Developing Standards and Guidelines for Establishing Speed Limits on Unpaved Roads By Joshua R. Jones, E.I.T Traffic Engineer Wyoming T 2 / LTAP University of Wyoming Laramie, WY 82071-3295 Joel Meena, P.E. State Traffic Engineer Wyoming Department of Transportation 5300 Bishop Blvd. Cheyenne, WY 82003 Khaled Ksaibati, Ph.D., P.E. Director Wyoming T 2 / LTAP Department of Civil and Architectural Engineering University of Wyoming P.O. Box 3295 Laramie, WY 82071-3295 Word Count: 5243 + 9 Tables/Figures * 250 Words = 7493 Words Submitted for Presentation and Publication at the 91 st Annual Meeting of the Transportation Research Board Washington, D.C., 2011 TRB 2012 Annual Meeting Original paper submittal - not revised by author.
21
Embed
Developing Standards and Guidelines for Establishing Speed
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Developing Standards and Guidelines for
Establishing Speed Limits on Unpaved Roads
By
Joshua R. Jones, E.I.T
Traffic Engineer
Wyoming T2 / LTAP
University of Wyoming
Laramie, WY 82071-3295
Joel Meena, P.E.
State Traffic Engineer
Wyoming Department of Transportation
5300 Bishop Blvd.
Cheyenne, WY 82003
Khaled Ksaibati, Ph.D., P.E.
Director Wyoming T2 / LTAP
Department of Civil and Architectural Engineering
University of Wyoming
P.O. Box 3295
Laramie, WY 82071-3295
Word Count: 5243 + 9 Tables/Figures * 250 Words = 7493 Words
Submitted for Presentation and Publication at the
91st Annual Meeting of the Transportation Research Board
Washington, D.C., 2011
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 1
ABSTRACT
The lack of national standards for establishing speed limits on unpaved roads has put pressure on
local governments to make judgment calls on their road networks. Unpaved roads have unique
characteristics that differ from paved roads. They usually are very low volume and were not
necessarily designed by engineers. This study was conducted to determine the necessary
standards and guidelines for establishing speed limits on unpaved roads in Wyoming. The
process used national standards and other state practices to ensure a comprehensive methodology
was followed. A focus group comprised of traffic and safety engineers was formed to find an
efficient process for establishing speed limits on unpaved roads. The group focused on four
main aspects for establishing speed limits: data collection requirements, roadway safety, finding
the appropriate speed limit and the declaration of the speed limit. Once the standards and
guidelines were developed, five different groups were asked to provide feedback to ensure that
all stakeholders had an opportunity to review them. The findings of this study will help states
and counties follow a similar process or be able to adopt the standards and guidelines with minor
adjustments.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 2
INTRODUCTION Establishing speed limits remains one of the more controversial tasks for the traffic engineering
profession. Engineers, public safety officials, and others involved in setting and enforcing speed
limits may disagree on the appropriate balance between safety and road-user convenience while
considering conditions of topography, weather, road surface, adjacent activities, and traffic.
Motorists, other road users, and roadway neighbors have their own perspectives on this balance
and may or may not abide by the professionals’ judgments. If the regulation of speed is to be
effective, the posted limit must be generally consistent with speeds that drivers feel are safe and
proper. Enforcement is widely recognized to be crucial to the success of speed limits as a means
of making roads safer. If law enforcement officers and the courts are confident that speed limits
have been developed on a reasonable basis, their enforcement of the limits will be more
effective. Generally, speed limits should be set at levels that are self-enforcing so that law
enforcement officials can concentrate their efforts on the worst offenders.
As of 2008, over 32% of the 4,058,000 miles of roads in the United States had unpaved
surfaces (1). Currently, there are no national standards or guidelines for setting speed limits on
unpaved roads. This has put pressure on local governments and counties to make judgment calls
on the correct speed limit. A lot of states use the same criteria for setting speed limits on
unpaved roads as on paved roads. This can create inconsistencies since unpaved roads have
many different characteristics including: very low traffic volumes, inadequate signing, varying
road conditions, narrow shoulders, varying road width, and many roadside hazards. In addition,
unpaved roads frequently evolved from trails or farm access lanes and were not designed by
engineers.
In Wyoming a new law came into effect in 2011 that reduced the statutory speed limit on
unpaved roads from 65 mph to 55 mph. Title 31, Chapter 5 – Article 3 of the Wyoming Statutes
establishes the speed regulations for all public roadways in Wyoming. The limits specified in
this subsection established a maximum lawful speed on an unpaved roadway at 55 mph. Any
speed limit, other than a statutory speed limit that is posted on a Wyoming unpaved road, must
be based on an engineering study. That study should include an analysis of free-flow traffic
speeds and a general roadway safety evaluation.
To help counties implement the new law, the Wyoming Department of Transportation
(WYDOT) and the Wyoming Technology Transfer Center (WYT²/LTAP) developed standards
and guidelines for establishing speed limits on unpaved roads. The standards and guidelines were
developed following a similar process that national agencies have used for paved roads. This
entailed a focus group that developed a decision process for establishing different speed limits.
Five different groups were contacted to get the necessary feedback from those who would be
directly impacted by the new standards and guidelines. These included WYDOT, local
governments at the transportation and safety congress, Wyoming County Commissioners
Association (WCCA), Wyoming Associations of County Engineers and Road Superintendents
(WACERS), and the general public.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 3
The main objective of this paper is to outline the standards and guidelines which were
developed. The standards establish proper study procedures for completing the engineering
investigation in accordance with the statutes, provide guidance in setting appropriate speed limits
based on the study data, and specify the documentation and notification procedures needed when
establishing new speed limits on unpaved roads. These standards and procedures summarize the
minimum requirements to establish speed limits on unpaved roads. A professional engineer can
consider additional information when establishing speed limits. The guidelines described in this
paper can be implemented by other agencies managing unpaved roads.
NATIONAL STANDARDS AND GUIDELINES The Manual on Uniform Traffic Control Devices (MUTCD) states that “when a speed limit is
posted, it should be within 5 mph of the 85th
percentile speed of free-flowing traffic”. However,
the manual indicates other factors may influence the appropriate speed limit, including: roadway
characteristics such as shoulder condition, grade alignment, and sight distance; pace speed;
roadside development and environment; parking practices and pedestrian activity; and reported
crash experience for at least a 12-month period (2). Even though the MUTCD indicates that
these other factors can influence the speed limit decision, it does not quantify the impact of those
factors on the final speed limit decision.
The National Cooperative Highway Research Program has developed a web based
application called US LIMITS2 which recommends a speed limit on a specific road based on
information from the user. The application was developed in part to quantify the other roadway
characteristics besides speed data. US LIMITS2 calculates two speed limits; the first one
depends solely on roadway characteristics and the second one is calculated by considering crash
data.
Figure 1a shows the decision tree developed by US LIMITS2 for setting speed limits on
roadways in undeveloped areas. This was highlighted because unpaved roads are more related to
roadways in undeveloped areas than any of the other US LIMITS2 categories. The decision tree
starts with user input to help calibrate the model. It then asks the user if crash data is available
and if it is, the model calculates two speed limits. Speed limit one is based solely on the
roadway characteristics. For road sections in undeveloped areas, the roadside hazard rating is the
only road characteristic that is analyzed. The rating looks at the width of clear zone, side slopes,
and recoverability of run off the road situations. The ratings rank from 1 being the safest to 7
having the most risk associated with it. Figure 1b shows how the speed limit one is chosen based
on the rating number. The second speed limit is found by comparing crash data on similar roads
and if the road section under consideration has a higher than expected crash rate, then the second
speed limit is usually set lower than the 85th
percentile speed. The final speed limit is then found
by taking the lower of either values.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 4
Figure 1a: Decision Process for setting speed limits in Undeveloped Areas (3).
Figure 1b: Speed Limit one Calculation on a Roadway Section in Undeveloped Areas (3).
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 5
Some of the shortcomings of the US LIMITS2 system for setting speed limits on unpaved
roads include the following:
There is no unpaved road option.
General process may not apply to Very Low Volume Roads (ADT < 400).
Roadside hazard rating may not apply to unpaved roads.
Higher variance between the 85th
and 50th
percentile speeds on unpaved roads is not
considered.
SPEED LIMIT PRACTICES IN OTHER STATES Michigan recently passed a law limiting speed limits on unpaved roads based on the number of
access points per half mile, as shown in Table 1. This law will stay in effect until Michigan State
Police finish their final report on guidelines for setting speed limits on gravel roads.
Table 1: Michigan Speed Limit Law on Gravel Roads (4).
Design Speed
Access Points
per 0.5 miles
25 60
35 45 - 59
45 30 - 44
55 < 30
In a study conducted by the Michigan DOT, The following recommendations were made
with regard to establishing speed limits:
Posting speed limits within 5 mph of the 85th percentile speed has a beneficial effect,
although small, on reducing total crashes but has a major beneficial effect on providing
improved driver compliance.
Posting speed limits more than 5 mph below the 85th percentile speed does not reduce
crashes and has an adverse effect on driver compliance.
Speed zoning should not be used as the only corrective measure at high crash locations in
lieu of other safety improvements (5).
An extensive gravel road study was done by Kansas State University in 2009. That study
focused on developing a system for determining the speed limit for a particular road section.
They found that the speeds being traveled on gravel roads had less to do with the actual speed
limit and were affected more by roadway geometrics, roadway characteristics, and drivers’
perceptions of safety (6). The study also found that some state DOTs, such as Oregon and
Minnesota, feel that because the conditions of gravel roads are always changing, it is not logical
to place a speed zone on such roads.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 6
CalTrans doesn’t provide guidelines for setting speed limits on unpaved roads but does
offer a general process for setting speed limits on their roads. When a speed limit is to be posted
in California, it should be established at the nearest 5 mph increment of the 85th
percentile speed
of free flowing traffic. The posted speed may be reduced by 5 mph from the nearest 5 mph
increment of the 85th
percentile speed. If the 5 mph reduction is applied, the conditions and
justification for the reduced speed limit need to be documented in writing and be approved by a
registered civil or traffic engineer (7).
Texas DOT sets their speed limits based on the 85th
percentile and doesn’t allow the
practitioner to lower it. The procedure states that “Arbitrarily setting lower speed limits at point
locations due to a perceived shorter than desirable stopping sight distance is neither effective nor
good engineering practice (8).”
For low volume roads, the Vermont DOT uses several time runs to estimate the speed
instead of gathering a sample of 100 vehicles because it might require too much time to obtain
the sample. They require three forms to be completed for every speed study, including those on
unpaved roads, to be able to verify in court that it was done to standards. This helps give
enforcement agencies the confidence to issue speeding tickets knowing that the speed limit can
be verified. They also state that a spot hazard such as a sharp curve or intersection shouldn’t be
used to lower the speed limit. An established speed limit is a speed that will allow the driver to
react to a variety of situations but does not guarantee that the speed can be maintained
throughout the zone (9).
WYDOT currently has standards and procedures for setting speed limits on paved roads.
Their policy follows the MUTCD fairly closely but sets the lower bound of the pace speed as the
lowest acceptable speed limit. The pace speed is the 10 mph increment that encompasses the
highest proportion of observed speeds. When there is a wide range that a recommended speed
limit can be set to, the speed limit decision relies mostly on the engineer’s judgment. This can
create inconsistencies of how speed limits are set across the state (10).
METHODOLOGY The WYT²/LTAP and WYDOT followed a systematic process to develop the standards and
guidelines for establishing speed limits on unpaved roads. It was important to get feedback from
various organizations to insure successful implementation. This was critical since unpaved roads
are usually the responsibility of local governments. The following is the process that was used:
Conducted analysis on the WYT²/LTAP gravel roads traffic data.
Met with WYDOT district traffic engineers.
Developed draft standards.
Obtained approval from WYDOT’s chief engineer.
Obtained counties’ feedback at the WYT²/LTAP Safety Congress.
Obtained county engineers’ and road supervisors’ feedback at the WACERS meeting.
Presented the standards and procedures to the Wyoming County Commissioners
Association.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 7
Incorporated input from the general public.
Implemented the standards and guidelines on July 1, 2011.
FOCUS GROUP The guidelines and standards were initially developed by conducting a focus group meeting at
the statewide traffic engineers meeting in Wyoming. The meeting had traffic and safety
engineers from multiple agencies as shown below:
State Traffic Engineer
District Traffic Engineers
FHWA Safety Engineer
State Highway Safety Engineer
WYT²/LTAP Engineers
Unbiased Survey
The meeting began by asking the group to fill out an unbiased survey. This was done to get the
individual members of the group to independently form opinions on setting speed limits on
unpaved roads before the meeting started. Table 2 shows the questions that were asked, the
percentage of participants’ choices, and any comments that were included. The group selected
the minimum value for the speed limit, access points, number of crashes and surface conditions
as critical variables when setting speed limits. A frequent comment was that many of the
variables are reflected in the 85th
percentile and don’t need to be considered separately as factors.
WYT²/LTAP Gravel Road Study The next step was a presentation of the Wyoming unpaved road speed data that was collected by
the WYT²/LTAP. Information on 82 Wyoming unpaved roads was collected to gain a better
understanding of traffic volumes and speeds. Table 3 shows that the average 50th
percentile
speed recorded was 31 mph while the average 85th
percentile speed was 40 mph. This shows
that, on average, there is a 9 mph gap between the two percentiles and showed that the 50th
percentile may be too low for the lower bound of the speed limit.
To understand the challenges of data collection requirements for unpaved roads, Figure 3
was presented to review the distribution of unpaved roads by ADT. It shows that the majority of
unpaved roads in Wyoming have less than 100 vehicles per day.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.