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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.
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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.
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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.
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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).
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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.
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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.
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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.
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Table 2: Unbiased Survey Results. Question Choice Comments
Are there any circumstances which would
justify recommending a speed limit higher
that the 85th percentile?
67% - Yes,
33% - No
When improper limits have been set and not enforced,
it should be higher than the 85th percentile.
When setting speed limits to the 85th
percentile, should it be the rounded down or
up 85th percentile?
75% - Both should
be used, 25% -
rounded up
Depends on roadway characteristics.
Check the boxes of all the characteristics that
would warrant a speed limit lower than the
85th percentile:
N/A Adverse horizontal curves, poor gravel condition, high
number of crashes, roadside hazards, high number of
access points were chosen
If the recommended speed limit is lower than
the 85th percentile speed, check the values
that can be used:
67% - Yes,
33% - No
Lower bound of pace speed (55%), Minus 5 from the
85th percentile, Minus 10 from the 85th percentile.
Should the recommended speed limit have a
minimum value that can be used?
67% - Yes,
33% - No
Allowing or documenting this will most always get
posted. This may lead to artificially posted speed limits.
Should a ball bank test be performed on all
adverse horizontal curves for unsigned roads
before recommending a speed limit?
56% - Yes,
22% - No,
11% Maybe
Advisory on curves that are lower than the speed limit,
Consider chevrons and delineation instead.
Should the Average Daily Traffic (ADT) be
considered for recommending speed limits?
44% - Yes,
33% - No,
22% - Maybe
Effect of ADT is observed in the 85th tile, As long as
there is a valid sample size.
Should Average Daily Truck Traffic (ADTT)
be considered for recommending speed
limits?
56% - No,
22% - Yes,
22% - Maybe
Included in the 85%tile, Higher ADTT roads should
generally have lower limits.
Should the segment length be considered for
recommending speed limits?
44% - Yes,
44% - No,
12% - Maybe
Attempt should be made to reduce short speed zones
Should roadside clearance be considered for
recommending speed limits?
44% - Yes,
44% - No,
12% - Maybe
Influence is seen in the 85%tile, If roadside clearance is
less than 5 feet than lower the speed limit.
Should the roadway classification (ie, rural
major collector, rural minor collector, and
rural local road) be considered for
recommending speed limits?
44% - Yes,
44% - No,
12% - Maybe
Major roads should keep high speeds, collectors should
generally have higher speeds than local roads.
Should the type of adjacent land-use (i.e.,
residential, commercial, mining, agriculture
and industrial) influence the speed limit
decision?
44% - Yes,
66% - No
Influence is seen in the 85%tile, adjacent land-uses
with high truck traffic should have a speed limit less
than the 85th percentile
Should the surface condition of the road be
considered when recommending speed
limits?
78% - Yes,
22% - No
Influence is seen in the 85%tile, this will take care of
itself when collecting data, speed limit should be set for
ideal conditions.
Should the number of access points
considered for recommending speed limits?
67% - Yes,
33% - No
20 access points should warrant a reduced speed limit,
influence is seen in the 85th percentile.
Should the number of crashes on a road be
used when setting speed limits?
78% - Yes,
22% - No
Minus 10 mph form the 85th percentile, no lower than
30 mph
Table 3: Wyoming Unpaved Road Speed Data - 82 Observations.
50th
percentile
speed
85th
percentile
Speed
Average 31 40
85th
Percentile 41 52
Standard Deviation
Deviation
9.1 11.1
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Figure 2: Distribution of Test Sections by ADT – 82 Observations
After the presentation of Wyoming unpaved road data, nine case studies were presented
where each study had different characteristics associated with it. The group individually
evaluated the first 2 while the next 7 were evaluated collectively. A “round table” discussion
followed where members discussed the pros and cons of each major decision. Table 4 shows the
speed data that was used for the case studies. All of the speed data was taken from actual roads
in Wyoming. On average only 44% of the vehicles recorded were in the pace speed. This
observation confirms that there is a higher variability in speeds on unpaved roads than on paved
roads.
05
10152025303540
Nu
mb
er
of
Ro
ads
Average Daily Traffic (vehicles/day)
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Table 4: Speed Data for Case Studies.
Case Study/
Scenario
85th %
speed
50th %
speed
Lower Bound
of Pace (mph)
Upper Bound of
Pace (mph)
Percentage in
Pace speed
1 57.8 46.7 42.8 52.2 38.5%
2 38.3 29.6 25.8 35.7 42%
3 45 28.6 33.4 43.3 45.9%
4 45.8 35.6 24.9 34.8 36%
5 48.1 40.3 34.1 44 53.3%
6 40.9 30.7 26.7 36.6 36%
7 29.3 22 16.3 26.2 59.5%
8 47 39 35.5 45.4 48%
9 60.6 48.9 45 54.9 37.1%
Averages 45.9 35.7 31.6 41.5 44%
OUTCOMES OF THE FOCUS GROUP MEETING
The discussions at the focus group meeting were instrumental in developing the first draft of the
standards and guidelines. The group focused on four main aspects: the determination of an
appropriate speed limit, data collection, roadway safety, and declaration of speed limit. For each
major decision, a procedure was proposed and voted on by the group. A major concern of the
group was the data collection requirements for speed data. Unpaved roads are generally very
low volume roads which make some of the data collection techniques unfeasible.
Data Collection A big difference for setting speed limits on unpaved surfaces compared to paved surfaces is the
data collection. Most unpaved roads have very low volumes which presents challenges to the
traditional spot speed study. Also, weekends can bring larger volumes and less familiar drivers
onto the roads. This affects the speeds that are recorded and more importantly the safety of the
road. The focus group approved data collection procedures which incorporated many of the
aspects that effect unpaved roads.
Procedure 1 - For the engineering studies, vehicle speeds and traffic volumes should be
determined using automated traffic counters. For most low volume unpaved roads, seven
consecutive days of traffic counts are required. For unpaved roads with more than 200 vehicles
per day, 2 days of traffic counts may be adequate. The automated traffic count values that are
required are: 85th
percentile, 50th
percentile, upper bound of the pace speed, the Average Daily
Traffic (ADT) and the Average Daily Truck Traffic (ADTT).
A special form was developed to assist with the collection and analysis of the traffic
count data and the safety aspects of the road as shown in Table 5. The form includes five parts:
general information, automated traffic count values, roadway characteristics, historical crash
data, and range of speed limit values. In the form, the clear boxes correspond to the input that
trained technicians or engineers are required to provide. The gray shaded boxes correspond to
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Jones, Meena, and Ksaibati 11
the calculations and information that the registered engineer is required to provide. When
collecting speed study data, the general information should be completed before proceeding to
the next steps. The required automated traffic count values can be obtained from the output file
that the automated traffic counters produce. As shown in Figure 3, the output file summarizes
the different speed and traffic volumes that the automated traffic counters produce.
Table 5: Unpaved Roads Speed Limit Data Collection Form
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Figure 3: Automated Traffic Counter Output (11)
Roadway characteristics are collected during a field inspection of the road. The roadway
characteristics that were determined to be critical to the speed limit decision are:
Roadway Length
Number of Access Points
Roadway Width
Adjacent Land-Use
Type of Terrain
Roadway length was chosen so that speed zones in very short segments could not be set
using the standards and guidelines. Access points is an important characteristic because unpaved
roads have unplanned access points on the road that can create risk. On some unpaved roads, the
roadway width is limited which can make vehicles drive on the shoulder to accommodate two-
way traffic.
Adjacent Land-Use was chosen to associate the roadway with a demographic of drivers.
The choices for the adjacent land use are shown below and were obtained from AASHTO’s
Guidelines on Geometric Design for Very Low Volume Roads (12). The descriptions of each
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adjacent land use highlight the type of vehicles and drivers that can be expected to traverse the
road.
Rural major access roads serve a dual function of providing access to abutting
properties as well as providing through or connecting service between other local road or
higher type facilities.
Rural minor access roads serve almost exclusively to provide access to adjacent
property. The length of minor access roads is typically short. Because their sole function
is to provide access, such roads are used predominantly by familiar drivers.
Industrial or commercial access roads serve developments that may generate a
significant proportion of truck or other heavy vehicle traffic. These roads are classified
separately from minor access roads, which they otherwise resemble, because of the
consideration for trucks and other heavy vehicles.
Recreation and scenic roads serve specialized land uses, including parks, tourist
attractions, and recreation facilities, such as campsites or boat-launch ramps. Their users
are often unfamiliar drivers and serve recreational vehicles including motor homes,
campers, and passenger vehicles pulling boats and other trailers.
Rural resource recovery roads are local roads serving logging or mining operations.
Resource recovery roads are distinctly different from the other functional subclasses of
very low-volume local roads in that they are used primarily by vehicles involved with the
resource recovery activities and the driving population consists primarily or exclusively
of professional drivers with large vehicles.
Rural agriculture roads are used primarily to provide access to fields and farming
operations. Vehicle types that use such roads include combines, tractors, trucks that haul
agricultural products, and other large and slow-moving vehicles with unique operating
characteristics.
Type of terrain refers to the general character of the road. This characteristic helps look
at sight distance and the alignment of the road in general terms so it can encompass the entire
road. This allows the technicians or engineers to speed up the data collection process and not
have to measure all of the sight distance and alignment issues. There are three different types of
terrain that can be chosen:
Level – Highway sight distances, as governed by both horizontal and vertical restrictions,
are generally long or can be made to be so without construction difficulty or major
expense.
Rolling – Natural slopes consistently rise above and fall below the road or street grade,
and occasional steep slopes offer some restriction to normal horizontal and vertical
roadway alignment.
Mountainous – Longitudinal and transverse changes in the elevation of the ground with
respect to the road or street are abrupt, and benching and side hill excavation are
frequently needed to obtain acceptable horizontal and vertical alignment. (12)
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Roadway Safety The focus group decided that a general road safety evaluation should be considered when
determining speed limits. Such an evaluation should include the following:
If there are a high number of crashes on the roadway under investigation, then a detailed
crash analysis should be performed to find the causes of the crashes. Once the causes are
determined, then additional steps should be taken to provide safety enhancements for
high crash locations. Reducing the speed limit by itself will not automatically reduce
crashes.
The road should be driven at a reasonable and safe speed to determine if any curves
require vehicles to slow down. Further safety enhancements such as signage, delineation,
shoulder widening, and guardrails should be considered at adverse curves.
When horizontal curves are signed, they should include either chevrons, delineators or
curve warning signs. Speed advisory signs are not required on unpaved roads.
Speed limits should not be reduced due to the presence of isolated fixed objects such as
utility poles, trees, rocks or narrow cattle guards. Consideration should be given to using
object markers, removing the hazard or widening the road at those locations.
The speed limit should never be lowered to reduce the number of signs needed on a road.
The general roadway safety section was meant to provide recommendations rather than
outlining required procedures. An important decision was to not require that all curves have a
“ball bank test” performed. It was thought that it would take too much time and effort to analyze
every curve with a ball bank test. On unpaved roads, gravel buildup and constant grade changes
lead to a speed that is not consistent around curves. Also trucks can travel faster around curves
than smaller vehicles making the ball bank test car specific and difficult to advise a speed for all
vehicles. Because of this, it was recommended to sign the adverse curves with delineators or
chevrons instead of advisory speeds.
It is recommended that 10 years of crash data be included in the evaluation. The
following three types of crashes are evaluated: number of fatalities, number of injuries, and the
number of Property Damage Only (PDO). The Equivalent Property Damage Only (EPDO) can
then be calculated by using Equation 1.
(Equation 1)
Once the EPDO is calculated, the EPDO per mile is determined by dividing the EPDO by
the roadway length. If there are a high number of crashes, then a detailed crash analysis should
be performed to determine if speed is a factor in the crashes.
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Determination of Appropriate Speed Limit
After collecting all the required data, a registered engineer can recommend a speed limit for
unpaved roads to the county commission. When recommending the speed limit, the focus group
felt that the registered engineer should consider the following procedures:
Procedure 2 - The preferred speed limit is the 85th
percentile speed rounded to the nearest
5 mph.
Procedure 3 - A professional engineer can consider other roadway characteristics to
justify a lower speed limit. But the lowest acceptable speed limit may only be up to 10
mph less than the 85th
percentile speed.
Procedure 4 - The recommended speed limit on unpaved roads shall never be higher than
the statutory speed limit of 55 mph
Declaration of Speed Limit Form Posted speed limits (other than statutory speed limits) on unpaved roads that have been
established by an engineering investigation must be documented by a Declaration of Speed
Limit. A declaration of speed limit form is shown in Figure 4.
When declaring a speed limit, the declaration of speed limit form must be signed and
sealed by a professional engineer. The form, with appropriate background information, is then
presented to the county commission where a resolution number approving the proposed speed
limit is issued. The resolution number will then be added to the declaration of speed limit form.
The form is then returned to appropriate county officials so that the necessary signing changes
can be made. Enforcement of the new speed limit cannot occur until the appropriate signs giving
notice thereof are in place. It is also recommended that copies of the resolution be sent to the
local law enforcement agency(ies) for help with enforcement.
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Jones, Meena, and Ksaibati 16
Figure 4: Declaration of Speed Limit Form
FEEDBACK ON THE STANDARDS AND GUIDELINES Five different groups provided feedback on the developed standards and guidelines after the
focus group meeting. They are:
WYDOT Chief Engineer.
Local governments attending the Transportation and Safety Congress.
Wyoming Associations of County Engineers and Road Superintendents (WACERS).
Wyoming County Commissioners Association (WCCA).
General Public. The Chief Engineer of WYDOT wanted to make sure that the counties and
commissioners had input since they were going to be the ones in charge of implementing the
standards and guidelines. He also wanted the WCCA to have input on what the correct process
was for declaring a speed limit. During the presentation at the WYT²/LTAP Safety Congress, it
was advised that all speed limits set before the standards and guidelines were finalized could be
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considered valid and wouldn’t need a new study as long as they were done in accordance with
WYDOT’s procedures. The WACERS committee recommended that the lower bound of the
recommended speed limit be changed from minus 5 mph to minus 10 mph from the rounded 85th
percentile. It was brought up to a vote and all 11 of the committee members voted for the
change. The WCCA recommended that a resolution number be assigned to every new speed
limit. That way if it is brought up in court they can find the resolution number and find all the
supporting information that is needed. It also simplified the process of declaring a speed limit.
SPEED LIMIT WORKSHOPS As part of the implementation of the standards and guidelines, the WYT²/LTAP will provide
training workshops to train county technicians and engineers on how to set speed limits on
unpaved roads. WYDOT worked out an agreement so that the WYT²/LTAP can establish speed
limits on local roads, if the counties collect the necessary data. A workshop training manual was
developed to explain all the steps and procedures in more detail. The following process is a
general outline on how the Center will assist and train counties and municipalities with setting
speed limits on unpaved roads:
1. Automated Traffic Counters
a. The Center will provide training on the equipment and the software.
b. The Center will provide training on the location and set up of the equipment.
2. Roadway Characteristics
a. The Center will provide training on how to collect the roadway characteristics that
are required for the study.
3. Safety Evaluation
a. The Center will provide training on the safety evaluation that should take place
during the speed study. This includes identifying low cost measures for
improving the safety of unpaved roads.
4. Declaration of Speed Limit Form
a. The Center will provide training on the steps to be taken when declaring a speed
limit. This includes the required documentation and signatures.
SUMMARY AND CONCLUSIONS The lack of national standards for establishing speed limits on unpaved roads proved to be a big
challenge for this study. However it also provided an opportunity for Wyoming to assist other
agencies that are pursuing the same type standards. The standards developed in this study can be
adopted with minor modifications by any agency managing unpaved roads.
The standards and guidelines for establishing speed limits on unpaved roads were
developed through an extensive process that allowed all stake holders a chance to provide timely
feedback. The standards and guidelines provide Wyoming counties with a step by step process
and the documentation needed for establishing speed limits on unpaved roads.
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Jones, Meena, and Ksaibati 18
According to the developed standards and procedures the following steps should be
followed when establishing speed limits on unpaved roads:
A registered engineer or a trained technician can collect the necessary data. For the
engineering studies, vehicle speeds and traffic volumes should be determined using
automated traffic counters. For most low volume unpaved roads, seven consecutive days
of traffic counts are required. For unpaved roads with more than 200 vehicles per day, 2
days of traffic counts may be adequate. The automated traffic count values that are
required are: 85th
percentile, 50th
percentile, upper bound of the pace speed, the Average
Daily Traffic (ADT) and the Average Daily Truck Traffic (ADTT).
The road should be driven at a reasonable and safe speed to determine if any curves
require vehicles to slow down. Further safety enhancements such as signage, delineation,
shoulder widening, and guardrails should be considered at adverse curves.
Speed limits should not be reduced due to the presence of isolated fixed objects such as
utility poles, trees, rocks or narrow cattle guards. Consideration should be given to using
object markers, removing the hazard or widening of the road at those locations.
The preferred speed limit is the 85th
percentile speed rounded to the nearest 5 mph.
A professional engineer can consider other roadway characteristics to justify a lower
speed limit. But the lowest acceptable speed limit may only be up to 10 mph less than
the 85th
percentile speed.
The recommended speed limit on unpaved roads shall never be higher than the statutory
speed limit of 55 mph.
The declaration of speed limit form needs to be sealed by a registered engineer then
approved by the county commission with a resolution number.
The training workshops will provide comprehensive knowledge to the county engineers
and technicians and let them know exactly what is expected of them. It also provides an
opportunity for practitioners to understand why the procedures are needed. The safety aspect of
the workshop will provide an opportunity to illustrate safety tools that can be used on all county
roadways. A major benefit is that the WYT²/LTAP will analyze the data free of charge if the
counties collect the necessary data.
This project will help create more uniform speed limits on unpaved roads and give the
public confidence that they were set appropriately. The enforcement of the speed limits will be
more efficient because police officers will feel that they were set appropriately and will only be
ticketing the worst offenders. It will also give the enforcement and judicial system confidence that
speed infractions will be upheld in the court of law.
ACKNOWLEDGMENTS The authors of this paper would like to thank WYDOT, Wyoming Association of County
Engineers and Road Superintendents, and the Wyoming County Commissioners Association for
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 19
their contributions in helping develop the standards and guidelines for establishing speed limits
on unpaved roads.
TRB 2012 Annual Meeting Original paper submittal - not revised by author.
Jones, Meena, and Ksaibati 20
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Notice No. 2011-1. February 2011.
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1995.
10. Wyoming Department of Transportation. WYDOT traffic studies manual. 2009.
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12. AASHTO, Guidelines for Geometric Design of Very Low-Volume Local Roads.
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TRB 2012 Annual Meeting Original paper submittal - not revised by author.