DSC TOC
Proceedings of IMECE2002
ASME International Mechanical Engineering
Congress
& Exposition
Proceedings
of IMECE2002
November
17-22,
2002,
New
Orleans,
Louisiana
ASME International Mechanical Engineering Congress & Exposition
November 17–22, 2002, New Orleans, Louisiana
IMECE2002-32479
IMECE2002-32479
ROBOTIC HIGHWAY SAFETY MARKERS
Shane M. Farritor
Assistant Professor
Department of Mechanical Engineering
University of Nebraska
Lincoln, NE 68588
sfarritor@unl.edu
ABSTRACT
Proper traffic control is critical in highway work zone safety.
Traffic control devices such as signs, barricades, cones, and
plastic safety barrels are often used. Accidents can occur
because of improper work zone design, improper work zone
housekeeping, and driver negligence. One solution is to
automate safety devices.
Mark E. Rentschler
Research Assistant
Department of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, MA 02139
mrentsch@mit.edu
Proper traffic control is critical to work zone safety. Traffic
control devices such as signs, barricades, traffic cones, and
plastic safety barrels are often used.
This paper presents a mobile safety barrel robot. The
Robotic Safety Barrels are the first elements of a team of
Robotic Safety Markers (RSM) that includes signs, cones, and
possibly barricades and arrestors.
To be practical the system must be reliable and have a low
per robot cost. A robot that malfunctions could enter traffic and
create a significant hazard. Also, multiple safety markers are
used and barrels are often struck by vehicles. Safety markers
with a high replacement cost are not practical.
This paper describes the motivation for the robotic safety
marker system and how it could improve work zone safety. The
design of three robot prototypes is presented. A control
architecture is discussed that has been implemented in
simulation and partially tested on the prototype robots.
INTRODUCTION
The safety of highway construction and maintenance workers
is extremely important. For every billion dollars spent on road
construction, more than 33 people die in accidents related to
that work [1]. Studies have found that accidents in work zones
have increased by 6.8% in the some states and as much as 119%
in the state of Virginia [2]. The nation’s highway system is
aging and is in need of constant repair. Increased maintenance
and construction will result in more injuries and more fatalities
unless new ways are found to prevent these accidents.
Figure 1: Robotic Safety Barrel
One method to improve work zone safety is to implement a
smart work zone consisting of automated devices that improve
safety. This paper presents a mobile robotic safety barrel,
Figure 1. Safety barrels are placed on the periphery of the work
zone to guide traffic and to serve as a visible barrier between
traffic and work crews. These barrels consist of a brightly
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Copyright © 2002 by ASME
colored plastic drum (approximately 130cm high and 50cm in
diameter) that is attached to a heavy base. Often, hundreds of
barrels are manually placed in a typical work zone. The
Robotic Safety Barrel (RSB) replaces the heavy base of a
typical safety barrel with a mobile robot. The mobile robot can
transport the safety barrel and robots can work in teams to
provide traffic control.
constant presence of safety markers leads to driver
complacency. In addition, deployment and retrieval activities
are extremely dangerous since the construction worker is very
close to passing motorists. In this country, the cost of
congestion to motorists in lost productivity is estimated to be
approximately $100 billion annually, not including the cost of
wasted fuel and environmental damage [3].
Independent, autonomous barrel motion has several
advantages. First, the barrels can self-deploy, eliminating the
dangerous task of manually placing barrels in busy traffic.
Second, the barrel positions can be quickly and remotely reconfigured as the work zone changes.
Barrels could
continuously follow work crews to maintain optimal placement
for safety.
Passing motorists are also partly to blame for accidents in
construction zones. Typically, motorists only slow 10-mi/hr
upon entering a work zone [4], therefore it is very important to
make the driver aware of dangerous conditions.
The Robotic Safety Barrels are the first elements of a team of
Robotic Safety Markers (RSM) that includes signs, cones, and
possibly barricades and arrestors.
To be practical the Robotic Safety Marker System must be
reliable and have a low per robot cost. A robot that
malfunctions could enter traffic and create a significant hazard.
The system will be designed so critical components (software
and hardware) have multiple redundancy. Cost per robot is also
critical. Multiple safety markers are used and barrels are often
struck by vehicles. The system is designed so that each
individual robot is simple, reliable, and inexpensive.
The proposed approach may have a higher equipment cost
than traditional systems, however, there are possible cost
reductions in labor and increased worker safety. The approach
is not fitted for every work zone but there are many situations
where it would be useful and practical.
BACKGROUND
Accidents can occur because of improper work zone design,
improper work zone housekeeping, and driver negligence.
Steps are being taken to improve work zone design. Many
agencies now require written safety plans. Also, technology is
being used to improve communication in work zones, leading to
better work control [1].
Work zone housekeeping may be the most important element
in reducing accidents [1]. A good safety plan is not helpful if it
is not properly executed. Work zone housekeeping involves
tasks such as covering and uncovering signs and moving
markers (barrels, cones, signs) as the work progresses.
While housekeeping is important, it comes at a very high
cost. Worker time is wasted setting up and tearing down
hundreds of construction zone safety devices. Safety markers
often need to be deployed and retrieved at the beginning and
end of each workday. Removing the markers or covering them
is time-consuming, but leaving them in open view around the
clock adds to unsafe conditions. Safety markers that are simply
moved to the side of the road remain highly visible. The
There are many ways to increase worker safety. For
example, every road under repair could be completely closed to
public traffic. Another solution may be to simply increase the
number of safety cones and barrels in work zones to better
channel traffic. While these solutions decrease the threat of
injury, the cost of materials, workers’ time, and driver
inconvenience is large. Obviously a reasonable balance must be
found between worker safety, cost, and driver convenience.
RELATED RESEARCH
There is much research investigating “smart” highway
systems and work zones. A large body of research exists on the
Intelligent Vehicle Highway Systems (IVHS). This system uses
information technology to provide information to travelers,
improve traffic control and congestion, and increase the
efficiency of commercial vehicle and transit operations [3].
Current IVHS research does not address traffic control through
construction and maintenance areas. Although there is great
potential for integration of IHVS and smart work zones.
Several projects have applied robotic technologies to
highway construction equipment. One project created a
Remotely Driven Vehicle (RDV). The RDV is a radiocontrolled dump truck that is used as a shadow vehicle during
slow-moving maintenance operations [5]. The shadow vehicle
follows the work crew to serve as a barrier. The RDV removes
the driver from being exposed to rear-end collisions while
driving the shadow vehicles.
Another highway construction robotics research project is the
development of automated paving machines [6]. This system
has increased the speed of these machines as well as the quality,
and the smoothness of the pavement.
The Advanced Highway Maintenance and Construction
Technology (AHMCT) Research Center at the University of
California, Davis has developed several automated highway
maintenance systems. One system is an automated laser-guided
lane striping machine [7]. This system is a traditional lanestripe-painting vehicle that uses sensors to partially automate
the stripe painting process. A second project creates tethered
robots for roadway crack sealing [8]. Normal crack sealing is a
dangerous, tedious, and slow process. In this system, a tethered
robot follows a work vehicle to rout pavement cracks prior to
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Copyright © 2002 by ASME
sealing. Other vehicle mounted robots can then seal the cracks.
A third system is a flying aerial bridge-inspection system with a
remote operator. This system inspects difficult to access areas
on bridges using a video camera [9].
Another research group has developed a robotic paintremoval system for bridges. This system consists of a robot
placed at the end of a long boom. The robot removes paint
while collecting debris. This prevents the removed lead paint
from polluting the environment [9].
There has been research in the automated placement of safety
devices. A tele-robotic system has been developed to apply
raised, reflective pavement markers [10]. Previously, the
markers were applied by hand, with the worker positioned in a
low-riding bay on a truck. This process was very time
consuming and very dangerous since the markers were applied
by the worker extending their hands into oncoming traffic.
Two other systems automate the placement of safety devices
with a safety cone dispenser. These systems automatically and
quickly deploy and remove rows of warning cones [11, 12].
The systems are modified maintenance vehicles (pickups) that
place cones as the vehicle travels down the road. The vehicle
can then drive along the line of cones and retrieve the markers.
Cones can be placed on either side of the vehicle and at various
distances between cones. These projects have focused on
smaller warning cones. Larger cones and barrels are often used
so the work zone is visible from a grater distance and from the
inside of a passing vehicle.
These pervious systems function in a fundamentally different
way than the proposed approach. The previous systems place
markers in a serial fashion as the deployment vehicle is driven.
The advantage of this approach is that the markers themselves
do not need to be modified. One "centralized" robotic system is
used to deploy/retrieve the markers. The cost and complexity
of the overall system is not related to the number of markers
deployed. The disadvantage of these systems is that the
markers are deployed serially so the time required for
deployment is linearly related to the number of markers placed.
Also, the location of the markers can not be changed once they
are deployed. Therefore the system cannot adapt to changes in
the work zone.
The proposed approach decentralizes the robotic system so
the markers themselves become "smart". The advantage of this
approach is that the markers can adapt to changes in the work
zone. If a crew moves, the safety markers can follow and
always maintain an optimal location. The markers can also be
moved/deployed/retrieved in parallel decreasing deployment
time. The disadvantage of this approach is that the cost of the
overall system linearly scales with the number of markers used.
Both approaches have advantages and there are work zone
situations where each would increase worker safety and
efficiency. The centralized approach (a single robot that
deploys numerous barrels) would lend itself to situations where
many barrels that are seldom re-configured are used. The
decentralized approach (where each barrel is a robot) would be
useful in situations where fewer barrels are used and are often
moved. For example, a situation might require a wedge of
barrels to follow a crew that patches a pothole every quarter
mile.
Individual mobile barrel robots could direct traffic quickly
and efficiently without endangering workers and wasting
worker effort. The robots could self-deploy, self-retrieve, and
self-reconfigure.
SYSTEM DESIGN
In this project three prototype robotic safety barrels were
designed, built and are being tested. The objective is to
demonstrate the usefulness in increasing worker safety and
decreasing traffic congestion delays through a more efficient,
better-marked work zone
Design Objectives
There are several physical requirements that a robotic safety
barrel must meet. The robot needs to be at least as stable as a
traditional safety barrel. The robot and barrel need to remain
stationary in 55mph winds and weigh less than 50 lbs. (so they
can be moved by workers). The prototypes must have the same
footprint as traditional barrels so current transport methods can
be used.
The device needs to cause minimal damage to vehicles if an
impact occurs. It is important that the robots do not become a
safety hazard.
The robot needs to climb slopes up to 7% grade and travel at
5mph (the speed of some maintenance operations such as
striping). The robot needs to traverse obstructions, bumps, and
depressions. The robot needs to operate in inclement weather.
The robot also needs to maneuver within the work zone.
There are two systems-level design requirements that need to
be addressed if the robot barrels are to be successful: 1) high
reliability and 2) low cost.
The system must be very reliable in hardware and software.
A robot that malfunctions could enter into traffic and create a
significant hazard. The system should be designed so critical
components have multiple redundancy. The multi-robot aspect
of the approach will be utilized to increase reliability-all robots
could monitor their neighbors. Also, an encoded Radio
Frequency (RF) carrier wave will be broadcast in the local area
so all robots could be stopped in an emergency. There is
extensive work in fault tolerant engineering. Robots are now
being used in critical operations such as surgery and many
systems with human interaction. All reliability issues were not
addressed with the prototype robots but are being fully
considered in design of the next generation.
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Copyright © 2002 by ASME
Cost per robot is also critical in a decentralized approach to
robot safety markers. Multiple safety markers are used in work
zones so costly markers are not practical. Also, barrels are
often destroyed when struck by passing vehicles. Safety
markers with a high replacement cost are not practical.
However, robots struck by cars have performed their function
by indicating to the driver that the vehicle is not in the correct
position.
Mechanical and Mobility Design
The robot mechanical design can be seen in Figure 2. The
robot consists of a platform that is similar to the base of a
traditional safety barrel. The robot has two 7 ½” in diameter
wheels that are independently driven by two motors. The rear
of the robot has a passive caster. This kinematic arrangement
allows the robot to turn on any radius including turning in place.
The motors each have a high gear ratio so the robot is not easily
moved when the motors are not powered. This allows the robot
to stay in place for long periods while using little or no power.
Since both the lead wheels are powered the robot can climb
small bumps and obstacles.
The barrel snaps onto a ring around the robot and encloses
all components. The robot itself stands less than one foot tall
and raises the barrel height by three inches.
robots or by a remote planning system. This allows the robot
processors to be simple, inexpensive, and redundant.
CONTROL OF THE ROBOTIC SAFETY BARRELS
The robotic safety barrels can be operated in many ways,
ranging from being simply remotely driven (tele-operation) to
operating autonomously.
One mode of control that has been implemented on the robot
prototypes is remote driving.
This simple approach is
advantageous because workers can deploy safety markers from
a safe remote location. A joystick is used to drive the robot to
its desired location. The base transmitter sends out encoded
commands that contain a robot identification number and two
desired wheel speeds. Each robot is constantly listening for
commands. The operator can change the robot identification
number and drive all robots individually.
Remote control is simple and increases worker safety by
moving the operator away from danger. However, it still
requires worker time and robots are deployed serially. An
autonomous system would be preferred.
Autonomous control is broken into two problems- global
planning and local control. The problem is divided in this
manner to reduce the per robot cost. With this approach each
robot does not need to possess high computational ability. Also,
this approach requires only low-bandwidth communication with
the central controller (or other robots).
Global Planning
Motion planning for mobile robots has been extensively
studied. Several approaches have been developed such as
behavior control [13], electrostatic potential fields [14],
probabilistic maps [15], genetic planning [16] and others.
Figure 2: Robot Mechanical Design
Electrical Design
The robot is powered by a 12-volt lead acid battery (shown
mounted above the caster in Figure 2). This application lends
itself to solar recharging, however, this option was not
considered because of the desire to reduce the per robot cost.
The barrel robot has two processors-one is used for robot
control and one to manage communication with the outside
world. Each robot has an RF transceiver that can transmit and
receive information. Each motor has an encoder that can be
used to detect wheel position or speed. The robot is designed to
perform local control (wheel speed and/or position) with highlevel planning being done either by the distributed system of
Previous work with multiple mobile robot control for
highway maintenance has been done for crack sealing robots [8,
17]. Here, up to four tethered mobile robots work together.
This system uses a host computer as the central intelligence and
each robot becomes a workstation in a Windows network. The
robots can access and share files but each remains independent.
This is done to create a highly fault-tolerant system. This
approach cannot be directly applied to the safety barrel robots
because each robot will not have high computational ability.
Also, the robots will not be tethered making high bandwidth
communication expensive.
Other research develops a hybrid control scheme for
formations of non-holonomic mobile robots [18]. This method
controls the motion of many robots while maintaining a
predetermined formation. This work also uses high bandwidth
communication and complex vision sensors on each robot.
The approach being developed for the robotic safety barrels
uses a single camera mounted on a maintenance vehicle. This
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Copyright © 2002 by ASME
camera would provide a view similar to the simulation image
shown Figure 3. The video system identifies the location of
each barrel robot with respect to itself using the highway stripes
as reference. The control system then transmits a desired
position to each robot. The robot itself moves to that position
using its local control system. For example in Figure 3, robot
#4 must move 1 meter forward and 1 meter to the left to join the
formation.
The desired positions are created based on a planning
algorithm that dictates a series of desired positions along a path.
The path can be created in many ways, here a polynomial is
used. The boundary conditions for this polynomial are the
beginning location and the desired end points and the initial
derivative (the robot begins with locally forward motion), see
Figure 5. This approach ensures the non-holnomic constraints
are considered. The final orientation of the path is not
constrained so a new destination can be given while maintaining
fluid motion.
Figure 3: Control Simulation
The global control scheme is currently under development in
simulation. The local controller, discussed in the following
section is currently being tested in a laboratory setting. Other
research has studied the use of video to identify barrel shaped
objects [19] and the use of cameras for robot control in outdoor
environments [20].
Local Control of Individual Robots
Figure 5: Path Planning
The control scheme also requires a relationship between
differential motion in Cartesian space and differential motion in
joint space. This jacobian relationship is nonlinear for the robot
safety barrels. To find this differential relationship, consider the
top
view
of
the
robot
in
Each barrel robot will receive a waypoint (i.e. desired
position or desired velocity) form the central controller. The
robot will then use a local control to obtain that position.
A simple inverse Jacobian Cartesian scheme that uses PID
control has been implemented. This is a common approach to
robot control where an error (δx) is created in Cartesian space
using a desired Cartesian position (xd=[xd, yd, ψd]) and a
measured position (x). This error is transformed into joint
space (δθ) using the inverse jacobian. This error is then acted
upon by a traditional PID control law to create left and right
torque commands (τ=[τl, τr]) for the robot's motors. Control of
this type has several well known limitations (ignores dynamic
effects, non-holonomic constrains) but initial tests have shown
acceptable results (5-10% position error, e.g. 5-10 cm in a 1
meter motion) for barrel placement.
Figure 6. Here the robot has moved an amount δx=[δx, δy, δΨ]
between times ti-1 and ti. During this time the wheels have
rotated an amount δθ=[ δθl, δθr].
Figure 4: Inverse Jacobian Cartesian Controller
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Copyright © 2002 by ASME
position of the robot is determined by accumulating differential
motions, equation ( 4 ).
X t = t i = X t = t i −1 + δX
(4)
This process (dead-reckoning) accumulates measurement
errors. Laboratory testing has shown this error to be within 510% of the overall motion. This error accumulation is not
critical since the global motion of the robot is determined by the
absolute external measurement of the video system used by the
global planner.
Figure 6 : Differential Robot Motion
During this differential time the robot will turn about a given
radius by an amount δΨ, see Figure 7.
The control scheme has been implanted and the results are
shown in Figure 8. The robot was asked to travel to a position
[x=1 meter, y=1 meter] in three seconds. The desired path is
shown as a solid line and the actual path is shown with a dashed
line. The robot arrives at the desired position with 9.2%
position error.
Figure 7: Turning Radius
Here, SL and SR are the paths of the left and right wheels. No
slip is assumed so the path lengths are related to the change in
wheel angles by S=δθr where r is the wheel radius and δθ is
given by the motor encoders. D is the distance between the
wheels and RL and RR are the turning radii for each wheel.
If small motions are assumed the differential motion in
Cartesian space can be found based on the differential motions
in joint space. Using geometry, the relationships are given by:
δx =
δy =
r
2
[δθ L + δθ R ]⋅ cos  r (δθ R − δθ L )
d
(1)

r
[δθ L + δθ R ]⋅ sin  r (δθ R − δθ L )
2
d

r
δ Ψ = (δθ R − δθ L )
d
(2)
(3)
The position of the robot in Cartesian space can only be
determined from the changes in wheel position for small
motions of the robot. For large motions the Cartesian position
of the robot is non-observable based on wheel angle. The
Figure 8: Desired and Actual Robot Path
FUTURE WORK
Future work on the robotic safety barrels will focus on
increasing the system's reliability, decreasing the per robot cost,
and the development of a global control scheme for a group of
robots.
The three prototypes cost approximately $700 per robot.
Cost was not explicitly considered in their design and off-theshelf components were used. Almost half the cost of the
prototypes was spent on the two drive motors. It is estimated
that the next generation prototypes will cost around half this
amount. It is hoped that a mass production approach can bring
the cost per robot to less than $200.
The next generation robots will also directly consider fault
tolerance with triple redundancy in every critical system. Each
robot will have several watchdog timers that must be frequently
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Copyright © 2002 by ASME
reset or the system will be shut down. The central controller
will broadcast a carrier wave so that all robots can be stopped if
a fault is detected. The robots may include three independent
processors that will vote on all commands. These and other
approaches will be directly incorporated into the next
generation design.
Other control approaches are being investigated. Namely, a
fully autonomous mobile robot is being prepared to serve as the
centralized controller. This robot is much more capable (large
computational ability, GPS, laser range finder, stereo camera,
etc) and will be a leader of several barrel robots. This
general/troops system has the potential for more autonomy.
SUMMARY AND CONCLUSIONS
This paper presents a mobile robotic safety marker system
that uses mobile robots to transport safety markers for highway
construction and maintenance. These robots work in teams to
provide traffic control.
Independent autonomous barrel robots have several
advantages. First, the barrels can self-deploy-eliminating the
dangerous task of manually placing barrels in busy traffic. The
barrel positions can be quickly and remotely re-configured as
the work zone changes. Barrels could continuously follow
work crews to maintain optimal placement.
This paper described the motivation for the robotic safety
marker system and how it could improve work zone safety. The
design of three robot prototypes was described. A control
architecture was briefly introduced and a jacobian matrix
derived.
The Robotic Safety Barrels are the first elements of a team of
Robotic Safety Markers (RSM) that includes signs, cones, and
possibly barricades and arrestors. Such devices are not suitable
for every work zone, however there are many situations where
they would useful and practical.
ACKNOWLEDGMENTS
This work is sponsored by the National Research Council
Transportation Research Board’s NCHRP IDEA program.
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