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21 The first step in converting the site-specific guardrail selec- tion guidelines shown in Appendices A, B, and C (not pub- lished herein) into a route-specific format was to examine the effects of each highway and roadside parameter on the rec- ommended barrier test level. Findings from this examination are summarized in the following sections. Findings Functional Class Highway functional class was found to have a major impact on the need for higher performance barriers. RSAP uses high- way functional class as a surrogate for operating speed. This ap- proach is based on a study by Mak, Sicking, and Ross (22) that showed functional class as the best indicator of encroachment speeds associated with ran-off-road crashes. High encroach- ment speeds greatly increase the number of vehicles that are predicted to penetrate through or over the top of a guardrail sys- tem. Thus the benefit of using higher, stronger barriers would be expected to increase significantly when a higher functional class raises predicted encroachment speeds. This effect is clearly observed when guardrail selection guidelines for freeways are compared to guidelines for lower functional classes. Higher bar- rier test levels were found to be consistently more cost benefi- cial for freeway application than for any other functional class. Hazard Severity As presented in Chapter 4, three different hazard severities were included in the study, severe, moderately severe, and moderate. Hazard severity proved to have a significant im- pact on test level selection. As shown in Table C1 (not pre- sented herein), TL-4 or TL-5 barriers were generally found to produce a B/C ratio of 4 or greater when a severe slope haz- ard was placed within 18 ft (5.5 m) of a freeway with traffic volume of 30,000 ADT or more. When the hazard was re- placed with a moderately severe slope, as shown in Table C2 (not presented herein), TL-4 barriers dropped off the table, and TL-5 barriers did not generally reach a B/C ratio greater than 4 until traffic volume exceeded 60,000 ADT. Finally, when the hazard was changed to a moderate severity slope, only TL-2 barriers were found to have B/C ratios greater than 4. This finding merely reflects the fact that hazard severity has a major impact on the risk of serious injury or fatality whenever a vehicle is predicted to penetrate through or over the guardrail. Recall that the primary benefit of increased guardrail test level is a reduction in the number of vehicles that penetrate through or over the barrier. Whenever the severity of going through the guardrail is increased, the benefits of using a stronger guardrail increase commensurately. Hazard Size Guardrail shielding of long hazards was found to be much more cost beneficial than treatment of point hazards. When viewed in terms of the benefits associated with a higher bar- rier test level, this finding is not surprising. As noted above, the benefit of increasing test level is primarily related to the risk of a vehicle striking a roadside hazard after penetrating through or over the barrier. When a vehicle penetrates through or over the portion of any guardrail placed upstream of an object, the risk of the vehicle continuing on to strike the hazard is still relatively modest. When a vehicle penetrates through a barrier immediately adjacent to an obstacle, however, it will almost certainly encounter the hazard. Because of the signifi- cantly different risks of a vehicle penetrating through or over the barrier and then striking the hazard, higher test level bar- riers are shown to be much more cost beneficial when placed adjacent to long hazards. Hazard Offset Ran-off-road crash frequencies have been shown to dimin- ish as roadside obstacles are moved farther from the travelway. C H A P T E R 6 Route-Specific Selection Guidelines
Thus, the potential benefit of installing guardrail dimin- ishes as hazards are moved further from the travelway. A sec- ondary factor that has the same effect on the benefit of using guardrail is the relationship between guardrail length and the offset to the back of the hazard. Whenever possible, guardrail is placed immediately adjacent to the hazard. However, whenever roadside slopes steeper than 10:1 are found in front of the haz- ard, the guardrail must be placed much closer to the roadway. In this situation, the length of guardrail required to adequately shield traffic from the hazard increases significantly, and both the cost of the guardrail installation and the number of impacts with the barrier increase proportionately. For long hazards, the site-specific guardrail selection guidelines do not show that off- set has as great an effect on test level selection as was originally anticipated. For the long slope hazards included in the study, increasing the offset made only modest increases in the traffic volume at which a higher test level barrier became more cost beneficial. Hazard offset had a much bigger impact on guardrail protection of point hazards. The RSAP analysis showed that in- creasing hazard offset made lower test level barriers more cost beneficial and that, for very high offsets, made no treatment the most cost beneficial alternative, even on freeways. Offset to Slope This parameter is the distance from the edge of travelway to the beginning of a moderate roadside slope. Although these slopes can cause some crashes, the severities are generally low. The primary effect is related to guardrail placement issues. Guardrails cannot be installed on even modest roadside slopes of 8:1 or steeper. Hence, if a modest roadside slope begins at the edge of the shoulder, the guardrail must be placed very near the travelway. This location requires more guardrail in order to properly treat the hazard and increases barrier crash frequency. Thus, as the offset to the slope diminishes, so does the benefit of using guardrail. This parameter was found to have the greatest effect for hazards with high lateral offsets. In this situation, the increase in guardrail crashes relative to haz- ard impacts reduced calculated B/C ratios when the barrier was moved closer to the roadway. Overall, the offset to slope parameter was found to be much less important than any of the parameters described above. Curvature Highway curvature has been shown to significantly in- crease the risk of ran-off-road crashes. However, for the eco- nomic analysis of guardrail installation, curvature proved to have a relatively limited impact. When the effects of curvature on guardrail protection of long hazards are studied, a barrier is found to be only modestly more cost beneficial when the hazard is placed on the outside of a left curve. When the effects of curvature on guardrail benefits are examined for point hazards, just the opposite is found. A barrier is found to be less cost beneficial when protecting motorists from point haz- ards placed outside of a curve. This effect is related to the risk of impacting a point hazard when a vehicle encroaches from a curved highway. For straight path encroachments, the risk of encountering a point hazard diminishes as encroachment angle increases. Further, the effective angle of encroachment increases as a vehicle moves away from a curved roadway. This increase in effective encroachment angle reduces the risk of striking small hazards and, thereby, tends to offset the ef- fects of increased encroachment frequency. Grade The effect of down grade on the RSAP analyses of guardrail applications was found to be very limited. RSAP adjusts en- croachment frequency upward to account for the effect of a down grade. An increase in encroachment frequency should translate into greater benefits for barrier installation. How- ever, the effect of grade on encroachment frequency is much less than the effect of curvature. Thus, the effects of grade were not considered when developing route-specific guardrail ap- plication guidelines. Guideline Development As summarized previously, highway functional class, haz- ard severity, hazard size, and hazard offset were found to be the most important parameters affecting the benefits of im- plementing higher performance guardrails. These parameters were chosen for evaluation during the process of develop- ing route-specific guidelines. Functional class was found to have such a dramatic impact on the benefit of implement- ing guardrail that it had to be implemented directly into the guidelines. However, the basic principle behind route-specific guidelines is that only one barrier system will be used for the entire length of a roadway section. Hence, hazard-specific parameters, including severity, size, and offset could not be directly implemented into the guidelines. Hazard severity and size were implemented indirectly by defining roadways in terms of typical terrain conditions. The RSAP analysis showed that high-performance barriers were most commonly cost beneficial when installed in front of long, severe hazards. These types of hazards are most commonly found in the form of steep roadside embankments. Steep road- side embankments are seldom encountered along highways across relatively flat terrain. However, severe roadside embank- ments are found along roadways through rolling terrain. Over the last 40 years, implementation of clear-zone pol- icy has produced a largely unobstructed region immediately adjacent to most modern roadways. Most roadside hazards 22
are found outside of this unobstructed region. The size of a typical unobstructed region varies by functional class and from one route to the next. Hazard offset was implemented by defining two ranges of unobstructed zone for each class of highway as shown in Table 14. Engineering judgment was then used to develop general route-specific guidelines for guardrail use based upon the site-specific guidelines presented in Appendices A, B, and C (not found herein). The resulting route-specific guidelines are presented in Tables 15 through 20. Note that the guide- lines developed for relatively flat terrain have been labeled general guidelines and are presented in Tables 15, 17, and 19 for B/C ratios of 2, 3, and 4, respectively. As presented in the previous chapter, the decision of which B/C ratio to imple- ment should be based upon comparisons with B/C ratios common to other types of highway construction projects. Guideline Application The route-specific guidelines shown in Tables 15 through 20 can only be implemented after a B/C ratio appropriate for guardrail application is identified. AASHTO or transporta- tion agency administrators should provide highway design- 23 Functional Class Classification Unobstructed Zone Width ft (m) Narrow <18 (5.5) Freeway Wide >18 (5.5) Narrow <12 (3.7) Rural Arterial Wide >12 (3.7) Narrow <8 (2.4) Rural Collector/Local Wide >8 (2.4) Narrow <8 (2.4) Urban Arterial Wide >8 (2.4) Narrow <8 (1.5) Urban Collector/Local Wide >8 (1.5) Table 14. Unobstructed zone widths. Traffic Volume (1,000 ADT) Functional Class WidthClass None TL-2 TL-3 TL-4 TL-5 Narrow 0-100 Freeway Wide 0-100 Narrow <20 >20 Rural Arterial Wide Any Narrow Any Rural Collector/Local Wide <1 >1 Narrow <20 >20 Urban Arterial Wide Any Narrow Any Urban Collector/Local Wide Any Table 15. General guardrail use guidelines, B/C 2. Traffic Volume (1,000 ADT) Functional Class WidthClass None TL-2 TL-3 TL-4 TL-5 Narrow <25 >25 Freeway Wide <33 >33 Narrow Any Rural Arterial Wide Any Narrow Any Rural Collector/Local Wide Any Narrow Any Urban Arterial Wide Any Narrow Any Urban Collector/Local Wide Any Table 16. Rolling terrain guardrail use guidelines, B/C 2. Traffic Volume (1,000 ADT) Functional Class Width Class None TL-2 TL-3 TL-4 TL-5 Narrow Any Freeway Wi de <20 >20 Narrow <35 >35 Rural Arterial Wi de Any Narrow <1 >1 Rural Collector/Local Wi de Any Narrow <30 >30 Urban Arterial Wi de Any Narrow Any Urban Collector/Local Wi de Any Table 17. General guardrail use guidelines, B/C 3. ers with a recommendation on this subject. As described in this section, after the appropriate B/C ratio is identified, high- way designers need only make three decisionsâdetermine the type of terrain, identify the highway functional class, and establish size of the unobstructed zoneâin order to deter- mine the guardrail test level appropriate for any given route. The designer must first determine the type of terrain through which the highway passes. Recall that the terrain clas- sifications are intended to represent the frequency and sever- ity of roadside embankments found adjacent to the highway as characterized in Table 4. Highways in flat terrain, or the gen- eral category, are expected to have very few severe roadside slopes and moderately severe slopes should not be common within the clear zone. A severe slope was represented in the RSAP analysis by a 26-ft (7.9-m) deep embankment with a slope of 1.5:1. A moderately severe roadside slope was repre- sented by a 2:1 embankment that was 20 ft deep. Most of the slope hazards encountered along highways falling into the flat or general category should be flatter or shallower than the definition of a severe slope. It is anticipated that most high- ways will fall into the flat or general category.
Highways through rolling terrain are expected to have a high proportion of moderately severe and severe roadside slopes within the clear zone. A significant number of these hazards would be expected to be encountered adjacent to almost every mile of the roadway. A designer must then identify the highway functional class associated with the route being evaluated. Most highway agen- cies have established functional classifications for all road- ways and a designer need only match the agency classification with one of the five classifications included in this study (free- way, rural arterial, rural collector/local, urban arterial, and urban collector/local). Designers must then determine the size of the unobstructed zone adjacent to the highway under consideration. With the exception of bridges, the majority of other hazards to be treated with guardrail should fall outside of the unstructured zone. After a B/C ratio has been selected and the type of terrain, highway functional class, and size of the unobstructed zone have been identified, a designer can determine the guardrail test level recommended for the highway under consideration directly from Tables 15 to 20. Consider, for example, that a state DOT has selected a B/C ratio of 3 as appropriate for guardrail implementation and a designer needs to identify the barrier test level appropriate for a rural interstate with 25,000 ADT. If the designer examines the roadway topography and finds few severe or moderately severe slope hazards adjacent to the roadway and the majority of hazards are less than 18 ft (5.5 m) from the edge of the travelway, Table 17 would show that a TL-3 guardrail is the most appropriate. This example shows how the guidelines are to be used but is not situation specific. The following example gives specific numbers to walk a user through each step in the guardrail selection process. A state has determined that safety projects should begin to be funded when the B/C ratio reaches 4.0. A rural freeway within the state crosses through an area of rolling hills with an average of more than four moderately severe or severe roadside slopes per mile. Further, the freeway has a somewhat constricted right-of-way and most of the roadside hazards are within 18 ft (5.5 m) of the travelway. The design year traffic volume of the highway is 20,000 ADT. Because of the frequency of moderately severe and severe roadside slopes, the appropriate guardrail test level for this roadway can be determined from Table 20. The first row in this table represents freeways with narrow clear areas. As shown in this row, a traffic volume of 20,000 ADT falls under the TL-4 column. Thus, TL-4 guardrails should be used on this route. 24 Traffic Volume (1,000 ADT) Functional Class Width Class None TL-2 TL-3 TL-4 TL-5 Narrow Any Freeway Wide <28 >28 Narrow Any Rural Arterial Wide Any Narrow <1.5 >1.5 Rural Collector/Local Wide Any Narrow <50 >50 Urban Arterial Wide Any Narrow <2 >2 Urban Collector/Local Wide Any Table 19. General guardrail use guidelines, B/C 4. Traffic Volume (1,000 ADT) Functional Class Width Class None TL-2 TL-3 TL-4 TL-5 Narrow <19 19-37 >37 Freeway Wide <46 >46 Narrow Any Rural Arterial Wide <12 >12 Narrow Any Rural Collector/Local Wide Any Narrow Any Urban Arterial Wide <12 >12 Narrow Any Urban Collector/Local Wide Any Table 20. Rolling terrain guardrail use guidelines, B/C 4. Traffic Volume (1,000 ADT) Functional Class Width Class None TL-2 TL-3 TL-4 TL-5 Narrow <28 >28 Freeway Wide <37 >37 Narrow Any Rural Arterial Wide Any Narrow Any Rural Collector/Local Wide Any Narrow Any Urban Arterial Wide Any Narrow Any Urban Collector/Local Wide Any Table 18. Rolling terrain guardrail use guidelines, B/C 3.
