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Understanding and Quantifying the Impact of Pavement Friction Demand on Traffic Safety.

Description:

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.

Friction demand 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.

Although the 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.

Objective:

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 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.

Benefits:

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.

Related Research:

The relation 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.

Several standards 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 including:


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 get reported.

Tasks:

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.

Implementation:

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
Date Posted:12/30/2019
Date Modified:01/08/2020
Index Terms:Pavements, Friction, Rolling contact, Traffic safety,
Cosponsoring Committees: 
Subjects    
Highways
Pavements
Safety and Human Factors

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