Understanding and Quantifying the Impact of Pavement Friction Demand on Traffic Safety.
There is a general
agreement that pavement friction has a direct impact on traffic safety and
pavement safety performance. As such,
pavement friction needs to be treated as an integral aspect of safety. A better understanding of friction demand is
needed with the ultimate goal of keeping vehicles on the highway, in their lane
and in control as a first line of defense for safety. Several studies have investigated the
relationship between pavement surface friction and crash risk, and could
quantify significant correlations between them. However, these correlations are
highly varying and can be delusive and challenging to derive due to the highly
varied and uncontrolled conditions during a crash or a near miss event due to
loss of control. Friction demand
provides a valuable tool for safety risk network screening, which will improve
these correlations and accordingly our understanding of the impact of friction
on traffic safety.
can be defined as the friction level required to provide sufficient
vehicle-pavement skid resistance to safely operate, maneuver, and navigate
about the highway given the road design, functional class, representative
vehicle characteristics (e.g., size and tire conditions), road geometry,
prevailing traffic condition (e.g., speed, traffic volume, and density), and
typical environmental conditions. In the United States, few agencies have
attempted to define friction demand categories taking into account some of the
mentioned variables. Other agencies are still facing the lack of guidance,
which constrains the proper development of friction management programs.
friction demand concepts sound appealing and straight forward, there are major challenges
in defining the factors affecting the friction demand. These factors include
the discrepancies in defining friction parameters with the different friction
measurement devices, the complexity of crash events, where friction is one of
the contributing factors in a complex event, and the lack of detailed crash
records mentioning the crash sequence or reporting the near misses. Moreover,
some past experiences are indicating unexpected crash rates when compared to
the friction demand and the measured pavement surface friction. With the
evolving concepts and improvements in pavement friction evaluation and safety
analysis, there is a need to revise the friction demand definitions and
standardize the practices and procedures to quantify it.
The objective of this research is to reevaluate
the concepts of friction demand and provide consistent definitions and
procedures to utilize friction demand with respect to traffic safety and
pavement surface friction requirements. The study will provide guidance for
State DOTs to facilitate the assessment of the friction demand at the network
level, and accordingly the possible investigatory levels and friction levels
requirements based on the mentioned variables and other possible variables
affecting the friction demand. This study should enhance the practices in
improving traffic safety through pavement surface improvements as part of crash
reduction programs. The research should:
Provide consistent definitions of friction demand and
extensive list of possible variables affecting the friction demand at the
Provide the ability to assign a friction demand level to
every discreet segment of a given highway network.
Provide guidance to utilize the friction demand in
setting investigatory and intervention friction levels and requirements.
Provide guidance on relating friction demand to safety
analysis and historical crash records at the segment level.
Understanding friction demand at the network
level is crucial to have an efficient and well-structured friction management
program which manages friction in a cradle to grave fashion. Mature
understanding will allow identification and prioritization of all segments on
the network with friction below requisite demand. A well-established pavement
friction management program will provide cost-effective alternatives to improve
safety and support the crash reduction programs across agencies.
between pavement friction and traffic safety has been recognized and considered
since the 1950s, where the research activities have increased rapidly. Since
then, studies have been focusing on understanding how pavement friction
develops, how to measure it, and how it relates to traffic safety. To assess
the impact of pavement friction on traffic safety, it has been suggested early
on that the measured values should be compared to a friction demand level,
which reflects the forces needed to safely operate, maneuver, and navigate
about the highway. The friction demand can be a function of several variables
including, but not limited to; the road geometry, where sharper curves will
require higher side forces to avoid run of the road accidents, prevailing
traffic speed, where a longer stopping distance is required at higher speed
segments, and the traffic levels.
have been developed internationally, including New Zealand, the United Kingdom,
and Australia. However, the progress is developing friction demand requirements
and relating them to the safety performance is still at infancy in the United
States. Several challenges are faced when developing friction demand concepts
The unique and inconsistent effects of pavement friction
on safety across states, this is due to the interaction between pavement
friction and the driver behavior and the network operational conditions.
The complicated impact of pavement friction during a
crash event, where in some cases segments deemed with a high friction demand
and low friction measurements can have low crash rates due to other safety
countermeasures that are reducing the overall crash rates, or the additional
safety features that reduces the severity to near misses, which do not get
reported. Additionally, not all crashes
The research should:
Provide consistent definitions of friction demand and extensive list of possible variables affecting the friction demand at the network level.
Provide the ability to assign a friction demand level to every discreet segment of a given highway network.
Provide guidance to utilize the friction demand in setting investigatory and intervention friction levels and requirements.
Provide guidance on relating friction demand to safety analysis and historical crash records at the segment level.
The target audience for the research findings
and products of this work will be State DOTs, MPOs, and other transportation
stakeholders. It should be useful across levels and disciplines of an
organization including executives and technical staff (communications,
planning, engineering and research staff) as well as field staff involved in
pavement evaluation and traffic safety.
The key decision-makers who can approve,
influence, or champion implementation of these research products are senior
staff, communications staff and executives of transportation agencies.
The AASHTO committees that will be involved
in the adoption and implementation of the results will be the AASHTO Committee
on Safety and the AASHTO Committee on Materials and Pavements and its subcommittee
on Pavement Measurement and Performance Measures.
|Sponsoring Committee:||AFD90, Pavement Surface Properties and Vehicle Interaction
|Research Period:||24 - 36 months|
|RNS Developer:||Brian Schleppi. Ahmad Alhasan, Magdy Mikhail|
|Index Terms:||Pavements, Friction, Rolling contact, Traffic safety, |
Safety and Human Factors