Category HIGHWAY ENGINEERING HANDBOOK

Testing criteria and test levels for bridge railings are the same as previously discussed for roadside barriers (Art. 6.4). The FHWA maintains a list of designs that have been tested to NCHRP Report 350 levels, as well as designs that were tested to earlier guidelines and have been assigned an equivalent Report 350 test level. Railings are seldom evaluated for TL-1. Examples of railings designs that meet higher levels are as follows:

TL-2. Thrie-beam bridge railing, side mounted. Consists of a thrie-beam rail cen­tered 22 in (550 mm) above the deck, supported on steel or wood posts mounted on the side of the bridge. It is intended for use on lower-volume secondary roads. Actually tested only to NCHRP 230 criteria, it is considered to meet TL-2.

TL-3. Wyoming Two-Tube bridge railing. Consists of two horizontal tubular rails, 6 X 2 X 0.25 in (152 X 51 X 6.4 mm), supported by fabricated steel posts on 10-ft (3-m) centers, mounted on and flush with concrete curb. Height of top rail is 29 in (740 mm) and height of bottom rail is 16 in (405 mm). A heavier version meets TL-4.

TL-4. (1) Solid concrete railings. The current New Jersey, F-shape, single-slope, and

vertical wall types meet TL-4 when adequately reinforced and built to a minimum height of 32 in (810 mm). (2) Massachusetts S3 Steel Bridge Railing. This is a beam and post system with three tubular rails on W6 X 25 (W150 X 37) posts, mounted flush on the outside edge of a sidewalk or on top of a curb. The tube size varies. Steel tube pickets are bolted to the back to provide an aesthetic look. (3) Wyoming Two – Tube bridge railing. Consists of two horizontal tubular rails, 6- X 4- X 0.25-in (152- X 102- X 6.4-mm) top rail and 152- X 76- X 6.4-mm (6- X 3- X 0.25-in) bottom rail], supported by fabricated steel posts on 10-ft (3-m) centers, mounted on and flush with concrete curb. Height of top rail is 33 in (830 mm) and height of bottom rail is 19 in (480 mm). (4) BR27C railing. This railing may be mounted on a curb or flush mounted

on the bridge deck. The lower portion consists of a concrete parapet, 24 in (610 mm) high by 250 mm (10 in) thick. The upper portion consists of steel tube posts 4 X 4 X 3/16 02 X 102 X 4.8 mm)/6.5 ft (2 m) spaced at on center. A horizontal steel tube

rail [4 X 3 X 0.25 in (102 X 76 X 6.4 mm)] is mounted to the face of the posts.

TL-5. Solid concrete railings. The current New Jersey, F-shape, single-slope, and vertical wall types meet TL-5 when adequately reinforced and built to a minimum height of 42 in (1070 mm).

TL-6. Texas Type TT (Tank Truck). This system consists of a very heavily rein­forced and widened concrete safety shape, with a heavily reinforced continuous concrete member and post system above.

The first step in an upgrading project is to identify potentially deficient systems. Bridge railing designs prior to 1964 are particularly suspect. Strength and performance should be documented. Verify critical details such as base plate connections, anchor bolts, material (strength, toughness, and condition), welding details, reinforcement development, etc. Open-faced railings may cause snagging. Curbs or sidewalks adjacent to a railing may cause an impacting vehicle to vault or roll over. Approach transitions may be inadequate.

Retrofits can be developed to address inadequacies. When possible, use crash-tested designs in such updating. One common improvement is to rebuild the approach barrier and transition to current standards, continuing the metal-beam rail element, for example, across the structure to provide continuity. If a curb is in place, a retrofit railing can

often be blocked out to minimize rollover and ramping. Some specific retrofit concepts are discussed in the following.

Concrete retrofit (safety shape or vertical). The concrete safety shape can be used most effectively when it can be constructed in front of an existing railing that can remain in place. A vertical-faced concrete shape creates an effective barrier when added on top of and flush with an existing safety curb. The structure must be evaluated for the extra dead load imposed, and for the development of the required anchorage to resist impact forces.

W-beam and thrie-beam retrofits. A partial solution sometimes used is to continue an approaching W-beam or thrie-beam roadside barrier across the bridge. It may not bring the bridge into full compliance with AASHTO criteria, but may be satisfactory as an interim solution, particularly on low-volume roadways. Adequate anchorage is provided by the continuous system. Gradual stiffening in the transition area is advised to avoid snagging.

Metal post-and-beam retrofit. Where a sidewalk is present, a steel post (S shape or channel shape) can be anchored to the top and a pair of steel tubes attached to the roadway side to provide a smooth traffic barrier between the sidewalk and the roadway. The tube elements must be in line with the face of the curb. The postattachment can be designed to resist the impact loads, or a yielding design can be developed to mini­mize possible bridge deck damage. The existing bridge railing on the outside of the sidewalk can be adapted to a pedestrian railing.

Bridges should provide a full, continuous shoulder that maintains uniform clearance with approaching roadside elements. However, if the bridge is narrower than the

AASHTO Designation

Test Level:

Nominal Barrier Height: Maximum Dynamic Deflection:

Remarks: This proprietary portable barrier system is suitable for both permanent (unbalanced traffic flow) and temporary applications. It is composed of a chain of safety-shape concrete barrier segments 37 in long which can be shifted laterally. Even though the cost is relatively high, the system becomes cost-effective when frequent lateral movement of the temporary barrier is required while maintaining traffic.

‘Deflections may be reduced by using CRTS or SRTS.

FIGURE 6.35 Movable concrete median barrier. Conversion: 1 in = 25.4 mm. (From Roadside Design

Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission)

approaching roadway and shoulder, the appropriate flare rate should be provided where the railing is within the shy distance (Art. 6.7.3). Curbs higher than 8 in (200 mm ) in front of bridge railings should be avoided. If a sidewalk is present, use a bridge railing between the traffic and the sidewalk to protect pedestrians, and a pedestrian railing along the outside of the bridge. End treatment of the bridge railing is difficult under these circumstances. If a crash cushion or other barrier cannot be used, a vertically tapered end section may be the best solution. The location and extent of the taper must be carefully considered for the conditions present.

Bridge railings are longitudinal barriers intended to prevent vehicles from running off the edge of a bridge. A metal post-and-rail system, a concrete safety shape, and various combinations have been used. Bridge railings are attached to the structure and designed to have minimal deflection under impact. The AASHTO Standard Specifications for Highway Bridges require that bridge railings meet specific geometric criteria and resist specified loads without exceeding allowable stresses. However, the AASHTO LRFD Bridge Design Specifications provide the most current design criteria, based on NCHRP Report 350. While AASHTO specifications do not prescribe bridge rail crash testing, the Federal Highway Administration (FHWA) does require that all bridge railings used on the National Highway System be of a crash-tested design. Existing railings designed to prior AASHTO specifications or crash-tested under earlier guidelines may be acceptable through evaluation of in-service performance.

AASHTO Designation:

Test Level:

Nominal Barrier Height: Maximum Dynamic Deflection:

Remarks: This barrier is suitable for both permanent and temporary applications, Its primary advantage is that the adjacent pavement can be overlaid several Limes without affecting the performance of the barrier. Its disadvantage is that greater vehicle damage occurs at shallower impact angles compared to other safety-shape barriers.

FIGURE 6.34 Single-slope concrete median barrier. Conversion: 1 in = 25.4 mm. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission)

6.11.1 Selection Considerations

The selection of a railing should include consideration of the following five factors:

Railing performance. There must be evidence that the system will provide the desired performance level. Design to current AASHTO LRFD Bridge Design Specifications and crash-testing to NCHRP Report 350 is recommended. Compatibility. A crashworthy transition section is required if the approach barrier significantly differs in strength, height, or deflection characteristics.

Cost. Life-cycle cost analysis is desirable to compare alternatives. Initial costs, maintenance costs, and the costs of accidents must be considered.

Field experience. Review in-service performance of existing systems to evaluate effectiveness and cost.

Aesthetics. Appearance is particularly important in scenic areas, but systems must be selected that meet required performance levels.

Either roadside barriers or median barriers may be appropriate for sloped medians, depending on conditions. If a relatively flat median (slope of 1:10 or flatter) free of rigid objects is available, a median barrier can be placed at the center. When such desirable conditions are not available, some additional guidelines should be considered. Figure 6.36 shows three basic types of median sections. Section I (illustrations 1-3) represents a depressed median or one with a ditch; section II (illustrations 4-6) represents a stepped median or a median that separates traveled ways with significant differences in elevation; and section III (illustration 7) applies to a raised median.

Section I. Check to see if the slopes warrant a barrier. If both slopes require shielding (illustration 1), place a roadside barrier near the shoulder on each side of the

median. If only one slope must be shielded, place a median barrier near the shoulder on that side; use a rigid or semirigid barrier, and install a rub rail on the ditch side of the barrier to prevent snagging of a vehicle that has crossed the ditch. If neither slope requires shielding but one is steeper than 1:10 (illustration 2), place a rigid or semirigid median barrier on the side with the steeper slope when warranted. If both slopes are rel­atively flat (illustration 3), place a median barrier (any type with dynamic deflection not greater than half median width) at or near the center of the median if vehicle over­ride is not likely.

Section II. If the embankment slope is steeper than 1:10 but traversable (illustration 4), place a median barrier near the shoulder on the high side of the slope. If the slope is not traversable (such as a rough rock cut, illustration 5), place a roadside barrier at the top and bottom of the slope. If a retaining wall is located at the bottom of the slope, contour the base of the wall to the exterior shape of a concrete safety shape. If the slope is flatter than 1:10 (illustration 6), place a median barrier near the center.

Section III. If the median is sufficiently high and wide (illustration 7), vehicles may be redirected without a barrier. If the slopes are relatively flat and traversable, place a semirigid median barrier at the apex. If the slopes are not traversable, place a roadside barrier on either side.

When a median barrier is warranted, it is best to use the same barrier throughout the length of need. In cases where a roadside barrier is required on both sides of the median

for some length and a centrally located median barrier is situated upstream and downstream, use a gradual transition between the systems proceeding in the direction of traffic.

Transition sections are used between adjoining median barriers having significantly different deflection characteristics, between a semirigid median barrier and a rigid barrier (such as a bridge rail), and in similar situations. The transition sections should provide impact performance similar to standard sections, and emphasis should be placed on designs to avoiding vehicle snagging. Structural details of special impor­tance include the following:

• Rail splices should develop the tensile strength of the weaker rail.

• Use a flared or sloped connection if the connection could snag an opposite-direction vehicle. Use a standard terminal connector to attach a W-beam or thrie-beam rail to a rigid bridge railing or parapet, or provide a recessed area in the parapet wall to receive the rail.

• In cases such as a strong-post W-beam transition to the concrete safety shape, use a blockout design and consider adding a rub rail or using a thrie-beam instead.

• Use a transition length 10 to 12 times the difference in lateral deflection of the two systems under consideration.

• Increase stiffness gradually from the weaker to the stronger system by means such as decreasing the post spacing, increasing the post size, and using nested sections of W-beam or thrie-beam.

Remarks: The concrete safety shapes are the only operational rigid barriers. The lower-sloped face redirects vehicles without damage under low-impact conditions. During moderate to severe impacts, some energy is dissipated when the vehicle is lifted off the pavement. The loss of tire contact with the pavement also aids redirection.

The details of the shape are critical. The distance from the pavement to the break between the upper and lower slopes should be kept at 13 in or below. Barrier performance under moderate to severe impact conditions is not significantly affected by overlays on the lower-sloped face, The overall height of the barrier, however, needs to be maintained at a minimum of 29 in.

The safety-shape barrier is suitable for narrow medians. Both faces can be flared away from the centerline to provide room for rigid objects to be installed in the medians.

*Very severe hits may destroy the barrier. Reinforcing is recommended to prevent shattering of concrete where the top of the barrier has a width less than 1 ft.

FIGURE 6.31 Concrete safety-shape median barrier. Conversion: 1 in = 25.4 mm. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2004, with permission)

Median barriers, similar to roadside barriers, should be introduced and terminated with safety in mind. Openings or breaks in barriers should be kept to a minimum to negate the need for end treatments. Where openings are required, shield barrier ends or, if the median

SGM04a (with non­steel blocks)

TL-3 W6x9 6ft-3 in Two steel W – sections

Two 6 in x 8 in x 14 in timber or plastic 27 in

Approximately 2 ft

Remarks: These systems are semirigid and are satisfactory for use in narrow medians. After typical impacts, the system remains serviceable. Some states use a W-section as a rubrail, centered at 10 in above grade. This modification is appropriate for both the SGM06a and b, and a higher SGM04a and b. By dividing any of these systems into parallel roadside barriers, assuming adequate deflection distance, fixed objects in the median can be effectively shielded,

*6 in x 8 in post and blockout is acceptable

FIGURE 6.29 W-beam (strong-post) median barrier. Conversions: 1 in = 25.4 mm, 1 ft = 0.305 m. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission) is sufficiently wide, flare or offset the barrier. In locations where impacts are likely, end treatments must be crashworthy. Also, they should safely redirect vehicles impacting from the rear, where hits from opposing traffic are likely. Many proprietary devices are available for terminals. Appropriate end terminals for W-beam barriers include the CAT or Brakemaster. For concrete barriers consider the ADIEM, the TRACC, the QuadGuard™, and the REACT-350®. Sand barriers can also be used to shield median barrier ends, particularly where medians are wide and the likelihood of impacts from opposing traffic is low.

FIGURE 6.30 Thrie-beam (strong-post) median barrier. Conversions: 1 in = 25.4 mm, 1 ft = 0.305 m. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission)

Like roadside barriers, median barriers can be classified as flexible, semirigid, or rigid as indicated in Table 6.7. Figures 6.26 through 6.35 show details of these various types of median barriers and factors to be considered in selection and application. Additional comments on several of the systems follow. In many of their characteristics they are similar to their roadside barrier counterparts.

Typical three-strand cable systems (Fig. 6.26) should be used only if there is ade­quate deflection distance, about 12 ft (3.5 m) in each direction. Performance is sensi­tive to mounting height. Proper end anchorage is critical. They are not well suited for areas hit frequently, on sharp curves, and on facilities with high truck volumes.

сл о 00

о ю 20 30 40 50 60 70

MEDIAN WIDTH (feet)

Deflection can be reduced by decreasing post pacing. The system shown meets the TL-3 requirements.

High-tension cable systems are installed with significantly greater cable tension. They reduce deflections to 6.6 to 9.2 ft (2 to 2.8 m) and often show less damage after impact. Several proprietary systems have been accepted by the FHWA.

The W-beam (weak-post) system (Fig. 6.27) is sensitive to mounting. Proper end anchorage is essential. It is not well suited where terrain irregularities exist or where frost heave or erosion is likely to alter the mounting height by more than 2 in (50 mm). However, it is suitable for relatively flat, traversable medians without curbs or ditches that could affect vehicle trajectory. This is a Tl-2 system.

The box-beam (weak-post) median barrier (Fig. 6.28) is a TL-3 system, most suit­able for traversable medians with no significant irregularities. Posts have to be repaired after most hits to maintain correct beam height, so it should not be used in areas where it is likely to be frequently hit.

None (the former single-strand cable “MBI” is obsolete)

TD3

S3 x 5.7 steel 16 ft

V4-in-dia. steel cable 30 in 11 ft-6 in

Remarks: Because of the high dynamic deflection for cable systems, they are not recommended for use in medians narrower than approximately 23 ft, nor in medians which contain rigid objects. The extensive damage done during moderate to severe impacts leaves a significant length of barrier inoperative until repairs can be made. Cable median barrier systems are recommended for use on irregular terrain and on wider medians where the need is only to prevent infrequent, potentially catastrophic cross-median crashes. For proper performance it is essential that this system be installed and maintained at the correct mounting height. This system is similar to the 3-strand cable roadside barrier, except that one of the cables is mounted on the opposite side of the post from the other two.

FIGURE 6.26 Three-cable median barrier. Conversions: 1 in = 25.4 mm, 1 ft = 0.305 m. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2004, with permission)

The blocked-out W-beam (strong-post) median barrier meets TL-3 or TL-2, depending upon the post type and blocking used. Figure 6.29 shows several variations of the system. Mounting heights of 30 in (760 mm) are sometimes specified but have not been tested. A separate rub rail (usually a steel channel or tube) has sometimes been added to minimize postsnagging problems with the higher mounting height.

The blocked-out thrie-beam (strong-post) median barrier meets TL-3 and the modified thrie-beam meets TL-4. The post type and blocking used affect the rating (Fig. 6.30). The thrie-beam is capable of accommodating a larger range of vehicle sizes than the W-beam because of its greater beam depth. Also, the deeper beam eliminates the need for a rubrail.

The concrete safety shape (Fig. 6.31) is the most common rigid median barrier because of low cost, effective performance, and low maintenance. Approved shapes include the New Jersey and F-shaped barriers, the single-slope barrier, and the vertical wall barrier.

Remarks: This barrier system is suitable for wide, flat medians where sufficient space is available to accommodate deflections. In order to place rigid objects within the median, the SGM02 must be divided into parallel SGR02 barriers with the objects centered in a 23-ft-plus gap or be transitioned to a semirigid system.

FIGURE 6.27 W-beam (weak-post) median barrier. Conversions: 1 in = 25.4 mm, 1 ft = 0.305 m. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission)

When adequately designed and reinforced, all of these meet the requirements of TL-4 at the standard height of 810 mm (32 in) and TL-5 at heights of 1070 mm (42 in) and higher. The New Jersey and F-shape barriers (Fig. 6.31), commonly referred to as safety shapes, differ in the height of the break point (change of slope of the face). The F-shape may per­form better with regard to vehicle roll when subjected to small vehicle impact. When pavement overlays exceed 3 in (75 mm), the height of the concrete above the break point must be increased to maintain an adequate height. Figures 6.32 and 6.33 show tall-wall safety-shape barriers (reinforced and nonreinforced concrete) that have been used success­fully. The single-slope barrier (Fig. 6.34) offers an advantage over others in that the pave­ment next to it can be overlaid several times, reducing the height to 42 in (1070 mm), without affecting performance. Foundation requirements do not appear critical, and there are many variations. Concrete median barriers can be slipformed, precast, or cast in place. A sand-filled metal version has been used in several states on an experimental basis.

Two important factors for safety-shape concrete barriers should be noted. Although the barrier does not deflect when hit, passenger vehicles may become air­borne and even reach the top in high-angle, high-speed impacts. Fixed objects on top of the barrier such as luminaire supports can cause snagging. Also, even for shallow-angle

A ASHTO Designation:

Test Level:

Post Type:

Post Spacing:

Beam Type:

Offset Brackets:

Mountings;

Nominal Barrier Height:

Maximum Dynamic Deflection:

Remarks: This barrier system is suitable for both wide and narrow medians and locations where the terrain is moderately irregular. Even moderate vehicle impacts cause a large number of posts to be damaged, Temporary supports may be used to maintain beam height until posts are replaced.

FIGURE 6.28 Box-beam median barrier. Conversions: 1 in = 25.4 mm, 1 ft = 0.305 m. (From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission) impacts, the roll angle of a high-center-of-gravity vehicle may be great enough to permit contact of the cargo box with objects on or just behind the barrier. Taller barriers offer improved characteristics in this regard.

The movable concrete barrier is an F-shaped barrier furnished in lengths of 37 in (940 mm) and arranged in a chain fashion with ends joined by pins. The proprietary Quickchange® system is shown in Fig. 6.35. The T segment at the top facilitates lifting. The system is often used in construction zones where traffic lanes are opened and closed frequently. Various other systems are available.

Table 6.6 provides a checklist that can be used to review existing barrier installations and determine adequacy for either structural or functional (design or placement) causes. Factors to be considered in determining the scope and extent of upgrading include the nature and extent of the deficiency, past accident history, and the cost-effectiveness of the recommended improvement. Remember to always consider the cost-effectiveness of eliminating or relocating the shielded feature.

6.6 MEDIAN BARRIERS

Longitudinal median barriers are used to separate opposing traffic on divided high­ways, to separate local and through traffic, or to separate traffic in designated lanes. Median barriers designed to redirect vehicles striking from either side require some different considerations from those for roadside barriers. However, performance requirements are the same as given in NCHRP 350 for roadside barriers.

Median barriers should be installed only if the consequences of striking the barrier are less severe than those of striking the feature in question. Figure 6.25 provides sug­gested warrants for median barriers on high-speed fully controlled-access roadways with relatively flat, traversable medians. The median width and the traffic volume dictate the need. There has been a trend to use median barriers for somewhat wider median widths than in the past as a result of studies of cross-median crash history. Site-specific data should also be considered. Also, special consideration should be given to barrier needs for medians separating roadways at different elevations.

The information presented in Arts. 6.6, 6.7, and 6.8 on selection, placement, and upgrading of roadside barriers applies generally to median barriers as well. Some additional information on transitions and placement follows in Arts. 6.9.2 and 6.10. End treatments are discussed in Art. 6.12.

I. Structural adequacy*

A. Longitudinal section

1. Standard barrier designf

2. Adequate post spacing

3. Rail element blocked out on strong-post system

4. Adequate splices in rail element

B. Terminal

1. Standard terminal designf

2. Adequate anchorage strength

C. Transition section

1. Standard transition designf

2. Adequate anchorage strength

3. Adequate stiffening in advance of rigid system

4. Adequate blockout and/or rubrail

II. Functional adequacy^

A. Longitudinal section

1. Adequate length to shield area of concern

2. Proper height of rail§

3. Proper flare rate

4. Barrier-to-object distance exceeds barrier deflection distance

5. Placement behind curb consistent with vehicle trajectory data

6. Placement on flat slopes (1:10) or on slopes up to 1:6 consistent with vehicle trajectory data

7. Beam backup plates present on steel strong-post system

B. Terminal

1. Adequate clear recovery area behind yielding terminal

2. Adequate offset of terminal end

*Structural adequacy is inherent in the barrier itself, rather than resulting from design, placement, or maintenance.

fStandard systems or elements are those which are currently an approved agency standard or have been successfully crash tested. Certain barriers that fall outside these categories may be left in place depending on the characteristics of the barrier and the results of an engineering analysis of the site.

^Functional adequacy results from barrier design or placement and is essential for barrier effectiveness.

§Generally, a 3-in (75-mm) variation from the nominal height is acceptable.

Source: From Roadside Design Guide, AASHTO, Washington, D. C.,

2002 and 2006, with permission.

The total length of a longitudinal barrier needed to shield an area of concern is referred to as the length of need. Figure 6.23 illustrates the variables that must be con­sidered, particularly the runout length LR and the lateral extent of the area of concern LA. The runout length is the theoretical distance needed for a vehicle that has left the road to come to a stop, measured as shown. Suggested values are given in Table 6.5 in terms of the traffic volume and the design speed. The lateral extent of the area of con­cern is the distance from the edge of the traveled way to the far side of the fixed object, or the outside edge of the clear zone LC of an embankment or fixed object that extends past the clear zone. After major variables are established, the length of the barrier will then depend on the tangent length L1, the distance from the traveled way L2, and the flare rate a:b. If a semirigid railing is connected to a rigid barrier, the tan­gent length should be at least as long as the transition section to reduce pocketing and increase likelihood of redirection. After variables have been selected, the required

length of need X in advance of the area of concern, for essentially straight sections of roadway, can be calculated from

La + (/a)/) – L2
(b/a) + (La/Lr)

The lateral offset Y from the edge of the traveled way to the beginning of the length of need is

R

The amount of rail installed should be a multiple of 12.5 or 25 ft (3.8 or 7.6 m), because metal-beam barriers are furnished in these lengths. A crashworthy end treat­ment must be added if the end treatment is located within the clear zone or in a loca­tion where it is likely to be struck. If the end treatment permits vehicle penetration, it must be extended upstream to preclude a vehicle from penetrating and striking the shielded feature.

Figure 6.24 shows the definition of variables of an approach barrier for opposing traffic. In this case, lateral dimensions are measured from the edge of the traveled way of the opposing traffic. This would be the centerline for a two-lane roadway or the edge of the driving lane next to the median for a two-way divided roadway. There are three ranges of clear zone width LC to consider for an approach barrier for opposing traffic:

• If the barrier is beyond the clear zone, no additional barrier or crashworthy end treatment is required.

• If the barrier is within the clear zone but the area of concern is beyond it, no addi­tional barrier is required but a crashworthy end treatment should be used.

• If the area of concern extends well beyond the clear zone, consider shielding only that portion which lies within the clear zone (set LA equal to LC).

The lateral placement of the approach rail should satisfy the criterion for embankment slopes. If steeper than 1:10, consider flattening the slope or decreasing the flare rate so the embankment criterion is not violated.