Chapter 7: Miscellaneous Design ElementsAnchor: #CHDIFCCA
Section 1: Longitudinal Barriers and Roadside Safety Hardware CriteriaAnchor: #i1012685
This section discusses the features and design criteria for longitudinal barriers and roadside safety hardware including the following subsections:
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- Concrete Barriers (Median and Roadside); Anchor: #GGTOSRLV
- Guardrail; Anchor: #QLBIBYPE
- Attenuators (Crash Cushions); and Anchor: #KAXPQSDQ
- Roadside Safety Hardware Crash Criteria.
Concrete Barriers (Median and Roadside)
Concrete barriers may be used to prevent the following:
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- Unlawful turns; Anchor: #PCVDCLGY
- Out of control vehicles from entering the opposing traffic lanes; Anchor: #HXEXVKIQ
- Unlawful crossing of medians by pedestrians; and Anchor: #PQUHMPVN
- Vehicles from encountering steep slopes or obstacles on the roadside.
On controlled access highways, concrete barriers will generally be provided in medians of 30-ft or less. On non-controlled access highways, concrete barriers may be used on medians of 30-ft or less; however, care should be exercised in order to avoid the creation of an obstacle or restriction in sight distance at median openings or on horizontal curves. Generally, the use of concrete barriers on non-controlled access facilities should be restricted to areas with potential safety concerns such as railroad separations or through areas where median constriction occurs. Concrete barriers may be considered in medians wider than 30-ft based on an operational analysis or safety analysis.
Medians for urban freeway sections generally are relatively narrow and flush. For new construction, an urban freeway usually includes a relatively narrow flush median (see Medians in Chapter 3 Section 2) with a concrete barrier.
In determining the type of barrier to be used, the primary consideration is safety, both for vehicular impacts and during maintenance activities. Field experience with concrete barriers indicates that, unlike guard fence systems, maintenance operations are not normally required following vehicular encroachment.
Reconstruction projects with median barriers should be considered on a project-by-project basis. Often, the structural capability of existing bridges may make the use of concrete median barriers infeasible due to increased dead load.
TxDOT’s design standards and standard construction specifications provide more information on the design and construction details for concrete barriers. See Appendix A, Longitudinal Barriers for additional information on roadside safety hardware applications.Anchor: #i1012774
Guardrail is considered a protective device for the traveling public and is used at points on the highway where a vehicle departure from the roadway would be a significant safety concern. Guardrail is designed to prevent impacts with roadside hazards by deflecting the vehicle so that it continues to move at a reduced velocity along the rail in the original direction of traffic. The limits of rail to be installed are shown on the plans; however, they may be adjusted in the field after the grading is completed to assure compliance with standard guardrail applications. See Appendix A for additional guardrail information.
Guardrail should be installed in areas where the consequence of an errant vehicle leaving the roadway is judged to be more severe than impacting the guardrail. Guardrail should be offset at least 4-ft and desirably 5-ft or more, measured from the back of post to the nearest edge of roadside hazard. Refer to Appendix A, Longitudinal Barriers for additional information on offset guidance. At overpasses, guardrail should be anchored securely to the structure.
Guardrail should be installed in accordance with current roadway standards.
Providing appropriate end treatments is one of the most important considerations in the design of guardrail. An untreated guardrail will stop a vehicle abruptly and can penetrate the passenger compartment. For more information on the installation of various types of end treatments, refer to Appendix A Section 5, End Treatment of Guard Fence and TxDOT’s standard construction specifications and roadway standards.Anchor: #i1012803
Attenuators (Crash Cushions)
Crash cushions or impact attenuators are protective devices that prevent errant vehicles from impacting fixed objects. This is accomplished by gradually decelerating a vehicle to a safe stop for head-on impacts or redirecting a vehicle away from the fixed object for side impacts.
Attenuators are ideally suited for use at locations where fixed objects cannot be moved, relocated, or made breakaway, and cannot be adequately shielded by a longitudinal barrier. A common application of a crash cushion is in an exit ramp gore where a bridge rail end requires shielding. Crash cushions are also frequently used to shield bridge columns as well as roadside and median barrier terminals.
In temporary work zone applications using a parallel rigid barrier with narrow shoulder widths, if feasible, the rigid barrier should be tapered slightly on the approach to allow additional shy distance before the placement of the crash cushion. This will reduce incidental impacts. The taper rate of the rigid barrier, and alignment and placement of the crash cushion should conform to AASHTO Roadside Design Guide criteria, and the manufacturer's specifications for the specific crash cushion type.
There are numerous types of attenuators that are in common use. When more than one system is under consideration, the designer should carefully evaluate the structural, safety, and maintenance characteristics of each candidate system. Characteristics to be considered include the following:
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- Impact decelerations; Anchor: #XDFKUJKE
- Redirection capabilities; Anchor: #MBLACATU
- Anchorage and back-up structure requirements; Anchor: #IBWTGTEK
- Debris produced by impact; and Anchor: #HMOKSBDV
- Ease and cost of maintenance.
Crash Cushion Categories
Crash cushions are classified in one of three categories based on the reusability of the product after a head-on impact: sacrificial, reusable, and low maintenance.
These units are typically filled with water or sand and typically require full replacement or substantial repairs either on-site or in a maintenance yard following an impact. These units have low initial costs and should be considered for sites that typically experience less than one impact every 18 months. Water-filled crash cushions are allowed for use in temporary work zones only.
These units typically feature side fender panels, cartridges, or cylinders that absorb an errant vehicle’s energy during impact. Typically, the cartridges, or cylinders and a nose piece will have to be replaced after an impact. These units should be considered in locations that typically experience impacts in the range between one impact every 18 months and less than 3 impacts per year.
These units typically utilize plastic cylinders, or hydraulic mechanisms to absorb energy. These require some maintenance after an impact to ensure proper performance during the next impact. These have high initial cost and should typically be considered at locations where they will experience 3 or more impacts per year.
For more detailed information on the installation of various types of attenuators, refer to TxDOT’s standard specifications and roadway standards.Anchor: #NDWCNRAV
Roadside Safety Hardware Crash Criteria
AASHTO’s Manual for Assessing Safety Hardware (MASH 2016), provides guidance for testing permanent and temporary highway safety features to assess safety performance of those features, replacing guidance defined in NCHRP Report 350. Guidance includes definitions of crash-test levels with specified vehicle, speed, and impact angle for each level.
Roadside hardware safety devices are categorized by Test Levels which define the impact conditions that the device is rated to withstand, based on structural adequacy, occupant risk, and vehicle trajectory. The standard MASH 2016 vehicles for testing categories include a small car (2,420-lbs.) and a large pick-up (5,000-lbs). TL-2 is used for low-speed roadways (45-mph or less); TL-3 is for high speed roadways (50-mph or greater); TL-4 includes the TL-3 criteria, plus additional testing for a delivery-type truck (22,000-lbs). The primary difference between MASH and the earlier NCHRP 350 criteria is the increase in size and height of the tested pick-up truck to account for the change in vehicle fleet, and to better simulate an SUV. Other changes include the small car weight, and angle of impact.
November 20, 2009, memorandum from David A. Nicol about AASHTO’s Manual for Assessing Safety Hardware (MASH). This AASHTO manual supersedes NCHRP Report 350 for the purposes of evaluating new safety hardware such as longitudinal barriers, transitions, end terminals, crash cushions, breakaway/yielding supports, truck mounted attenuators, and work zone traffic control devices. It sets guidelines for crash testing and evaluation criteria for assessing test results. The joint AASHTO/FHWA implementation plan stated that all highway safety hardware accepted under the criteria in NCHRP Report 350 does not need to be retested to MASH criteria; may remain in place; and may continue to be manufactured and installed. However, all new hardware that is developed must be tested and evaluated according to MASH.
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- May 21, 2012, memorandum from Tony Furst on the subject of Roadside Safety Hardware - Federal-Aid Reimbursement Eligibility Process and related Frequently Asked Questions. The memo establishes that States can certify that roadside safety hardware has been tested by an accredited crash test laboratory and meets MASH criteria, and can thus be eligible for reimbursement. Anchor: #UKKHHUTD
- January 7, 2016 memorandum from Thomas Everett on the subject of AASHTO/FHWA Joint Implementation Agreement for Manual for Assessing Safety Hardware (MASH). The memo discusses the agreement between AASHTO and FHWA that requires all new installations of safety hardware on the NHS to be evaluated using the 2016 edition of MASH.
Current TxDOT MASH Implementation Timetable/Policy
As product manufacturers and developers have tried to develop MASH 2016 compliant products, the FHWA in coordination with AASHTO has allowed additional flexibility with respect to the implementation of MASH 2016 compliant products by the respective states. The following are the categories of Roadside Safety Hardware Products and current TxDOT policy:
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- W-Beam barriers and cast-in-place concrete barriers: Effective 12/31/2017, for all new permanent installations and full replacements, all W-Beam Barriers and cast-in-place concrete barriers shall be MASH 2016 compliant for projects let after this date. Anchor: #IRTRMREK
- Guardrail End Treatments (SGTs): Effective Feb 28, 2018 all new permanent installations and full replacement SGTs must be MASH 2016 compliant regardless of project letting date. Anchor: #NSBPEISI
- Cable barriers, cable barrier terminals, and crash cushions: In December 2019, the FHWA in collaboration with AASHTO provided updated guidance that allows the continued use of NCHRP 350 or MASH 2009 compliant devices for those categories of devices where a MASH 2016 alternative may not be available. The TxDOT Roadway Standards webpage provides standards (whether MASH 2016, MASH 2009, or NCHRP 350) that are available for use until further notice. Anchor: #OMNRICKV
- Bridge rails, transitions, all other longitudinal barriers (including portable barriers installed permanently), all other terminals, sign supports, and all other breakaway devices: In December 2019, the FHWA in collaboration with AASHTO provided updated guidance that allows the continued use of NCHRP 350 or MASH 2009 compliant devices for categories of devices where a MASH 2016 alternative may not be available. Note that all current Bridge Railing Standards (BRG), Permanent Sign Support Standards (TRF), and Mailbox Standards (MNT) are MASH 2016 compliant. Anchor: #BGKINLLN
- Temporary work zone devices (including portable barriers and water-filled crash cushions): Devices manufactured after 12/31/2019 are required to be MASH 2016 compliant. Such devices manufactured on or before 12/31/2019, and successfully tested to NCHRP 350 or MASH 2009, may continue to be used throughout their normal service lives. Note that certain temporary sign supports do not meet MASH 2016 criteria. Testing is on-going with these products. Also, trailer-type work zone devices such as arrow boards, and electronic portable message signs are not MASH 2016 compliant, but FHWA has currently exempted these devices due to safety benefits offered by their use.
All of the standards available on the respective TxDOT Division standards webpage are available for use until future notice. As additional MASH 2016 compliant items become available, the remaining NCHRP 350 or MASH 2009 items will migrate off the lists of available standards and the Districts will continue to be notified accordingly. A list of all available MASH compliant roadside safety hardware items and associated memoranda are available on the DES Division's (Roadway Design Section) Webpage.