DISTRIBUTION: Electronic Recipients List TRANSMITTAL LETTER NO. (15-01) MINNESOTA DEPARTMENT OF TRANSPORTATION. MANUAL: Road Design English Manual

Transcription

1 DISTRIBUTION: Electronic Recipients List MINNESOTA DEPARTMENT OF TRANSPORTATION DEVELOPED BY: Design Standards Unit ISSUED BY: Office of Project Management and Technical Support TRANSMITTAL LETTER NO. (15-01) MANUAL: Road Design English Manual DATED: April 15, 2015 SUBJECT: Section (Median Barriers) A list of changes is attached to this update. INSTRUCTIONS: 1. Record this transmittal letter number, date and subject on the transmittal record sheet located in the front of the ENGLISH manual. The last Transmittal Letter was 14-01, dated June 3, Remove from the ENGLISH manual: 10-7(21) through 10-7(24) 3. Insert into the ENGLISH manual: 10-7(21) through 10-7(24) All updated sheets are dated March, The Road Design Manual and associated Transmittal Letters are available online in PDF format at Any technical questions regarding this transmittal should be directed to Mike Elle, Design Standards Engineer, at (651) , or by to DesignStandards.DOT@state.mn.us Michael Elle, P.E. Design Standards Engineer

2 THIS PAGE INTENTIONALLY LEFT BLANK

3 Summary of Changes MnDOT Road Design Manual Chapter 10 Section o Guidance for median barrier warrants is clarified with emphasis on barrier consideration and recommendation. Minor corrections regarding the use of rigid median barriers are made. Page 10-7(23) o Figure A Modified graph to match the Roadside Design Guide.

4 THIS PAGE INTENTIONALLY LEFT BLANK

5 JANUARY, 2001 ROAD DESIGN MANUAL (ENGLISH) 10-7(21) APPROACH GUARDRAIL LENGTH OF NEED Figure E BRIDGE APPROACH RAILINGS (WITH AND WITHOUT SHOULDERS) Figure F

6 10-7(22) MnDOT ROAD DESIGN MANUAL MARCH, Median Barriers Service Requirements and Performance Criteria A median barrier is a longitudinal traffic barrier primarily used in "narrow" medians to prevent acrossthe-median, head-on collisions between vehicles in opposing traffic. The barrier shall be designed, using established criteria, to perform several functions: 1. Restrain the design vehicle and not allow the vehicle to climb over or wedge under the installation. 2. Stop or redirect the vehicle in such a manner that passengers restrained by seat belts can survive, preferably uninjured. 3. Redirect or stop the vehicle in such a manner to minimize the hazard to following or adjacent traffic. Ideally, the vehicle should remain close to the barrier and not be directed back into the traffic stream. 4. Function so that barrier fragments do not endanger vehicle occupants or other traffic. Generally, while crash severity and fatalities decrease after a traffic barrier has been installed in a median, the crash frequency increases. This increase is attributed to the decrease in maneuvering space for run-off-theroad vehicles. Thus, the barrier must not only be structurally adequate to prevent penetration through the barrier, it must also provide for the safety of the occupants of impacting vehicles. These two interdependent factors, structural strength and occupant safety, must be considered simultaneously in order to achieve optimum barrier performance. The safety of the maintenance crews that must repair damaged barriers must also be considered. The design should not inhibit general maintenance of the barrier, particularly in narrow medians where high-speed traffic would be impeded and traffic would endanger the repair crew. Median barriers may be divided into three major types according to lateral stiffness. These are rigid, semi-rigid and flexible barriers. The rigid barrier is used where no lateral deflection is acceptable. Rigid barriers are typically made of concrete with a shape that meets safety criteria. The most common shapes are the F-shape, single slope, and the vertical wall barrier. The most common semi-rigid barrier is the strong-post/strong-beam system (the W-beam guardrail system). This type is used where small lateral deflections are allowable. The flexible barrier, represented by the cable barrier, is used where large lateral deflections can be tolerated. Median barriers must meet the same performance criteria applied to other roadside barriers. Rigid barriers are discussed in Section The semi-rigid and flexible barriers are discussed in Section General Warrants for Median Barriers As a general rule, a median barrier should be installed only when it has a potential of significantly reducing the occurrence of cross-median crashes and the overall severity of median related crashes. It should be kept in mind that the barrier is a hazard itself and should be used only where a collision with the barrier is of less consequence than the collision potential beyond the barrier. The terminal points in barrier systems are generally critical elements in the performance of the system. An unprotected median barrier terminal is essentially a fixed-object hazard to a motorist. For this reason the use of median barriers are generally restricted to full access-controlled facilities where there are no median crossings requiring discontinuities in the barrier. Special designs may be required where at-grade crossings are permitted and median barrier is used. Crash cushions or energy-absorbing end terminals are typically used to shield median barrier ends. The main warrants for median barriers are determined from median width and traffic volume. An additional consideration is the design speed of the highway. On existing roads, accident experience may also be a warrant. Figure A presents the need for a median barrier based on median width and traffic volume for high-speed roadways. Figure A indicates that median barrier: 1. Is Recommended - when Average Daily Traffic (ADT) is at or above 20,000, for medians at or less than 30 ft. (10 m) wide. 2. Is Considered - when ADT is at or above 20,000, and median widths are more than 30 ft. (10 m) and at or less than 50 ft. (15 m) wide. 3. Is Optional - when ADT is below 20,000, or when median widths are greater than 50 ft. (15 m) wide.

7 MARCH, 2015 MnDOT ROAD DESIGN MANUAL 10-7(23) MEDIAN BARRIER WARRANTS FOR HIGH SPEED FULLY CONTROLLED ACCESS ROADWAYS Figure A When considering the placement of barrier, a cost/benefit analysis or an engineering study evaluating such factors as traffic volumes, vehicle classifications, median crossover history, crash incidents, vertical and horizontal alignment relationship, and median/terrain configurations may be conducted to determine the appropriate application for the median barrier installations. For optional areas, a barrier may still be included, for special site-specific considerations, such as a location (or geometric type) with a known history of cross-median crashes. For any divided highway, regardless of other warrants, the median roadside must also be examined for other barrier warranting factors, such as hazards and lateral drop-offs, the same as for any roadside Rigid Median Barriers The preferred rigid barriers for median installations are the F -shape concrete barrier, and the Single Slope barrier. Additional placement criteria, for special conditions, are discussed in the AASHTO Roadside Design Guide, Chapter 6. Lighting guidelines can be found in the MnDOT Lighting Design Manual Sloped Medians As mentioned previously, a level median is necessary for optimum barrier performance. A rigid or semi-rigid barrier must have an adjacent graded median with slopes of 1V:10H or flatter, to facility vehicle stability prior to impact. Where these conditions do not exist, special placement is necessary as discussed in the AASHTO Roadside Design Guide, Chapter 6. Some flexible barrier systems can be installed on steeper slopes. Refer to the specific flexible barrier type to determine slope design guidance.

8 10-7(24) ROAD DESIGN MANUAL (ENGLISH) SEPTEMBER, Traffic Screens and Glare Screens General A severe problem on high volume divided highways is gawking (rubber-necking) at accidents or other incidents in the opposing traffic lanes. This can result in congestion and secondary accidents. In order to lessen this effect, traffic screens can be used on median barriers. Vehicular headlight glare can pose some problems to opposing traffic even in lighted sections. It can momentarily blind drivers and disorient them, especially on curves or through unlighted underpasses. Continual exposure leads to driver fatigue and reduces the driver's alertness. This can be expected to become a greater problem as the population ages. In order to lessen the effect, glare screen can be used on median barriers. Traffic screens and glare screens can create horizontal sight distance problems when placed on curves. In the design process there should be an evaluation of the degree of this restriction based on the stopping sight distance (Sections and 3-4.0) and an engineering judgment made on whether they should be placed or not. A distinction should be made between traffic screens (intended to reduce gawking), glare screens (intended to reduce headlight glare), and traffic barriers (intended to reduce errant vehicle crossovers) Locations 1. Traffic Screens Opaque traffic screens should be provided in medians to reduce congestion due to gawking at incidents in adjacent roadways on those freeways and expressways which have or will have concrete median traffic barriers and carry high volumes of traffic exceeding 1,200 vehicles per lane per hour. 2. Glare Screens Normally, glare screens should be provided on all concrete traffic median barriers. However, the final decision as to deviations to this policy should be made by the district in consultation with the Office of Traffic, Safety & Technology. In rural areas, glare screen should be erected in spots where experience has shown a glare problem exists. These usually will be on horizontal curves or where a pavement transitions from a four-lane divided roadway to a two-lane roadway. Whenever possible, plantings should be used for a glare barrier in these situations. However, caution should be exercised in using plantings and other glare screens so that their use does not present a greater hazard than what was originally present. If plantings are to be used, the designer shall check with the Landscape Architecture Unit of the Office of Environmental Stewardship to determine what types of plantings are compatible in the area. The guidelines above should cover most instances when a glare screen will be appropriate. In addition, a glare screen may be required between a two-way frontage road and a mainline road if the two are in close proximity. This would prevent glare from approaching traffic and also would prevent some driver confusion as to what roadway the frontage road traffic is actually on. The designer should choose the type of screen intended for use, basing this decision on the economics of the screen and the demands of that location Concrete Screen Slip Formed The slip formed concrete screen is the only type of screen currently being used. This screen is cast integrally with the concrete median barrier; see the Standard Plates Manual, Series Selection of Screen Type A screen may be used based on either Case I, designed to reduce gawking or gawking and glare, or Case II, designed to reduce glare only. 1. Case I - Gawking, or Gawking and Glare Use the concrete traffic screen with concrete traffic barrier where horizontal sight distance to traffic ahead is not a problem; these screens should be 56 in. high. See the Standard Plates Manual, Series Case II - Glare Use of the concrete glare screen is recommended where horizontal sight distance is not a problem. At locations on curves where screens could restrict sight distance, such curves should be flattened, or the barrier should be moved back from the pavement, if practical. If not, standard concrete safety-shape barriers without a glare screen, in most cases, permit continued visual contact of traffic flow ahead on curves.