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Figure shows a traversable pipe grate on a concrete box culvert constructed to match the 1V:6H side slope. For intermediate-sized pipes and culverts whose inlets and outlets cannot be readily made traversable, designers often extend the structure so the obstacle is located at or just beyond the suggested clear zone.

While this practice reduces the likelihood of the pipe end being hit, it does not completely eliminate that possibility. If the extended culvert headwall remains the only significant fixed object immediately at the edge of the suggested clear zone along the section of roadway under design and the roadside is generally traversable to the right-of-way line elsewhere, simply extending the culvert to just beyond the suggested clear zone may not be the best alternative, particularly on freeways and other high-speed, access-controlled facilities.

On the other hand, if the roadway has numerous fixed objects, both natural and man-made, at the edge of the suggested clear zone, extending individual structures to the. However, redesigning the inlet or outlet so that it is no longer an obstacle is usually the preferred safety treatment.

Each mm [30in. The safety pipe runners are Schedule 40 pipes spaced on centers of mm [30in. For major drainage structures that are costly to extend and whose end sections cannot be made traversable, shielding with an appropri- ate traffic barrier often is the most effective safety treatment. Although the traffic barrier is longer and closer to the roadway than the structure opening and is likely to be hit more often than an unshielded culvert located farther from the through traveled way, a properly designed, installed, and maintained barrier system may provide an increased level of safety for the errant motorist.

Parallel drainage culverts are those that are oriented parallel to the main flow of traffic. They typically are used at transverse slopes under driveways, field entrances, access ramps, intersecting side roads, and median crossovers. Most of these parallel drainage cul- verts are designed to carry relatively small flows until the water can be discharged into outfall channels or other drainage facilities and carried away from the roadbed.

However, these drainage features can present a significant roadside obstacle because they can be struck head-on by impacting vehicles. As with cross-drainage structures, the designer's primary concern should be to design generally traversable slopes and to match the culvert openings with adjacent slopes. On low-volume or low-speed roads, where crash history does not indicate a high number of run- off-the-road occurrences, steeper transverse slopes may be considered as a cost-effective approach.

Using these guidelines, safety treatment options are similar to those for cross-drainage structures, in order of preference: 1. Eliminate the structure.

Use a traversable design. Move the structure laterally to a less vulnerable location. Shield the structure. Delineate the structure if the above alternatives are not appropriate.

Unlike cross-drainage pipes and culverts that are essential for proper drainage and operation of a road or street, parallel pipes some- times can be eliminated by constructing an overflow section on the field entrance, driveway, or intersecting side road. To ensure proper performance, care should be taken when allowing drainage to flow over highway access points, particularly if several access points are closely spaced or the water is subject to freezing. This treatment usually will be appropriate only at low-volume locations where this design does not decrease the sight distance available to drivers entering the main road.

Care also should be exercised to avoid erosion of the entrance and the area downstream of the crossing. This usually can be accomplished by paving the overflow section assuming the rest of the facility is not paved and by adding an upstream and downstream apron at locations where water velocities and soil conditions make erosion likely. Closely spaced driveways with culverts in drainage channels are relatively common as development occurs along highways approach- ing urban areas.

Because traffic speeds and roadway design elements are usually characteristic of rural highways, these culverts may constitute a significant roadside obstacle. In some locations, such as along the outside of curves or where records indicate concentra- tions of run-off-the-road crashes, it may be desirable to convert the open channel into a storm drain and backfill the areas between adjacent driveways.

This treatment will eliminate the ditch section as well as the transverse slopes with pipe inlets and outlets. As emphasized earlier in this chapter, transverse slopes should be designed while considering their effect on the roadside environment. The designer should try to provide the flattest transverse slopes practical in each situation, particularly in areas where the slope has shown a high probability of being struck head-on by a vehicle.

Once this effort has been made, parallel drainage structures should match the selected transverse slopes and, if possible, should be safety treated when they are located in a vulnerable position relative to main road traffic. Although many of these structures are small and present a minimal target, the addition of pipes and bars perpen-. Research has shown that for parallel drainage structures, a grate consisting of pipes set on mm [24 in.

It also is recommended that the center of the bottom bar or pipe be set at to mm [4 to 8 in. Generally, single pipes with diameters of mm [24 in.

When a multiple pipe installation is in- volved, however, a grate for smaller pipes may be appropriate. Reference may be made to the Texas Transportation Institute Research Study , Safe End Treatment for Roadside Culverts 13 , in which researchers concluded that a passenger vehicle should be able to traverse a pipelslope combination at speeds up to 80 k m h [50 mph] without rollover. To achieve this result, the roadway or ditch foreslope and the driveway foreslope both should be 1V:6H or flatter and have a smooth transition between them.

Ideally, the culvert should be cut to match the driveway slope and fitted with cross members perpendicular to the direction of traffic flow as described previously.

This study suggests that it could be cost-effective to flatten the approach slopes to 1V6H and match the pipe openings to these slopes for all sizes of pipes up to mm [36 in. The addition of grated inlets to these pipes was considered cost-effective for pipes mm [36 in.

Because these numbers were based in part on assumptions by the researchers, they should be interpreted as approxi- mations and not as absolute numbers. Figure illustrates a possible design for the inlet and outlet end of a parallel culvert. When channel grades permit, the inlet end may use a drop-inlet type design to reduce the length of grate required.

A mm [ in. The recommended grate design may affect culvert capacity if significant blockage by debris is likely; however, because capacity is not normally the governing design criteria for parallel structures, hydraulic efficiency may not be an overriding concern.

A report issued by the University of Kansas suggests that a 25 percent debris blockage factor should be sufficiently conservative to use as a basis for culvert design in these cases 8. This report also suggests that under some flow conditions, the capacity of a grated culvert may be. In those locations where headwater depth is critical, a larger pipe should be used or the parallel drainage structure may be positioned outside the clear zone, as discussed in the following section.

Some parallel drainage structures can be moved laterally farther from the through traveled way. This treatment often affords the designer the opportunity to flatten the transverse slope within the selected clear-zone distance of the roadway under design. If the embankment at the new culvert locations is traversable and likely to be encroached upon by traffic from either the main road or side road, safety treatment should be considered.

It is suggested that the inlet or outlet match the transverse slope regardless of whether additional safety treatment is deemed necessary. Figure 1 shows a suggested design treatment, while Figure shows a recom- mended safety treatment for parallel drainage pipes. Flow in Drainage Channel. In cases in which the transverse slope cannot be made traversable, the structure is too large to be safety treated effectively, and reloca- tion is not feasible, shielding the obstacle with a traffic barrier may be necessary.

Specific information on the selection, location, and design of an appropriate barrier system is in Chapter 5. Drop inlets can be classified as on-roadway or off-roadway structures.

On-roadway inlets are usually located on or alongside the shoulder of a street or highway and are designed to intercept runoff from the road surface. These include curb opening inlets, grated in- lets, slotted drain inlets, or combinations of these three basic designs. Because they are installed flush with the pavement surface, they do not constitute a significant safety problem to errant motorists. However, they should be selected and sized to accommodate design water runoff.

In addition, they should be capable of supporting vehicle wheel loads and should be pedestrian and bicycle compatible. Off-roadway drop inlets are used in medians of divided roadways and sometimes in roadside ditches.

Although their purpose is to col- lect runoff, they should be designed and located to present a minimal obstacle to errant motorists. This goal can be accomplished by building these features flush with the channel bottom or slope on which they are located. No portion of the drop inlet should project more than mm [4 in. The opening should be treated to prevent a vehicle wheel from dropping into it; however, unless pedestrians are a consideration, grates with openings as small as those used for pavement drainage are not necessary.

Neither is it necessary to design for a smooth ride over the inlet; it is sufficient to prevent wheel snagging and the resultant sudden deceleration or loss of control. Discussion-The available recovery area of 8. If the culvert headwall is greater than mm [4 in. If the foreslope contains rough outcroppings or boulders and the headwall does not significantly increase the obstruction to a motorist, the decision to do nothing may be appropriate.

A review of the highway's crash history, if available, may be made to determine the nature and extent of vehicle encroachments and to identify any specific locations that may require special treatment.

Discussion-The available recovery area of 1. If this section of road has a significant number of run-off-the-road crashes, it may be appropriate to consider shielding or removing the entire row of trees within the crash area.

If this section of road has no significant history of crashes and is heavily forested with most of the other trees only slightly farther from the road, this tree would probably not require treatment. If, however, none of the other trees are closer to the roadway than, for example, 4. If a tree were 3.

This example emphasizes that the clear-zone distance is an approximate number at best and that individual objects should be analyzed in relation to other nearby obstacles. Discussion-Since the non-recoverable foreslope is within the recommended suggested clear-zone distance of the 1VH foreslope, a runout area beyond the toe of the non-recoverable foreslope is desirable.

Using the steepest recoverable foreslope before or after the non-recoverable foreslope, a clear-zone distance is selected from Table In this example, the 1V:SH foreslope beyond the base of the fill dictates a 9 to 10 m [30 to 32 ft] clear-zone distance. Since 7 m [23 ft] are available at the top, an additional 2 to 3 m [7 to 10 ft] could be provided at the bottom. Since this is less than the 3 m [lo ft] recovery area that should be provided at the toe of all the non-recoverable slopes the 3 m [lo ft] should be applied.

All foreslope breaks may be rounded and no fixed objects would normally be built within the upper or lower portions of the clear-zone or on the intervening foreslope. Discussion-Since the critical foreslope is within the suggested clear-zone distance of 9 to However, if this is an isolated obstacle and the roadway has no significant crash history, it may be appropriate to do little more than delineate the drop-off in lieu of foreslope flattening or shielding.

Discussion-The available recovery area of ;I is 0. If much of this roadway has a similar cross section and no significant run-off-the-road crash history, neither foreslope flattening nor a traffic barrier would be recommended. On the other hand, even if the 1V5H foreslope were 3 m [lo ft] wide and the clear-zone re- quirement were met, a traffic barrier might be appropriate if this location has noticeably less recovery area than the rest of the roadway and the embankment was unusually high.

Discussion-Since the range for the flatter slope of 26 to 30 ft extends past the slope break onto the steeper slope, the upper end of this range should be considered. However, the range for the steeper slope of 32' to 40' might be considered conservative since the majority of the clear zone area is on the flatter slope.

Thus the lower range of this slope might be considered. An appropriate range for this combination slope could be 30 to 32 ft. In this example, it would be desirable to have no fixed objects constructed on any part of the 1V5H foreslope. Natural obstacles such as trees or boulders at the toe of the slope would not be shielded or removed. However, if the final foreslope were steeper than 1V4H, a clear runout area of 3 m [lo ft] should be considered at the toe of the foreslope. The designer may choose to limit the clear-zone distance to 9 m [30 ft] if that distance is consistent with the rest of the roadway template, a crash analysis or site investigation does not indicate a potential run-off-the-road problem in this area, and the distance selected does not end at the toe of the non-recoverable foreslope.

Discussion-For channels within the preferred cross-section area of Figures or , the clear-zone may be determined from Table 3- 1. However, when the suggested clear-zone exceeds the available recovery area for the foreslope, the backslope may be considered as additional available recovery area. The range for the suggested clear zone for the foreslope of 6 to 7. Since the backslope cut has a suggested clear-zone of 5 to 5. In addition, fixed objects should not be located near the center of the channel where the vehicle is likely to funnel.

An appropriate range for this combination slope could be 20 to 24 R. Because the tree is located beyond the suggested clear zone, removal is not required. Removal should be considered if this one ob- stacle is the only fixed object this close to the through traveled way along a significant length.

Drainage channels not having the preferred cross section see Figure or should be located at or beyond the suggested clear zone. However, backslopes steeper than 1V:3H are typically located closer to the roadway. If these slopes are relatively smooth and unobstructed, they present little safety problem to an errant motorist. If the backslope consists of a rough rock cut or outcropping, shielding may be warranted as discussed in Chapter 5.

Discussion-The ditch is not within the preferred cross section area of Figure and is 0. However, if the ditch bottom and backslope are free of obstacles, no additional improvement is suggested. A similar cross section on the outside of a curve where encroachments are more likely and the angle of impact is sharper would probably be flattened if practical.

Discussion-The rock cut is within the given suggested clear-zone distance but would probably not warrant removal or shielding unless the potential for snagging, pocketing, or overturning a vehicle is high. Steep backslopes are clearly visible to motorists during the day, thus lessening the risk of encroachments.

Roadside delineation of sharper than average curves through cut sections can be an effective countermeasure at locations having a significant crash history or potential. Highway Clear Zone. Highway Clear Zone f 4- Cross Section. The suggested clear zone should be the greater of the two clear zones. Refer to the bold line in the above figure for the overall suggested clear zone.

Refer to Examples K and L for the ramp clear zones. I Shoulder. Discussion-Refer to the bold line in the above figure for the overall suggested clear zone. As an alternative, the clear zones for ramp may be set at 9 m [30 ft] if previous experience with similar projects or designs indicates satisfactory experience.

See Example-J for the speed-change lane clear zones. Shoulder I Ramp Shoulder. Radius: m [1, ft] Suggested clear-zone distance for 4: 1 foreslopes along the inside of curve: 2. Radius: m [1, ft] Suggested clear-zone distance for 6:l foreslopes along the inside of curve: 3.

Acceleration Lane Clear Zone. See Example J for the speed-change lane clear zones. Highway Safety Design and Operations Guide. To be released Fall 20 Graham, J. Highway Engineering Series No. McEnroe, B. KU- University of Kansas, Lawrence, KS, Olson, R. Weaver, H. Ross, and E. Jackson, and D. Sicking, T. Hirsch, H. Cooner, J. Nixon, S. Fox, and C. Safety Treatment ofRoadside Drainage Structures.

Transportation Research Record Bielenberg, J. Rohde, J. Reid, R. Faller, and K. Safety Gratesfor Cross-Drainage Culverts. Weaver, G. Marquis, and R.

Although a traversable and unobstructed roadside is highly desirable from a safety standpoint, some appurtenances simply should be placed near the traveled way. Man-made fixed objects that frequently occupy highway rights-of-way include highway signs, roadway lighting, traffic signals, railroad warning devices, intelligent transportation systems ITS , mailboxes, and utility poles.

According to the Insurance Institute for Highway Safety IIHS 7 , since the proportion of vehicle deaths involving collisions with fixed objects has fluctuated between 19 and 22 percent see Section 1. Approximately 50 percent 4, of all fixed-object fatalities involve crashes with trees, 5 percent involve sign and lighting supports, and 12 percent 1, involve utility poles. Although they are less frequent, collisions with other roadside hardware are frequently severe as well.

Figure shows the percent distribution of fixed-object crash deaths in by object struck. This chapter is not intended to provide technical design details.

Similarly, information on existing operational hardware is included only to the extent necessary to familiarize the designer with the types of breakaway devices available and how each is intended to function. The highway designer is charged with providing the safest facility practicable within given constraints.

As noted in Chapter 1, there are six options for mitigation of objects within the design clear zone: 1. Remove the obstacle 2. Redesign the obstacle so it can be traversed safely 3. Relocate the obstacle to a point where it is less likely to be struck 4. Reduce impact severity by using an appropriate breakaway device 5. Shield the obstacle with a longitudinal traffic barrier designed for redirection or use a crash cushion or both if it cannot be elimi- nated, relocated, or redesigned 6.

Delineate the obstacle if the above alternatives are not appropriate While the first two options are the preferred choices, these solutions are not always practical, especially for highway signing and light- ing, which should remain near the roadway to serve their intended functions. This chapter deals primarily with the fourth option: the use of breakaway hardware, which has become a cornerstone of the forgiving roadside concept since its inception in the mids.

Emphasis is placed on the selection of the most appropriate device to use in a given location and on installing the support needed to ensure acceptable performance when the device is hit. The final section of this chapter addresses the problems associated with trees and shrubs and provides the designer with some guidelines to follow on this frequently sensitive topic.

The term breakaway support refers to all types of sign, luminaire, and traffic signal supports that are designed to yield, fracture, or separate when impacted by a vehicle. The release mechanism may be a slip plane, plastic hinge, fracture element, or a combination of them. Newly developed supports tested under MASH will un- dergo an additional crash test with a pickup truck; this test is intended to evaluate potential for windshield penetration with a taller pas- senger vehicle than has been used for testing in the past.

NCHRP Report and MASH criteria require that a breakaway support perform in a predictable manner when struck head-on by an kg [ lb] andlor kg [ Ib] vehicle, or its equivalent, at speed from 30 kmih [19 mph] to kmlh [62 mph]. Limits are placed on the transverse and longitudinal components of the occupant impact velocity and the crash vehicle should remain stable and upright during and after the impact with no significant deformation or intrusion of the windshield or passenger compartment.

These specifications also establish a maximum stub height of mm [4 in. The appropriate procedures for acceptance testing of breakaway supports are described in Sections 2. Full-scale crash tests, bogie tests, and pendulum tests are used in the acceptance testing of breakaway devices.

In full-scale testing, an actual vehicle is accelerated to the test speed and impacted into the device being tested. The point of initial impact is the front of the vehicle, either at the center or quarter point of the bumper. Full-scale tests produce the most accurate results, but their main disadvan- tage is cost. Bogie vehicles also are used to test breakaway hardware. A bogie is a reusable, adjustable surrogate vehicle used to model actual vehicles. A nose, similar to a pendulum nose, is used to duplicate the crush characteristics of the vehicle being modeled.

To reduce testing costs, pendulum tests are used to evaluate breakaway hardware. Pendulum nose sections have been developed that model the fronts of vehicles. Pendulum tests typically have been used to test luminaire support hardware. This extrapolation method should not be used with base-bending or yielding supports.

Sign, luminaire, traffic signal, and similar supports first should be structurally adequate to support the device mounted on them and to resist ice, wind, and fatigue loads as specified in the AASHTO Standard SpeciJicationsfor Structural Supports for Highway Signs, Luminaires and Trafic Signals 3.

Other concerns are that supports be properly designed and carefully located to ensure that the breakaway devices perform properly and to minimize the likelihood of impacts by errant vehicles. For example, supports should not be placed in drainage ditches where erosion and freezing could affect the proper operation of the breakaway mechanism. It also is possible that a vehicle entering the ditch could be inadvertently guided into the support.

Signs and supports that are not needed should be removed. If a sign is needed, then it should be located where it is least likely to be hit.

Whenever possible, signs should be placed behind existing roadside barriers beyond the design deflection distance , on existing structures, or in similar non-accessible areas.

If this cannot be achieved, then breakaway supports should be used. Only when the use of breakaway supports is not practicable should a traffic barrier or crash cushion be used exclusively to shield sign supports.

As a general rule, breakaway supports should be used unless an engineering study indicates otherwise. However, concern for pedes- trian involvement has led to the use of fixed supports in some urban areas. Because pedestrian activity tends to concentrate during. Examples of sites where breakaway supports may be imprudent are those adjacent to bus shelters or that have extensive pedestrian concentrations.

Supports placed on roadside slopes should not allow impacting vehicles to snag on either the foundation or any substantial remains of the support. Surrounding terrain should be graded to permit vehicles to pass over any non-breakaway portion of the installation that remains in the ground or rigidly attached to the foundation. Figure , adopted from these specifications, illustrates the method used to measure the maximum stub height.

Ground Line. The MUTCD requires that breakaway supports housing electrical components utilize electrical disconnects to reduce the risk of fire and electrical hazards after the structure is impacted by a vehicle. Upon knockdown, the supportlstructure should electronically dis- connect as close to the pole base as possible. Breakaway support mechanisms are designed to function properly when loaded primarily in shear.

Most mechanisms are designed to be impacted at bumper height, typically at about mm [20 in. If impacted at a significantly higher point, the bending moment in the breakaway base may be sufficient to bind the mechanism, resulting in non-activation of the breakaway device. For this reason, it is critical that breakaway supports not be located near ditches, on steep slopes, or at similar locations where a ve- hicle is likely to be partially airborne at the time of impact.

Supports placed on a foreslope of 1 to 6 or flatter are acceptable. Supports placed on foreslopes that are 1 to 4 through 1 to 6 are only acceptable when the face of the support is within mm [24 in.

The type of soil also may affect the activation mechanisms of some breakaway supports. Fracture-type supports e. Usually this occurrence is not a problem for a fracture-type support embedded less than 1 m [3 ft] because the support will likely pull out of the soil, unless a special anchor plate prevents it. For fracture-type supports with pull- out-resisting anchors, supports embedded more than 1 m [3 ft], or any other support that might be sensitive to foundation movement, consideration should be given to qualifying them through crash testing in "weak soil" in addition to qualifying them through "standard soil" crash tests.

As explained in the Commentary on Chapter 3 in the MASH The weak soil should be used, in addition to the standard soil, for any feature whose impact performance is sensi- tive to soil-foundation or soil-structure interaction if: 1 identifiable areas of the state or local jurisdiction in which the feature will be installed contain soil with similar properties, and 2 there is a reasonable uncertainty regard- ing performance of the feature in the weak soil.

Tests have shown that some base-bending or yielding small sign supports readily pull out of the weak soil upon impact. For features of this type, the strong soil is generally more critical and tests in the weak soil may not be necessary.

Special anchor plates or design details also may be used to accommodate expected wind loads. Because these design details could affect proper performance, it is recommended that these designs also be tested in both soils. To affect a truly cost-effective program of breakaway supports, other items need to be considered. Availability of a particular support will affect installation costs and re- placement costs. Durability of the support will affect the expected life of a non-struck support.

Also, some supports can be reused after being impacted by a vehicle, which may be more cost-effective even though the initial costs are high. Thus, the expected impact frequency and simplicity of maintenance may influence an agency's selection. Roadway signs can be divided into three main categories: overhead signs, large roadside signs, and small roadside signs. The hardware and corresponding safety treatment of sign supports varies with the sign category.

Some states shield all supports regardless of offset. The large mass of these support systems and the potential safety consequences of the systems falling necessitate a fixed-base design that cannot be made breakaway. Where possible, these supports should be located behind traffic barriers protecting nearby overpasses or other existing structures, or the signs should be mounted on the nearby structure.

All overhead sign supports located within the clear zone should be shielded with a crashworthy barrier. The location of the support should provide clearance between the back of the rail and the face of the support to ensure that the rail will function as intended when struck by a vehicle.

This clearance will vary depending on the type of rail used. They typically have two or more breakaway support posts. The basic concept of the breakaway sign support is to provide a structure that will resist wind and ice loads, yet fail in a safe and predictable manner when struck by a vehicle. Figure shows the loading conditions for which the support should be designed, while Figure depicts the desired impact performance. To achieve satisfactory breakaway performance, the following criteria should be met: The hinge should be at least 2.

A single post, spaced with a clear distance of 2. The total mass [weight] below the hinge but above the shear plate of the breakaway base should not exceed kg [ Ib]. For two posts spaced with a clear distance of less than 2. No supplementary signs should be attached below the hinges if such placement is likely to interfere with the breakaway action of the support post or if the supplemental sign is likely to strike the windshield of an impacting vehicle.

The breakaway mechanisms of large roadside sign supports are of either a fracture or a slip-base type. Fracture mechanisms consist of either couplers or wood posts with reduced cross sections. Most couplers are considered to be multidirectional; that is, they are expected to work satisfactorily when struck from any direction. Figure shows one type of multidirectional coupler in common use. Slip-base type mechanisms activate when two parallel plates slide apart as the bolts are pushed out under impact.

The designs may be either unidirectional or multidirectional. Horizontal slip bases using the four-bolt pattern shown in Figure are unidirectional.

The upper hinge design for unidirectional impacts consists of a slotted h s e plate on the expected impact side and a saw cut through the web of the post to the rear flange.

The rear flange then acts as a hinge when the post rotates upward. Figure shows this commonly used design. Slotted plates may be used on both sides of the post if impacts are expected from either direction. Moment reaction is small Moment Moment. Vehicle passes underneath sign Vehicle force. Proper fimctioning of the slip base and fuse plate designs requires proper torque of the bolts.

If the bolted connection is too tight, friction forces between the plates may prevent activation of the breakaway base under intended loading conditions. If the bolts are under-torqued, the posts may "walk" off the base under wind and other vibration loads.