Aashto roadside design guide 2006 pdf free download

Because roadside safety design is intended to minimize the consequences of a motorist leaving the roadway inadvertently, the designer should consider the speed at which encroachments are most likely to occur when selecting an appropriate roadside design standard or feature. Design values are presented in this document in both metric and U. The relationship between the metric and U.

Customary values is neither an exact soft conversion nor a completely rationalized hard conversion. The metric values are those that would have been used had the guide been presented exclusively in metric units; the U.

Customary values are those that would have been used if the guide had been presented exclusively in U. Therefore, the user is advised to work entirely in one system and not to attempt to convert directly between the two.

The reader is cautioned that roadside safety is a rapidly changing field of study, and changes in policy, criteria, and technology are certain to occur after this document is published. Beginning in the early s, as more Interstate highways and other freeways were opened to traffic, the nature and characteristics of the typical rural highway crash began to change.

Instead of head-on crashes with other vehicles or crashes involving trees immediately adjacent to the roadway, many drivers were running off the new freeways and colliding with man-made objects such as bridge piers, sign supports, culverts, ditches, and other design features of the roadway.

This document became known as the "Yellow Book" and its principles were widely applied to highway COIlstruction projects, particularly high-speed controlled access facilities.

A second edition of the Yellow Book, published by AASHTO in , stated that "for adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as wide as practical on a specific highway section.

Studies have indicated that on highspeed highways, a width of9 meters [30 feet] or more from the edge of the through traveled way permits about 80 percent of the vehicles leaving a roadway out of control to recover. Many obstacles located within this clear-zone distance were removed, relocated, redesigned, or shielded by traffic barriers or crash cushions. It soon became apparent, however, that in some lim-. Conversely, on most low-volume or low-speed facilities, a 9 III [30 ft] clear-zone distance was considered excessive and could seldom be justified for engineering, environmental, or economic reasons.

The AASHTO Guide for Selecting, Locating and Designing Traffic Barriers 2 modified the earlier clearzone concept by introducing variable clear-zone distances based on traffic volumes, speeds and roadside geometry. Figure 3. However, Figure 3. The curves are based on limited empirical data that was extrapolated to provide information for a wide range of conditions.

The designer must keep in mind site-specific conditions, design speeds, rural versus urban locations, and practicality. The distances obtained from Figure 3. I should suggest only the approximate center of a range to be considered and not a precise distance to be held as absolute. The designer may choose to modify the clear-zone distance for horizontal curvature obtained from either Figure 3.

These modifications are normally considered only when crash histories indicate a need, or a specific site investigation shows a definitive crash potential that could be significantly lessened by increasing the clear-zone width, and when such increases are cost effective. Horizontal curves, particularly for high-speed facilities, are usually superelevated to increase safety and provide a more comfortable ride.

Increased banking on curves where the superelevation is inadequate is an alternate method of increasing roadway safety within a horizontal curve, except where snow and ice conditions limit the use of increased superelevation. For relatively flat and level roadsides, the clear-zone concept is simple to apply. However, it is less clear when the roadway is in a fill or cut section where roadside slopes may be either positive, negative, or variable, or where a drainage channel exists near the through traveled way.

Consequently, these features must be discussed before a full understanding of the clear-zone concept is possible. Each of these features has an effect on a vehicle's lateral encroachment and trajectory as discussed in the following sections. Recoverable foreslopes are 1VAH or flatter. If such slopes are relatively smooth and traversable, the suggested clear-zone distance may be taken directly from Figure 3. Motorists who encroach on recoverable Ioreslopes can generally stop their vehicles or slow them enough to return to the roadway safely.

Fixed obstacles such as culvert headwalls will normally not extend above the foreslope within the clear-zone distance. Examples of suggested roadside design practices for recoverable foreslopes and the application of the clear-zone concept are in Section 3. A non-recoverable foreslope is defined as one that is traversable, but from which most vehicles will be unable to stop or to return to the roadway easily.

Vehicles on such slopes typically can be expected to reach the bottom. Since a high percentage of encroaching vehicles will reach the toe of these slopes, the clearzone distance cannot logically end on the slope. Fixed obstacles will normally not be constructed along such slopes and a clear runout area at the base is desirable. Section 3. Example C provides an example for a clear-zone computation.

A critical foreslope is one which a vehicle is likely to overturn, Foreslopes steeper than 1V:3H generally fall into this category. If a foreslope steeper than 1V:3H begins closer to the through traveled way than the suggested clear-zone distance for that specific roadway, a barrier might be warranted if the slope cannot readily be flattened.

Barrier warrants for critical foresIopes are discussed in Chapter 5. Clear zones may be limited to 9 III for practicality and to provide a consistent roadway template if previous experience with similar projects or designs indicates satisfactory performance.

Recovery of high-speed vehicles that encroach beyond the edge of the shoulder may be expected to occur beyond the toe of slope. Deterruinntion of the width of the recovery area at the toe of slope should take into consideration right-of-way availability, environmental concerns, economic factors, safety needs, and crash histories.

Also, the distance between the edge of the through traveled lane and the beginning of the IV :3H slope should influence the recovery area provided at the toe of slope. While the application may be limited by several factors, the foreslopc parameters which may enter into determining a maximum desirable recovery area arc illustrated in Figure 3.

Clear zones may be limited to 30 fl lor practicality and to provide a consistent roadway tempi ate if previous experience with similar projects or designs indicates satisfactory performance. Determination of the width of the recovery area at the toe of slope should take into consideration right-of-way availability, environmental concerns, economic factors, safety needs, and crash histories, Also, the distance between the edge of the through traveled lane and the beginning of the lV:3H slope should influence the recovery area provided at the toe of slope.

While the application may be limited by several factors, the foreslope parameters which may enter into determining a maximum desirable recovery area arc illustrated in Figure 3. The clear-zone correction factor is applied to the outside of curves only. Curves flatter than III [ It] do not require an adjusted clear zone.

Such a cross section is more economical than providing a continuous flat foreslope from the edge of the through traveled way to the original ground line and is generally perceived as safer than constructing a continuous steeper foreslope from the edge of the shoulder. Example clear-zone calculations for this type of cross section are also included in Section 3. Warrants for the use of a roadside barrier in conjunction with backslopes are included in Chapter 5. These are generally more critical to errant motorists than foreslopes or backs lopes because they are typically struck head on by run-off-the-road vehicles.

Transverse slopes of 1V:6H or flatter are suggested for high-speed roadways, particularly for that section of the transverse slope that is located Immediately adjacent to traffic.

This slope can then be transitioned to a steeper slope as the distance from the through traveled way increases. Transverse slopes of 1V: desirable; however, are their practicality may be limited by width restrictions and the maintenance problems associated with the long tapered ends of pipes or culverts.

Transverse slopes steeper than 1V:6H may be considered for urban areas or for lowspeed facilities. Figures 3. Safety treatments for parallel drainage structures are discussed in Section 3. Some alternative designs for drains at median openings are shown in Figure 3. The water flows into a grated drop inlet in the median to a cross-drainage structure or.

If the foreslope between the roadway and the base of the bnckslope is traversable lV:3H or flatter and the backslope is obstaclefree, it may not be a significant obstacle, regardless of its distance from the roadway. On the other hand, a steep, rough-sided rock cut should normally begin outside the clear zone or be shielded.

A rock cut is normally considi ered to be rough-sided when the face will cause excessive. Section A -A 'Use of the IIattest possible median cross slopes on high-speed highways, particularly within the appropriate clear-zone area, can provide an improved roadside. Safety treatment of culverts as discussed in Seclion 3. This eliminates the two pipe ends that would be exposed to traffic in the median. Channels must be designed to carry the design runoff and to accommodate excessive storm water with minimal highway flooding or damage.

However, channels should also be designed, built, and maintained with consideration given to their effect on the roadside environment. Cross sections shown in the shaded region of each of the figures are considered to have traversable cross sections. Channel sections that fall outside the shaded region are considered less desirable and their use should be limited where high-angle encroachments can be expected, such as the outside of relatively sharp curves.

Channel sections outside the shaded region may be acceptable for projects having one or more of the following characteristics: restrictive right-of-way; rugged terrain; resurfacing, restoration, or rehabilitation 3R construction projects; or on low-volume or low-speed roads and streets, particularly if the channel bottom and backslopes are free of any fixed objects.

If practical, drainage channels with cross sections outside the shaded regions and located in vulnerable areas may be reshaped and converted to a closed system culvert or pipe or, in some cases, shielded by a traffic barrier. Warrants for the use of roadside barrier to shield nontraversable channels within the clear zone are included in Chapter 5. A basic understanding of the clear-zone concept is critical to its proper application.

The numbers obtained from Figure 3. Again, the curves are based all limited empirical data that was then extrapolated to provide data for a wide range of conditions.

Thus, the numbers ob-. In some cases, it is reasonable to leave a fixed object within the clear zone; in other instances, an object beyond the clearzone distance may require removal or shielding. Use of an appropriate clear-zone distance amounts to a compromise between maximizing safety and minimizing construction costs. Appropriate application of the clear-zone concept. The following sections are intended to illustrate a process that may be used to determine if a fixed object or non-traversable terrain feature warrants relocation, modification, removal, shielding, or no treatment.

The guidelines in this chapter may be most applicable 0 new construction or major reconstruction. On resurfacing, rehabilitation, or restoration 3R projects, the pri, mary emphasis is placed on the roadway itself. The actual. Consequently, it may not be cost-effective or practical because of environmental impacts or limited right-of-way to bring a 3R project into full compliance with all of the clear-zone recommendations provided in this guide.

Because of the scope of such projects and the limited funding available, emphasis. Bodies of water and escarpments are the types of areas that may be considered for special emphasis.

Example C demonstrates the method for calculating the desirable runout area. They will cause most vehicles to overturn and should be treated if they begin with ill the clear-zone distance of a particular highway and meet the warrants for shielding contained in Chapter 5.

Examples C, D, and E illustrate the application of the clear-zone concept to critical foreslopes. The clear-zone distance for recoverable foreslopes of IV:4H or flatter may be obtained directly from Figure 3. On new construction or major reconstruction, smooth slopes with no significant discontinuities and with no protruding fixed objects are desirable from a safety standpoint. It is desirable to have the top of the slope rounded so an encroaching vehicle remains in contact with the ground.

It is also desirable for the toe of the slope to be rounded to make it essentially traversable by an errant vehicle. Designing smooth cross slopes is normally accomplished by using standard or typical cross sections. The flatter the selected slope, the easier it is to mow or otherwise maintain and the safer it becomes to negotiate. The examples at the end of this chapter illustrate the application of the clear-zone concept to recoverable foreslopes.

If an adequate recovery area as determined from Figure 3. Clear-zone distances for embankments with variable foreslopes ranging from essentially flat to 1V:4H may be averaged to produce a composite clear-zone distance. Slopes that change from a foresJope to a backslope cannot be averaged and should be treated as drainage channel sections and analyzed for travers ability using Figure 3.

Although a "weighted" average of the foreslopes may be used, a simple average of the clear-zone distances for each foreslope is accurate enough if the variable slopes are approximately the same width.

If one foreslope is significantly wider, the clear-zone computation based on that slope alone may be used. However, because many vehicles on slopes this steep will continue Oil to the bottom, a clear runout area beyond the toe of the non-recoverable foreslope is desirable. The extent of this clear runout area could be determined by first t1nding the available distance between the edge of the through traveled way and the breakpoint of the recoverable foreslope to the non-recoverable foreslope.

See Figure 3. This distance is then subtracted from the recommended clear-zone distance based on the steepest recoverable foreslope before or after the non-recoverable foreslope. The result is the desirable clear runout area that should be provided beyond the non-recoverable foreslope if practical.

The clear runout area may be reduced in width based on existing conditions or site investigation. Such a variable sloped typical section is often used as a compromise between roadside safety and economics. By providing a relatively flat recovery area immediately adjacent to the roadway, most errant motorists can recover before reaching the steeper foreslope beyond.

The foreslope break may be liberally rounded so that an encroaching vehicle does not become airborne. It is suggested that the steeper slope be made as smooth as practical and rounded at the bottom.

It is important that roadside hardware not be located in or near ditch bottoms or on the backslope near the drainage channel. Any vehicle leaving the roadway may be funneled along the drainage channel bottom or encroach to some extent on the backslope, thus making an impact more likely.

Breakaway hardware may not func-. Non-yielding fixed objects should be located beyond the clear-zone distance for these cross sections as determined from Figure 3.

Effective drainage is one of the most critical elements in the design of a highway or street. However, drainage features should be designed and built with consideration given to their consequences on the roadside environment. In addition to drainage channels, which were addressed in Section 3.

The remaining sections of this chapter identify the safety problems associated with curbs, pipes and culverts, and drop inlets, and offer recommendations concerning the location and design of these features to improve their safety characteristics without adversely affecting their hydraulic capabilities.

The information presented applies to all roadway types and projects. However, as with many engineering applications, the specific actions taken at a given location often rely heavily on the exercise of good engineering judgment and on a case-by-case assessment of the costs and benefits associated with alternative designs. Curb designs are classified as vertical or sloping. Vertical curbs are defined as those having a vertical or nearly vertical traffic face nun [6 in.

These are intended to discourage motorists from deliberately. Sloping curbs are defined as those having a sloping traffic face mm [6 in. These can be readily traversed by a motorist when necessary, but a designer may prefer a height for sloping curbs of no greater than mm [4 in. In general, curbs are not desirable along high-speed roadways.

If a vehicle is spinning or slipping sideways as it leaves the roadway, wheel contact with a curb could cause it to trip and overturn, Under other impact conditions, a vehicle may become airborne, which may result in loss of control by the motorist. The distance over which a vehicle may be airborne and the height above or below normal bumper height attained after striking a curb may become critical if secondary crashes occur with traffic barriers or other roadside appurtenances.

If a curb is used in conjunction with a metal beam traffic barrier, it should ideally be located flush with the face of the railing or behind it. Where the curb height is mm [6 in. Curbs should not be used in front of sloping faced concrete barriers because such placement may result in unsatisfactory barrier performance. Refer to Chapter 5, Section 5.

A National Cooperative Highway Research Program project, scheduled for completion in March , is under way to develop design guidelines for the use of curbs and curbbarrier combinations. When obstructions exist behind curbs, a minimum horizontal clearance of 0. This offset may be considered the minimum allowable horizontal clearance or operational offset , but it should not be COIlstrued as an acceptable clear zone distance.

Since curbs do not have a significant redirectional capability, obstructions behind a curb should be located at Of beyond the minimum clear-zone distances shown in Table 3. In many instances, it will not be feasible to obtain the recommended clear zone distances on existing facilities. On new construction where minimum recommended clear zones callnot be provided, fixed objects should be located as far from traffic as practical on a project-by-project basis, but ill case closer than 0.

Cross-drainage structures are designed to carry water underneath the roadway embankment and vary in size from nun [18 in.

Typically, their inlets and outlets consist of concrete headwalls and wingwalls for the larger structures and beveled-end sections for the smaller pipes. While these types of designs are hydraulically efficient and minimize erosion problems, they may represent an obstacle to motorists who run off the road. This type of design may result in either a fixed object protruding above an otherwise traversable embankment or an opening into which a vehicle can drop, causing an abrupt stop.

Such a facility would resemble a landing strip or runway at an airport. Thus, it is readily apparent from the start that roadside design must be a series of compromises between "absolute" safety and engineering, environmental, and economic constraints. The designer should strive for embankments as smooth or traversable as practical for a given facility. As indicated in Sections 3. If a foreslope is traversable, the preferred treatment for any cross-drainage structure is to extend or shorten it to intercept the roadway embankment and to match the inlet or outlet slope to the foreslope.

For small culverts, no other treatment is required. For cross-drainage structures, a small pipe culvert is defined as a single round pipe with a mm [36 in.

Extending the pipe results in warping the foreslopes in or out to match the opening, which produces a significantly longer area that affects the driver who has run off the road.

Matching the inlet to the foreslope is desirable because it results in an extremely small "target" to hit, reduces erosion problems, and simplifies mowing operations.

Single structures and end treatments wider than I m 3 ft] can be made traversable for passenger size vehicles by using bar grates or pipes to reduce the clear opening width. Modifications to the culvert ends to make them traversable should not significantly decrease the hydraulic capacity of the culvert. Safety treatments must be hydraulically efficient.

In order to maintain hydraulic efficiency, it may be necessary to apply bar grates to flared wingwalls, flared end sections, or to culvert extensions that are larger in size than the main barrel. The designer should consider shielding the structure if significant hydraulic capacity or clogging problems could result.

This spacing does not significantly change the flow capacity of a pipe unless debris accumulates and causes partial clogging of the inlet. This underscores the importance of accurately assessing the clogging potential of a structure during design and the importance of keeping the inlets free of debris. It is important to note that the toe of the foreslope and the ditch or stream bed area immediately adjacent to the culvert must be more or less traversable if the use of a grate is to have any significant safety benefit.

Normally, grading within the right-of-way limits can produce a satisfactory runout path. For median drainage where flood debris is not a concern and where mowing operations are frequently required, much smaller openings between bars may be tolerated and grates similar to those commonly used for drop inlets may be appropriate. It should also be noted that both the hydraulic efficiency and the roadside environment may be improved by making the culverts continuous and adding a median drainage inlet.

This alternative eliminates two end treatments and is usually a practical design when neither median width nor height offill are excessive. The salety pipe runners are Schedule 40 pipes spaced on centers of mm [30 in. For intermediate sized pipes and culverts whose inlets and outlets cannot readily be made traversable, an option often exercised by the designer is to extend the structure so the obstacle is located at or just beyond the appropriate clear zone. While this practice reduces the likelihood of the pipe end being hit, it does not completely eliminate the possibility.

As noted in Section 3. If the extended culvert headwall remains the only significant man-made fixed object immediately at the edge of the clear zone along the section of roadway under design, and the roadside is generally traversable to the right-ofway line elsewhere, simply extending the culvert to the edge of the clear zone may not be the best alternative, particularly on freeways and other high-speed, accesscontrolled facilities.

On the other hand, if the roadway has numerous fixed objects, both natural and man-made, at the edge of the clear zone, extending individual structures to the same minimum distance from traffic may be appropriate. 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 lowspeed 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.

For major drainage structures that are costly to extend and whose end sections cannot be made traversable, shielding with an appropriate traffic barrier is often 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.

They are typically used at transverse slopes under driveways, field entrances, access ramps, intersecting side roads, and median crossovers. Most such culverts 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.

Unlike cross-drainage pipes and culverts which are essential for proper drainage and operation of a road or street, parallel pipes can sometimes be eliminated by constructing an overflow section on the field entrance, driveway. 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 will usually be appropriate only at low-volume locations where this design does not decrease the sight distance available to drivers entering the main road. Care must also be exercised 0 avoid erosion of the entrance and the area downstream of the crossing. This can usually 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 approaching urban areas. Since 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 concentrations of run-off-the-road crashes, it may be desirable to convert the open channel into a storm drain and.

This treatment will eliminate the ditch section as well as the transverse slopes with pipe inlets and outlets. 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-Oil by a vehicle, Once this has been done, parallel drainage structures should match the selected transverse slopes and should be safety treated if possible when they are located in a vulnerable position relative to main road traffic.

While many of these structures are small and present a minimal target, the addition of pipes and bars perpendicular to traffic can reduce wheel snagging in the culvert opening. Research has shown that, for parallel drainage structures, a grate consisting of pipes set on mm [24 in. It is recommended that the center of the bottom bar or pipe be set at to nun [4 to 8 in. Generally, single pipes with diameters of mm [24 in. However, when a multiple pipe installation is involved, consideration of a grate for smaller pipes may be appropriate.

Reference may be. To achieve this result, both the roadway or ditch foreslope and the driveway foreslope should be IV: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 above. This study suggests that it could be cost-effective to flatten the approach slopes to 1V:6H and match the pipe openings to these slopes for all sizes of pipes up to nun [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 approximations and not as absolute numbers.

When channel grades permit, the inlet end may use a drop-inlet type design to reduce the length of grate required. 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. A report issued by the University of Kansas suggested that a 25 percent debris blockage factor should be sufficiently conservative for use as a basis for culvert design in these cases 5.

This report also suggests that under some flow conditions, the capacity of a grated culvert may be equal to that of a standard headwall design as a result of decreased entrance turbulence. 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. In cases where the transverse slope cannot be made traversable, the structure is too large to be safely treated effectively, and relocation is not feasible, it may be necessary to shield the obstacle with a traffic barrier.

Specific information on the selection, location, and design of an appropriate barrier system is contained in Chapter 5. 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 inlets, slotted drain inlets, or combinations of these three basic designs.

Since they are installed flush with the pavement surface, they do not constitute a significant safety problem to errant motorists. However, they must be selected and sized to accommodate design water runoff. In addition, they must be capable of supporting vehicle wheel loads and present no obstacle to pedestrians or bicyclists. Off-roadway drop inlets are used in medians of divided roadways and sometimes in roadside ditches. While their purpose is to collect runoff, they should be designed and located to present a minimal obstacle to errant motorists.

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 either main road or side road traffic, safety treatment should be considered. It is suggested that the inlet or outlet match the transverse slope regardless of whether or not additional safety treatment is deemed necessary.

A suggested design treatment is shown in Figure 3. This 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 mill [4 in. The opening should be treated to prevent a vehicle wheel from dropping into it; but unless pedestrians me 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 associated with it.

Texas Transportation Institute. University of Kansas. Discussion: The available recovery area of 8. If the culvert headwall is greater than mm [4 i Jin height and is the only obstruction on an otherwise traversable foreslope, it should be removed and the inlet modified to match the I V:5H foreslope. 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.

When an area 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 4. 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.

Using the steepest recoverable fares lope before or after the non-recoverable foreslope, a clear-zone distance is selected from Figure 3. I or Table 3. In this example, the 1V:8R foreslope beyond the base of the fill dictates a 9 to IO m [30 to 32 fl] clear-zone distance.

All foreslope breaks may be rounded and no fixed objects would normally be built within the upper or lower portions of the clearzone or on the intervening foreslope. It may be practical to provide less than the entire 4 to 5 m [13 to 17 ft] at the toe of the non-recovernble foreslope.

A smaller recovery area could be applicable based on the rounded foreslope breaks, the flatter IV:IOH foreslope at the top, or past crash histories. A specific site investigation may be appropriate in determining an approximate runout area beyond the toe of the non-recoverable foreslope. 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.

Although a "weighted" average of the foreslopes may be used, a simple average of the clear-zone distances for each foreslope is accurate enough if the variable foreslopes are approximately the same width. If one foreslope is significantly wider, the clear-zone computation based OIl that foreslope alone may be used. 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 1V:5H foreslope were 3 m [to ft] wide and the clear-zone requirement 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. The IV:8H foreslope and the 1V:5H foreslope may be averaged taking into account the distance available on each foreslope.

The distance 5 m [17 ftJ along the IV:5H foreslope is multiplied by the slope of The resulting distances are added together and divided into the sum of the two distances 6 m [20 ft] plus 5 m [17 ft] available. The result is an "average" foreslope which may be used in Figure 3. Decimal results of 0. The calculations are given below:. Since the example has II III [37 ft] available on the two foreslopes, it is acceptable without further treatment.

In this example, it would be desirable to have no fixed objects constructed on any part of the 1V:5H 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 1VAH, a clear runout area 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, 11 crash analysis or site investigation does 1I0t indicate a potential run-off-the-road problem in this area, and the distance selected does not end at the toe of the lion-recoverable foreslope.

Recommended dear-zone distance for IV:6H foreslope fill : 6 to 7. Discussion: For channels within the preferred cross-section area of Figures 3. However, when the recommended clear zone exceeds the available clear zone for the foreslope, an adjusted clear zone may be determined as follows: i 1. Calculate the percentage of the recommended clear-zone range available from the edge of through traveled way to the PVI of the foreslope 4.

Add the available clear zone on the foreslope to the range of values determined in Step 2 4. The adjusted clear-zone range is 5. Because the tree is located beyond the adjusted clear zone, removal is not required. Removal should be considered this one obstacle is the only fixed object this close to the through traveled way along a significant length. To determine the recommended clear zone for the foreslope in the trapezoidal calculated. See Example F for the method of foreslope averaging.

Drainage channels not having the preferred cross section see Figure 3. 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. However, if the ditch bottom and backslope are free of obstacles, 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. Ij 6 Discussion: The rock cut is within the given 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.

Although a traversable and unobstructed roadside is highly desirable from a safety standpoint, some appurtenances simply must be placed near the traveled way.

Manmade fixed objects that frequently occupy highway rightsof-way include highway signs, roadway lighting, traffic signals, railroad warning devices, motorist-aid callboxes, mailboxes, and utility poles. Approximately 15 percent of all fixed-object fatalities each year involve crashes with sign and lighting supports and utility poles.

Although of a lesser order of magnitude, collisions with other roadside hardware are frequently severe as well. Finally, it must be recognized that approximately 3, motorists a year are killed as a result of crashes with trees and other vegetation. This chapter is not intended [0 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 6 options from which to choose a safe design. In order of preference, these are: 2. Redesign the obstacle so it can be traversed. Conversely, there will be other instances where roadway conditions will prevent most motorists from driving as fast as the design speed.

Customary values are those that would have been used if the guide had been presented exclusively in U. Therefore, the user is advised to work entirely in one system and not to attempt to convert directly between the two. The reader is cautioned that roadside safety is a rapidly changing field of study, and changes in policy, criteria, and technology are certain to occur after this document is published. Newer Post Older Post Home.