizontal alignment signs,
advisory speed plaques, and/or supplemental guidance, these
horizontal alignment of the highway. The language should TCDs shall be used.
read as follows:
Standard: Guidance:
After an engineering study has been made in accordance with The factors that should be considered in determining the
established traffic engineering practice or where engineering system of TCDs to be displayed when there is a change in the
judgment determines the need for horizontal alignment horizontal alignment of the highway include
signs, advisory speed plaques, and/or supplemental guidance,
these TCDs shall be used. · The difference in the posted speed limit and the 85th-
percentile speed of free-flowing traffic (or a 16° ball-bank
Guidance: reading);
· The approach sight distance to the beginning of the curve;
The factors that should be considered in determining the · The visibility around the curve;
system of TCDs to be displayed when there is a change in the · Unexpected geometric features within the curve, such as
horizontal alignment of the highway include the following: an intersection or a change in the curve radius;
· Curve and roadway geometry;
· Accident history; and
· The difference in the posted speed limit and the 85th-
· As appropriate, the position of the most critical curve in a
percentile speed of free-flowing traffic;
sequence of relatively closely spaced curves.
· The approach sight distance to the beginning of the curve;
· The visibility around the curve;
· Unexpected geometric features within the curve, such as It is also recommended that further definition of the factors to
an intersection or a change in the curve radius; and be included in engineering studies related to horizontal curves
· The position of the most critical curve in a sequence of
relatively closely spaced curves. be included in appropriate sections of the Traffic Control
Device Handbook.
In the 2003 edition, the section on speed limit signs was It has been argued here that the existing definition of an
renumbered (from §2B.11 in the ME to §2B.13 in the 2003 "engineering study" is too general for the purposes of placing
edition) although the content remained basically the same. As TCDs on horizontal curves; it is thus recommended that the
noted, the factors to be considered in an engineering study are inclusion of a more explicit list of factors to be considered be
not explicitly defined in either the ME or 2003 edition. How- included in the definition. The current definition does little to
ever, in both editions, there are factors implied in the guidance help practicing engineers know what to look at when under-
section on the use of the advisory speed plaque (ME §2C.42; taking such a study. Indeed, there are other instances in the
2003 edition §2C.46): MUTCD where additional factors to be considered in other
situations are explicitly listed. For example, in §2B.13 regard-
ing speed limit signs, six factors are listed including road char-
Because of changes in conditions, such as roadway geo-
metrics, surface characteristics, or sight distance, might affect acteristics, pace speed, roadside development and environment,
the recommended speed, each location should be periodi- parking practices and pedestrian activity, and crash experience.
cally evaluated and the Advisory Speed plaque changed if Signal and other warrants are also, in essence, lists of factors
necessary. to be considered in engineering studies. In addition, just under
two-thirds of county road and DOT respondents to the final
In this context, defining the factors that should be considered survey regarding the efficacy of the suggested recommenda-
in an engineering study doesn't really increase the level of tions thought this recommendation should be adopted.
