A separate clear zone is not necessary for speed-change lanes on conven- tional highways and where the auxiliary lane does not h c t i o n as a through lane e. Refer to Example at the end of this chapter for an example of a freeway speed-change lane. The suggested clear-zone distance along the ramp may be based on the speed, volume, horizontal curvature, and roadside geometry along the ramp.
Because ramps are of limited length, often contain very sharp curves, and tend to be overdriven by motorists, design- ers should use a conservative approach to determining the clear-zone distance.
For the purpose of determining this suggested clear- zone distance, the design speed along the ramp proper, which excludes a transition curve of m [I ft] or greater, should be determined from the simplified curve formula in Chapter 3 of AASHTO's A Policy on Geometric Design for Highways and Streets 4.
Transition curves of m [I ft] or more can act as extensions of the speed-change lane and should have speeds similar to the adjacent tangent or speed-change lane. For simple ramps, such as loop and diagonal ramps, the design speed and volume of the ramp proper should be used to determine the suggested clear-zone distance.
When compound and reverse curves are used, the clear-zone distance recommended for the higher- speed curve excluding transition curves may be used for the entire ramp. Refer to Example 3-K for more detailed information. For complex ramps with multiple radii and variable operating speeds, a separate clear-zone distance may be determined for each unique segment of the ramp. Refer to Example 3-L for more detailed information.
Alternately, clear zones for ramps may be set at 9 m [30 ft] if previous experience with similar projects or designs indicates satisfac- tory experience.
This method provides a consistent template that can be more practical to design and maintain. 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 while considering their consequences on the roadside environment. In addition to drainage channels, which were. In general, the following options, listed in order of preference, are applicable to all drainage features: Eliminate non-essential drainage structures a Design or modify drainage structures so they are traversable or present a minimal obstruction to an errant vehicle If a major drainage feature cannot be effectively redesigned or relocated, shield it by using a suitable traffic barrier if it is in a vulnerable location The remaining sections of this chapter identify the safety problems associated with curbs, pipes and culverts, and drop inlets, and they offer recommendations about 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 engineeringjudgment 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 those having a vertical or nearly vertical traffic face mm [6 in. They are intended to discourage motorists from deliberately leaving the roadway. Sloping curbs are those having a sloping traffic face mm [6 in.
Sloping curbs, especially those with heights of mm [4 in. Curbs higher than mm [4 in. However, if higher curbs are used, they are not normally regarded as fixed objects that would require mitigation. In general, curbs are not desirable along high-speed roadways 9. If a vehicle is spinning or slipping sideways as it leaves the road- way, wheel contact with a curb could cause it to trip and overturn.
In 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 ap- purtenances. Refer to Section 5. When obstructions exist behind curbs, a minimum lateral offset of 0. A minimum lateral offset of 0.
This lateral offset should not be construed as a clear-zone distance. Because curbs do not have a significant redirectional capability, obstructions behind a curb should be located at or beyond the suggested clear-zone distances shown in Table In many instances, obtaining the suggested clear-zone distances on existing facilities will not be feasible.
On new construction for which suggested clear-zone dis- tances cannot be provided, fixed objects should be located as far from the traveled way as practical on a project-by-project basis, but in no case closer than 0. Cross-drainage structures are designed to carry water underneath the roadway embankment and vary in size from mm 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. Although 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.
The options available to a designer to minimize these obstacles are 11 : Using a traversable design, Extending the structure so that it is less likely to be hit, and. Shielding the structure. Each of these options is discussed in the following subsections.
To maintain a traversable foreslope, 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 a single round pipe with a mm in. Extending culverts to locate the inlets or outlets a fixed distance from the through trav- eled way is not recommended if such treatment introduces discontinuities in an otherwise traversable slope.
Extending the pipe results in the warping of the foreslopes in or out to match the opening, which produces a significantly longer area that affects the motorist who has run off the road. Matching the inlet to the foreslope is desirable because it results in a much smaller target for the errant vehicle to hit, reduces erosion problems, and simplifies mowing operations. Single structures and end treatments wider than 0. Modifications to the culvert ends to make them traversable should not significantly decrease the hydraulic capacity of the culvert.
Safety treatments should be hydraulically efficient. To maintain hydraulic efficiency, it may be necessary to apply bar grates to flared wingwalls, flared end sections, or culvert extensions that are larger than the main barrel. The designer should consider shielding the structure if significant hydraulic capacity or clogging problems could result. Full-scale crash tests have shown that automobiles can traverse cross-drainage structures with grated-culvert end sections constructed of steel pipes spaced on mm [30 in.
This spacing does not significantly change the flow capacity of the culvert 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.
Figure shows recommended sizes to support a full-sized automobile and is based on a mm [in. More recently, two full-scale crash tests were conducted to examine the safety performance of a 6. The first test involved a P pick-up truck impacting the upstream portion of the grate.
The second test involved an C small car striking the culvert grate with the left-side tires while the right-side tires encountered the slope above the grate. These scenarios were determined to be the worst testing conditions.
This testing clearly demonstrated that the culvert safety grate recommended in Figure meets the safety performance evaluation guidelines set forth in NCHRP Report for a test level 3 TL-3 device. Further, these findings clearly support historical studies that show culvert grates provide the most cost-beneficial safety treatment for cross-drainage culverts. It is important to note that the toe of the foreslope and the ditch or stream bed area immediately adjacent to the culvert should be more or less traversable if the use of a grate is to have any significant safety benefit.
For median drainage where flood debris is not a concern and where mowing operations are frequently required, much smaller open- ings between bars may be tolerated and grates similar to those commonly used for drop inlets may be appropriate. In addition, 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 the median width nor the height of fill is excessive.
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. Two of the blends, one coarse and one fine, included silt as mineral filler, and the other two blends included clay aazhto mineral filler.
The laboratories were requested to follow Method B xashto Method D of T to compact the three replicates of the four aashto m blends at 5 different moisture contents. The aadhto process involving breaking up the compacted soil and adding water increments for further re-compactions was continued until sufficient test points were acquired to draw the compaction curve.
The laboratories were asked to take aashto samples of m kg 25 lbs and 7 kg 16 lbs for testing from the coarse and fine blends, respectively. The instructions and the aasjto entry sheet are provided in Appendix A. Do you enjoy reading reports from the Academies online for free?
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This edition presents an updated framework for geometric design that is more flexible, multimodal, and performance-based than in the past. The document provides guidance to engineers and designers who strive to make unique design solutions that meet the needs of all highway and street users on a project-by-project basis. File Name: aashto lrfd bridge design specifications. AASHTO noted that this 9th edition replaces the 8th edition — published in — and includes revisions to almost all of its specification sections.
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