09 Chapter 4 Design Placement
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Barrier Design and Placement
November 2005
CHAPTER 4 BARRIER DESIGN AND PLACEMENT
4.1 OVERVIEW OF THE AASHTO ROADSIDE DESIGN GUIDE DESIGN PROCESS
Chapter 5 of the AASHTO Roadside Design Guide (RDG) contains roadside barrier layout and design guidance. It is important to understand the philosophy behind the design process presented in the RDG. If a vehicle leaves the roadway atapproximately 10 degrees in the vicinity of the upstream end of a roadside barrier and the driver then attempts to correct and return to the pavement, the vehicle could be traveling parallel and behind the barrier. This design process is intended to allow sufficient room for a vehicle to come to a stop before striking the hazard if it should get into this situation. An important part of the layout process isto allow a clear zone behind the barrier upstream of the hazard. This is also an important concept to remember in the construction and maintenance of the roadway.
4.1.1 Design Variables
Figure 4.1 shows the variables that are considered in the RDG design process.
Figure 4.1: Barrier Design Variables
Clear Zone
LR X Length of Need when Flared L1 Length of Need when Straight Y L3 L2 Edgeof Traveled Way LS
LC LA
H A Z A R D
b
a
The variables used in the design process are defined below:
________________________________________________________________________ Overview of AASHTO Roadside Design Guide Design Process 4-1
Barrier Design and Placement
November 2005
LA is the lateral distance from the edge of the traveled way to the back of the hazard. LC isthe clear zone width, measured from the edge of the traveled way. LC serves as a check on LA. It is not necessary to shield a hazard beyond the clear zone, so LA does not have to be greater than LC. L3 is the lateral distance from the edge of the traveled way to the front edge of the hazard. L2 is the offset of the roadside barrier, measured from the edge of the traveled way to the front face ofthe barrier. The designer must select the barrier offset. Factors to consider in selecting L2 are listed in Section 4.1.2. LR is the runout length, measured longitudinally from the upstream extent of the hazard along the edge of pavement. LR is the stopping distance off the pavement. LR values are found in Table 4.1. LS is the shy line offset. Rigid objects such as roadside barriers close to thepavement tend to intimidate drivers, causing them to slow down or shift positions. This may result in a loss in capacity that can be a concern for high volume roads. Although it is preferable to locate barriers at or beyond the shy line offset, it is seldom an important factor for low volume conditions. Shy line values are found in Table 4.2. If the barrier is placed on a flare, the flare isdescribed as a: b in the RDG. Placing a barrier on a flare is a design decision. Benefits are that less barrier is needed (improving both safety and costing less) and the end treatment is moved further away from traffic. The ability to include a flare is usually limited by the site terrain. Slopes in front of a barrier should be 1V: 10H or flatter, which is often difficult to achieve. The flare a: b isin the standard section of the barrier and is not related to any flare that may be required for an end treatment. End treatments must be laid out from the projection of the barrier at the point of beginning of the end treatment. If a barrier is laid out on the maximum flare, it may be necessary to exceed the maximum flare because an additional flare for the end treatment is introduced. It isacceptable to exceed the maximum flare rate for this purpose. When possible, very flat flare rates should be used when the barrier is located within the shy line offset. Chapter 5 of the RDG has more detail on suggested flare rates in this case. If a flare in the standard section is used, L1 is the tangent length of the barrier and defines the beginning point of the flare, measured from the upstream...
November 2005
CHAPTER 4 BARRIER DESIGN AND PLACEMENT
4.1 OVERVIEW OF THE AASHTO ROADSIDE DESIGN GUIDE DESIGN PROCESS
Chapter 5 of the AASHTO Roadside Design Guide (RDG) contains roadside barrier layout and design guidance. It is important to understand the philosophy behind the design process presented in the RDG. If a vehicle leaves the roadway atapproximately 10 degrees in the vicinity of the upstream end of a roadside barrier and the driver then attempts to correct and return to the pavement, the vehicle could be traveling parallel and behind the barrier. This design process is intended to allow sufficient room for a vehicle to come to a stop before striking the hazard if it should get into this situation. An important part of the layout process isto allow a clear zone behind the barrier upstream of the hazard. This is also an important concept to remember in the construction and maintenance of the roadway.
4.1.1 Design Variables
Figure 4.1 shows the variables that are considered in the RDG design process.
Figure 4.1: Barrier Design Variables
Clear Zone
LR X Length of Need when Flared L1 Length of Need when Straight Y L3 L2 Edgeof Traveled Way LS
LC LA
H A Z A R D
b
a
The variables used in the design process are defined below:
________________________________________________________________________ Overview of AASHTO Roadside Design Guide Design Process 4-1
Barrier Design and Placement
November 2005
LA is the lateral distance from the edge of the traveled way to the back of the hazard. LC isthe clear zone width, measured from the edge of the traveled way. LC serves as a check on LA. It is not necessary to shield a hazard beyond the clear zone, so LA does not have to be greater than LC. L3 is the lateral distance from the edge of the traveled way to the front edge of the hazard. L2 is the offset of the roadside barrier, measured from the edge of the traveled way to the front face ofthe barrier. The designer must select the barrier offset. Factors to consider in selecting L2 are listed in Section 4.1.2. LR is the runout length, measured longitudinally from the upstream extent of the hazard along the edge of pavement. LR is the stopping distance off the pavement. LR values are found in Table 4.1. LS is the shy line offset. Rigid objects such as roadside barriers close to thepavement tend to intimidate drivers, causing them to slow down or shift positions. This may result in a loss in capacity that can be a concern for high volume roads. Although it is preferable to locate barriers at or beyond the shy line offset, it is seldom an important factor for low volume conditions. Shy line values are found in Table 4.2. If the barrier is placed on a flare, the flare isdescribed as a: b in the RDG. Placing a barrier on a flare is a design decision. Benefits are that less barrier is needed (improving both safety and costing less) and the end treatment is moved further away from traffic. The ability to include a flare is usually limited by the site terrain. Slopes in front of a barrier should be 1V: 10H or flatter, which is often difficult to achieve. The flare a: b isin the standard section of the barrier and is not related to any flare that may be required for an end treatment. End treatments must be laid out from the projection of the barrier at the point of beginning of the end treatment. If a barrier is laid out on the maximum flare, it may be necessary to exceed the maximum flare because an additional flare for the end treatment is introduced. It isacceptable to exceed the maximum flare rate for this purpose. When possible, very flat flare rates should be used when the barrier is located within the shy line offset. Chapter 5 of the RDG has more detail on suggested flare rates in this case. If a flare in the standard section is used, L1 is the tangent length of the barrier and defines the beginning point of the flare, measured from the upstream...
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