ROAD SAFETY FUNDAMENTALS September 2005
FIELD REFERENCE GUIDE
Office of Safety FHWA SA-05-011
Local Technical
Assistance Program
Notice
This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. The contents of this report reflect the views of the authors, who are respon sible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official policy of the Department of Transportation. This report does not constitute a manual, handbook, standard specification, or regulation. The United States Government does not endorse products or manufacturers. Any trademarks or manufacturer’s names that appear herein are only because they are considered essential to the object of this document.
Field Reference Guide for Road Safety Fundamentals
Table of Contents
BASIC HUMAN FACTORS . . . . . . . . . . . . . . . . . . . . . . . 1
User Information Needs . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Information Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Displaying Information . . . . . . . . . . . . . . . . . . . . . . . . . . 2
TRAFFIC CONTROL DEVICES . . . . . . . . . . . . . . . . . . . . . 3
Placement Information . . . . . . . . . . . . . . . . . . . . . . . . . . . Advance Posting Distance . . . . . . . . . . . . . . . . . . . . . . . . Sign Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pavement Marking and Delineators . . . . . . . . . . . . . . . . Parking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field Inspection Checklist . . . . . . . . . . . . . . . . . . . . . . . .
3
4
5
6
6
6
ROAD DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Stopping Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Road Design Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Paved Road Surface Condition Checklist . . . . . . . . . . 10
Unpaved Road Surface Condition Checklist . . . . . . . . 10
ROADSIDE SAFETY . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Clear Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Treatment of Roadside Hazards . . . . . . . . . . . . . . . . . . Barrier Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Guardrail Maintenance Checklist . . . . . . . . . . . . . . . . . Ditches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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12
16
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18
INTERSECTION SAFETY . . . . . . . . . . . . . . . . . . . . . . . . 18
Intersection Sight Distance . . . . . . . . . . . . . . . . . . . . . . . 18
Intersection Safety Checklist . . . . . . . . . . . . . . . . . . . . . 20
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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Field Reference Guide for Road Safety Fundamentals
Preface
The purpose of this field reference guide is to identify the core concepts local and Tribal road agency professionals can use to identify, evaluate, and improve their road safety operations. This reference guide is an abridged version of Road Safety Fundamentals, a guidebook that was developed by the Federal Highway Administration to help local and Tribal road agency pro fessionals understand the critical relationships between roads, roadside, road user behavior, and safety. This reference guide is not all-encompassing. However, it will provide use ful information to help you identify safety problems and make informed safety improvements in the field. Refer to the references at the end of the manual, or refer to the full Road Safety Fundamentals guidebook, to seek addi tional information about the issues described in this field guide.
Acknowledgements Funding to develop this reference guide was provided by the U.S. Department of Transportation, Federal Highway Administration, Office of Safety. This document was prepared by BMI-SG, a VHB Company. Tech nical editors include Dr. Eric Donnell of The Pennsylvania State University, Dr. Hugh McGee of BMI-SG, and Dr. Ronald Eck of West Virginia University.
Field Reference Guide for Road Safety Fundamentals
BASIC HUMAN FACTORS Human factors refer to people—in this case drivers—and the things they do, or fail to do, that can cause traffic crashes. A few key concepts to be aware of when evaluating your roads and traffic control devices are discussed below.
User Information Needs Road users combine the information they gather with their driving experi ence; then they make a decision. It takes skill and experience to make the right decision. Keep in mind that novice drivers are still learning driving skills while older drivers sometimes find it difficult to process large amounts of information. In summary, consider the following basic driver information principles to improve road safety: •
Stress important information that could lead to a crash if informa
tion is not received (e.g., make STOP AHEAD sign visible in favor
of a guide sign if the signs must be placed in close proximity).
•
Do not overload the driver with information—a series of simple
decisions is better than a few complex ones.
•
Spread driver decision-making points by keeping information
processing consistent.
•
Require drivers to make decisions that are expected. For instance, sudden lane drops on roadways will increase confusion and could result in crashes.
Information Sources Two types of information sources exist to alert drivers of necessary actions— formal and informal. Examples of formal information include traffic control devices such as traffic signs and pavement markings. These devices have standard shapes and colors – it is important to rely on the Manual on Uniform Traffic Control Devices (MUTCD)1 for information on signs and markings. Drivers also make navigation decisions based on informal information. Examples of informal information include vegetation or telephone poles near a roadway that provide visual cues to drivers. Telephone poles typi cally parallel a roadway. When poles, such as those shown in figure 1, fol low a straight path when the road curves, it can provide a misleading visual cue to the driver. It is important that appropriate traffic control devices are present to help drivers negotiate such situations.
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Field Reference Guide for Road Safety Fundamentals
Figure 1. Visual information cues
Displaying Information Primacy is the relative importance of each level of driver performance—con trol, guidance, and navigation—and the information associated with each level. A principal criterion upon which primacy is assessed is the level of consequence if the motorist fails to see or comprehend the information. As a guideline, in order of importance, signs fall into the following categories: 1. Regulatory signs that control traffic, such as STOP, YIELD, or DO
NOT ENTER.
2. Critical warning signs, such as STOP AHEAD or CROSS TRAFFIC DOES NOT STOP. 3. Other warning signs, such as CURVE AHEAD, LOW CLEARANCE, and SOFT SHOULDER signs. 4. Guide signs and route markers. 5. Other regulatory signs, such as speed limits or parking regulations. Redundancy refers to giving the same kind of information in more than one way. Redundancy can be accomplished by repeating certain signs, putting signs on both sides of the road or using pavement markings to supplement signs. Redundant information can help drivers make safe and proper maneuvers. Older drivers need more driving information than younger drivers. Complex highway driving environments, such as interchanges and
Field Reference Guide for Road Safety Fundamentals intersections, are situations where redundant information can be helpful. Work zones are another example where repetitive signing can reduce driver error. As drivers gain experience they expect things to happen as they always have. For example, drivers expect that a green light on a traffic signal will lead to a yellow light. This is called expectancy. If a signal changes from green to red, or a curve becomes suddenly tighter halfway through, a driver’s ex pectancy is violated, and the driver may react in an erratic or incorrect way thus causing crashes. Removing expectancy violations from the roadway environment can improve safety.
TRAFFIC CONTROL DEVICES Traffic control devices are driver information sources—signs, traffic signals, pavement markings, delineators, work zone devices—along the roadway. The MUTCD is the FHWA’s official standard regulating how road agencies throughout the United States use traffic control devices. The MUTCD(1) can viewed at http://mutcd.fhwa.dot.gov
Placement Information Traffic control device placement, size, and retroreflectivity are all important considerations in road safety. All traffic signs should be placed to ensure maximum visibility and effectiveness. Figure 2 shows recommended sign placement (i.e., lateral offset and height) guidelines for rural areas. In urban areas, signs should be at least 7-feet above the edge of the roadway.
Figure 2. Rural sign placement guidelines
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Other sign location guidelines are as follows: •
Avoid placing signs in dips, beyond hillcrests, or at other places where motorists cannot see them in enough time to react safely.
•
Consider the possibility that a sign could be hidden by parked trucks or summer foliage or be a hazard to pedestrians.
•
Try to erect signs individually, except when they supplement each other, and always try to space signs far enough apart so that motor ists have time to make reasonably safe decisions.
•
Use larger signs to provide added emphasis.
•
Materials used for signs should provide nighttime visibility equal to daytime visibility.
Advance Posting Distance The MUTCD developed two advance posting distance category guidelines for traffic signs. Category A signs are warning signs used when drivers need to reduce speed or change lanes in heavy traffic. MERGE and RIGHT LANE ENDS warning signs are examples of Category A signs. Table 1 shows advance posting distances for Category A signs. Table 1. Advance posting distance for Category A signs Posted or 85th percentile speed (mph)
25
30
35
40
45
50
55
60
Advance posting distance (ft)
325
450
550
650
750
850
950
1100
65
70
1200 1250
Category B warning signs are used with advisory speeds. Curve signs and turn signs are common examples. Table 2 shows advance posting distances for Category B signs.
Field Reference Guide for Road Safety Fundamentals Table 2. Advance posting distance for Category B signs Posted or 85th percentile speed (mph)
a b
Deceleration to listed advisory speed (mph) for the conditiona 0b
10
20
30
40
50
40
125
45
175
125
50
250
200
150
100
55
325
275
225
175
100
60
400
350
300
250
175
65
475
425
400
350
275
175
70
550
525
500
425
350
250
60
150
Typical conditions are when road users must reduce speed to perform a maneuver. Typical condition is to warn of a potential stop.
Sign Supports Various types of breakaway post systems have been developed to allow ve hicles leaving the roadway to recover or minimize crash impact severity. For breakaway signposts to work properly, the top of the stub must be less than 4 in above the ground. During your sign inventory inspection check basepost heights in areas where erosion could occur. If enough soil washes away, the breaking point of a correctly installed breakaway post may be higher. Other signpost installation guidelines are shown in figure 3.
Figure 3. Sign support placement guidelines
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Field Reference Guide for Road Safety Fundamentals
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Pavement Markings and Delineators Pavement markings guide and regulate traffic. The MUTCD contains recom mended pavement marking application guidelines. Reapply pavement markings as needed to maintain good visibility. New markings provide much better guidance than worn ones, especially at night. Delineators are retroreflective roadside markers used to guide traffic at night, during adverse weather, through work zones, and at locations with confusing alignment features. Delineators can be embedded in the centerline or edgeline or mounted on posts at curves. Delineators are a good safety device because they are less affected by inclement weather than pavement markings, and delineators do not have to be replaced as often because traffic does not drive on them. Post-mounted delineators are normally placed between 2 and 8 ft beyond the edge of the shoulder. They are mounted 4 ft above the roadway edge on suitable posts. Normal spacing is 200 to 530 ft apart, but space them closer together on curves.
Parking Parking can be a factor on local road crashes, but eliminating parking can anger local business owners and residents. Guidelines for when to consider vehicular parking prohibition on a roadway are as follows: 1. Vehicles are parked on a curve with limited sight distance. 2. Parked vehicles limit passing sight distance of motorists on main roadway. 3. Parked vehicles limit intersection sight distance of turning vehicles. 4. Parked vehicles encroach onto the travel lanes. 5. Roadway infrastructure (pavement) is damaged. 6. Traffic control devices(s) is blocked or intersecting roadway is not visible. 7. Parked vehicle violates parking ordinance for local community.
Field Inspection Checklist The following checklist should be used to perform field inspections of sign ing, pavement markings and delineators: •
Is the sign placed where drivers can easily see it?
•
Is there any foliage or other objects blocking the view of the sign?
•
Can the sign be read at a safe distance?
•
Is the sign color faded or lost its retroreflectivity?
Field Reference Guide for Road Safety Fundamentals •
Is the sign in a location where it does not impair safe sight distance?
•
Is the sign properly located with respect to mounting height and
lateral position?
•
Are the pavement markings so worn that they are no longer effective, especially at night?
•
Are there any damaged, non-standard or improperly located delinea tion devices (post-mounted delineators, chevrons, object markers)?
ROAD DESIGN Roadway features such as travel lanes, shoulders, roadside slopes, horizontal curves, and vertical curves all influence safety. Road agencies use many refer ences for road design standards. AASHTO’s A Policy on Geometric Design of Highways and Streets, also called the Green Book, is one of the most commonly used highway design references.(2) If the road carries fewer than 400 vehicles a day, use the AASHTO’s Geometric Design Guidelines for Very Low-Volume Local Roads.(3)
Consistency Many traffic crashes occur at places where the road character changes. Transi tions from straight to curved alignments, reduced lane width, and borders be tween rural and built-up areas are examples of character changes that could cause safety problems. Where possible, maintain a constant road width and character. Where change is necessary, provide as much sight distance as pos sible and use signs and pavement markings to warn drivers about the change and guide them through it.
Stopping Sight Distance A driver’s ability to see the roadway ahead is critical for safety reasons. The available stopping sight distance is the distance necessary for a motorist to see an object in the road, react by braking and then bring the vehicle to a stop, if necessary, before it reaches an object in its path. Vehicle braking distances are longer on downhill sections and shorter on uphill sections. Trucks are larger and heavier than cars so it is reasonable that they need longer distances to stop than other vehicles. To measure stopping sight distance, use an average eye height of 42 in and an object height of 24 in (see figure 4). On horizontal curves, measure stopping sight distance along the center of the inside lane, as shown in figure 5.
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Field Reference Guide for Road Safety Fundamentals
Figure 4. Measuring stopping sight distance on a horizontal crest
Figure 5. Measuring stopping sight distance (S) on a horizontal curve
Minimum stopping sight distances are shown in table 3.
Field Reference Guide for Road Safety Fundamentals Table 3. Stopping sight distance on grades
Design
Stopping Sight Distance (ft)
Speed
Level
Downgrade
Upgrade
(mph)
0%
3%
6%
9%
3%
6%
9%
20
112
116
120
126
109
107
104
25
152
158
165
173
147
143
140
30
197
205
215
227
200
184
179
35
246
257
271
287
237
229
222
40
300
315
333
354
289
278
269
45
359
378
400
427
344
331
320
50
423
446
474
507
405
388
375
55
491
520
553
593
469
450
433
60
564
598
638
686
538
515
495
65
642
682
728
785
612
584
561
70
725
771
825
891
690
658
631
Road Design Checklist The following checklist should be used to perform field inspections of road design conditions: •
Is the road section free of inconsistencies, e.g. intermittent loss of shoulder, that could cause safety problems?
•
Are narrow bridges, lane drops, and other reduced road widths properly marked?
•
Are there any sharp curves that follow long, tangent roadway segments?
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Field Reference Guide for Road Safety Fundamentals
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Paved Road Surface Condition Checklist The following checklist should be used to perform field inspections of paved road surface conditions: •
Is the pavement free of defects that may cause loss of steering con trol or other safety problems (such as frost heaves, pot holes, washboarding, etc.)?
•
Are changes in surface type (pavement begins, changes type, or ends) free of poor transitions?
•
Does the pavement have adequate skid resistance, particularly on curves, steep grades, and approaches to intersections? (Look for polished aggregate or bleeding asphalt.)
•
Is the pavement free of areas where water ponding or sheet flow could cause hydroplaning or other safety problems?
•
Is the pavement free of loose soil or gravel that may cause safety problems?
•
Are pavement edges free of drop-offs more than 3 in high?
•
Are shoulders firm and free of loose material?
Unpaved Road Surface Condition Checklist The following checklist should be used to perform field inspections of unpaved road surface conditions: •
Is the pavement free of defects that may cause loss of steering con trol or other safety problems?
•
Is the pavement free of areas where water ponding or sheet flow could cause safety problems?
•
Is the pavement free of loose soil or gravel that may cause safety problems?
•
Are there changes in surface type (such as where pavement ends or begins) free of drop-offs or poor transitions?
ROADSIDE SAFETY Single-vehicle, run-off-road crashes account for approximately 30 percent of highway fatalities. These fatalities result from vehicles colliding with a fixed object (such as trees, utility poles, or unshielded bridge rail ends) or rolling down or off dangerous slopes.
Field Reference Guide for Road Safety Fundamentals
Clear Zones The best advice for preventing run-off-crashes into fixed objects is to have a obstacle-free roadside or clear zone. The clear zone is the area along the roadside that is free of fixed objects or dangerous slopes. The desired mini mum clear zone width depends on the nature of the road and is based on factors like the amount of traffic, the design speed (or operating speed if design speed is not known), slope of the roadside, and how the road curves. For example, on rural local roads, try to include clear zones of 10 ft from the lane edgeline. Consult the AASHTO Roadside Design Guide (4) or the AASHTO Guidelines for Geometric Design of Very Low-Volume Local Roads (3) for recommended clear zones on other road types.
Treatment of Roadside Hazards Ask yourself the following questions as you consider what to do with a hazard that reduces the available clear zone distance: •
Is the hazard dangerous?
•
Can you remove the hazard?
•
Can you relocate it to a place where it is less likely to be hit?
•
Can you reduce crash severity by redesigning the hazard, e.g. make
it breakaway?
•
If you can’t remove, relocate, or modify the hazard, will adding
guardrail make the road safer?
•
Would delineation help guide drivers around the hazard?
Is the Hazard Dangerous?—When you know something on the roadside is a hazard, do something about it. To decide how dangerous the hazard is to drivers, ask yourself: •
Is there a fixed object in the clear zone?
•
Can the object in the clear zone penetrate the passenger compart
ment, cause sudden deceleration if struck, or cause the vehicle to
roll over?
•
Is there a critical slope near the road?
•
Is there a fixed object at or near the bottom of a nonrecoverable slope?
Can You Remove the Hazard?—Your best option is to eliminate the hazard. Can You Relocate the Hazard to a Place Where it is Less Likely to be Hit?—Moving an object significantly farther from the road or from the outside of a curve to the inside can reduce the chances that the hazard will be hit.
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Field Reference Guide for Road Safety Fundamentals Can You Reduce Crash Severity?—If you can’t remove or relocate the hazard, then try to reduce the severity of a crash. There are three main ways to do this: 1. Install signposts, light and utility poles on breakaway bases to reduce collision forces. 2. Upgrade drainage features so that vehicles leaving the roadway can drive over them. 3. Use crash cushions and impact attenuators to soften the collision with hard objects. Will Guardrail Improve Safety?—Remember that striking guardrail can cause injuries, so install it only where crashing into the hazard would be worse than striking the guardrail. Would Delineation Guide Drivers Around the Hazard?—If you can’t remove, relocate, modify, or shield a hazard with guardrail, then install signs and delineation to warn drivers that they need to be alert for the hazard. Is the Solution Feasible and Cost-effective?—Determine when the cost of an im provement is more than the crash cost savings you can expect from it. If the cost is greater than the benefit, it is likely not cost-effective to make that par ticular improvement and another improvement should be evaluated.
Barrier Installation Roadside barriers are meant to protect traffic from hazardous objects or slopes. If hitting a barrier would be worse than hitting the hazard, then do not install it. Warrants for installing barriers can be found in the Roadside Design Guide(4). Once you install guardrail, you’re responsible for maintain ing it. The following represent different applications of guardrail to common highway safety problems. Slopes—Barriers are not effective when installed on any slope steeper than 6:1. You can use cable guardrail at any offset on slopes as steep as 6:1. If guardrail other than cable is to be installed on a 6:1 slope, it must be at least 12 ft beyond the breakpoint (where the slope gets steeper). Guardrail and Curb—When a vehicle hits a curb, it will usually bounce
upward. If possible, don’t use curb with guardrail.
Flexible Systems—These systems tend to be forgiving when hit because they deflect considerably under the impact, which reduces the collision forces on vehicle occupants. But the systems have limitations as noted below.
Field Reference Guide for Road Safety Fundamentals •
Cable guardrail is inexpensive, but it requires maintenance because it loses effectiveness when cable tension is not maintained. After it is hit by a motor vehicle, snowplow, or agricultural equipment, the whole length of the rail is ineffective until crews repair it. Figure 6 illustrates a cable guardrail used in a median; for roadside installa tion, all cables are on the traffic side of the posts.
Figure 6. Cable guardrail
•
Weak-post W-beam guardrail is also inexpensive, but costs more than cable. Collision damage tends to be more localized than a simi lar impact with cable guardrail. W-beam guardrail can also act as a snow fence and may cause significant snow drifting on the road. Figure 7 shows a weak-post w-beam guardrail.
Figure 7. Weak-post w-beam guardrail
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Semi-rigid Systems—Semirigid systems deflect less than flexible systems. This means they are more likely to cause an injury in a crash, but they can be used when the hazard is closer to the road. They are also more durable, and often remain serviceable after minor impacts. •
Blocked-out strong-post W-beam guardrail can survive multiple low-speed crashes before losing effectiveness, but its rigidity in creases occupant injury risk. It is also more expensive than flexible systems. Figure 8 illustrates a strong-post w-beam guardrail.
Figure 8. Strong-post w-beam guardrail
• Thrie-beam guardrail is stronger than the w-beam which increases its ability to redirect vehicles. Figure 9 shows a thrie-beam guardrail.
Figure 9. Thrie-beam guardrail
Field Reference Guide for Road Safety Fundamentals Barrier Length—Guardrail has to be long enough to protect traffic from the hazard it shields. General rules-of-thumb are as follows: •
Avoid short gaps between adjacent barrier installations.
•
On two-way roads, the trailing end of the guardrail run must be
long enough to protect traffic going off the left side of the road.
•
For bodies of water, large stands of trees, or steep embankments that
aren’t easily bypassed, the rail should be long enough so that when
a vehicle gets behind the rail it will be able to come to a safe stop
before reaching the hazard.
Terminals—Guardrail terminals have two main functions. First, they anchor the ends of the rail to resist the tension in the rail when struck. Second, they must be crashworthy. The best way to end a guardrail is to anchor it in a backslope. Mounting Height—Table 4 shows acceptable guardrail mounting heights of barrier types mentioned earlier.
Table 4. Acceptable guardrail heights(5)
Barrier type
Normal Mounting Height, in1
Acceptable Mounting Height, in
Cable2
30
27
Weak-post W-beam3
30
32
Strong-post W-beam3
27
28
Strong-post Thrie-beam4
32
34
Notes: 1 Height normally measured from the ground directly below the barrier. 2 Measured to the center of the top cable at the mounting point. 3 Measured to the top of rail at a post. 4 Measured to the top of the barrier.
Deflection Distance—When impacted by a vehicle, guardrail will deflect. It is important that the deflection distance is known for a roadside barrier so that no fixed objects are within this space. Table 5 shows common guardrail deflection distances.
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Transitions—Be very careful when making transitions from one type of guardrail to another or from guardrail to bridge rail. Avoid sudden changes in stiffness or lack of continuity.
Table 5. Guardrail deflection distance(5)
Rail class Flexible
Semirigid
Rail type
Post spacing, ft
Deflection distance, ft
Cable
16.0
11.5
Modified W-beam (weak post)
12.0
6.5
Blocked-out W-beam (strong post)
6.0
5.5
Blocked-out Thrie-beam (strong post)
6.3
2.9
Guardrail Maintenance Checklist Guardrail maintenance includes inspection for corrosion, rust, or rotting in posts or rail element. Periodic inventories should include the following checks: • • • • • • •
• •
Crash damage. Rail set too high or too low (see table 4). Insufficient deflection distance (see table 5). Gaps or breaks. Poor transition from one rail type to another (sudden increase in
stiffness).
Rail behind curbs. Obsolete and nonstandard rail types, such as:
- Cobbled-up, nonstandard rail.
- Rail mounted on concrete posts.
- External-splice box beam.
Obsolete or no end sections. Rail and post condition.
- Rust.
- Rot in wooden posts.
- Loose or missing bolts.
- Cable tension.
- Loss of support caused by erosion.
Field Reference Guide for Road Safety Fundamentals
Ditches In rural areas, ditches typically carry water away from the roadbed. When ditch slopes are too steep, they become safety problems. Steep foreslopes can cause rollover crashes while steep backslopes can snag vehicles and spin them violently upon impact. Preferred ditch cross-sections are shown in figure 10. Be careful when cleaning ditches so that you do not create a roadside hazard.
Figure 10. Preferred ditch cross-sections(7)
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Bridges Bridge piers and abutments are fixed objects on roadways under bridges. Bridges narrower than the approach roadway can adversely affect safety. Bridge rails are also fixed object hazards situated close to the roadway. Con sider the following safety enhancements at or approaches bridges on local roads: •
Base appropriate signs and markings on the MUTCD.
•
Install delineation on roadway approach and bridge.
•
Illuminate roadway approach and bridge if power is accessible.
•
Install crashworthy transition guardrail.
•
Remove fixed objects from roadway approaches.
•
Provide adequate sight distance on roadway approaches.
•
Improve cross slopes on bridge to provide adequate drainage.
INTERSECTION SAFETY Intersections are safety trouble spots because they are where vehicle paths cross. Channelized-Y, skewed, and multileg intersections are the most likely to have safety or operational problems. Three-leg or T-intersections can be safer than four-way intersections because there are fewer conflict points for crashes to happen. Build intersections so the corner angle is between 75 and 105 degrees.
Intersection Sight Distance There are two sight distances that affect the safety of an intersection: •
Stopping sight distance to the intersection.
•
Intersection sight distance.
Stopping sight distance was described earlier. Drivers stopped at an inter section must be able to see far enough to tell if there is a large enough gap in traffic for them to pull into the intersection safely. This is called intersection sight distance. The distance needed depends on the type of intersection con trol and the maneuver being made. At two-way stop-controlled intersec tions, Tables 6 and 7 provide the desirable intersection sight distance for the left-turn, and right and crossing maneuver from the minor road, re spectively; Table 8 provides the same for the left-turn from the major road. The values contained in Tables 6 through 8 are based on flat grades and no medians on either intersection roadway—refer to the AASHTO Green Book for more details.
Field Reference Guide for Road Safety Fundamentals Table 6. Intersection sight distance for left turns from stop condition on minor road
Intersection Sight Distance for Left Turns from Stop Condition on Minor Road (feet) Design Vehicle
Design Speed on Major Road (mph) 25
35
45
55
65
75
Passenger Car
280
390
500
610
720
830
Single-unit Truck
350
490
630
770
910
1050
Tractor-trailer
425
595
765
930
1100
1270
Table 7. Intersection sight distance for right turn and crossing maneuvers on minor road
Design Vehicle
Intersection Sight Distance for Right Turns and Crossing Maneuvers from Stop Condition on Minor Road (feet) Design Speed on Major Road (mph) 25
35
45
55
65
75
Passenger Car
240
335
430
530
625
720
Single-unit Truck
315
440
565
690
815
940
Tractor-trailer
390
545
695
850
1005
1160
Table 8. Intersection sight distance needed for left turns off the major road
Intersection Sight Distance for Left Turns from Stop Condition on Major Road (feet) Design Vehicle
Design Speed on Major Road (mph) 25
35
45
55
65
75
Passenger Car
205
285
365
445
530
610
Single-unit Truck
240
335
430
530
625
720
Tractor-trailer
280
390
500
610
720
830
19
20
Field Reference Guide for Road Safety Fundamentals When measuring intersection sight distance in the field, the passenger car is assumed to stop 10 ft back from the edge of the intersecting road. The sight line for intersection sight distance is based on the position of the driver’s eye, which is assumed to be 14.4 to 17.8 ft from the same edge. Figure 11 illustrates this concept.
Figure 11. Sight distance at intersections
Intersection Safety Checklist • Are intersections free of sight restrictions that could create safety
problems?
• Are intersections free of abrupt elevation or surface condition
changes?
• Are advance warning signs (STOP AHEAD, YIELD AHEAD, TRAFFIC CIRCLE AHEAD or SIGNAL AHEAD) installed where drivers can’t see intersection traffic control at a safe distance ahead of the intersection? • On through roads, are intersection warning signs installed where driv ers can’t see the intersection at safe distance ahead of the intersection?
Field Reference Guide for Road Safety Fundamentals
REFERENCES 1. Manual of Uniform Traffic Control Devices. U.S. Department of Trans portation, Federal Highway Administration, Washington, DC 2003. 2. A Policy on Geometric Design of Highways and Streets. American Asso ciation of State Highway and Transportation Officials (AASHTO), Washington, DC, 2001. 3. Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT < 400). AASHTO, 2001. 4. Roadside Design Guide. AASHTO, 2002. 5. Highway Safety Design and Operations Guide. AASHTO, 1997. 6. Guidance for Implementation of the AASHTO Strategic Highway Safety Plan. NCHRP Report 500, Volumes 1 through 13. Transportation Research Board, The National Academies, Washington, DC, 2003. 7. Manual of Transportation Engineering Studies. Institute of Transporta tion Engineers, Washington, DC, (H. D. Robertson ed.), 2000. 8. Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities. The Access Board, Washington, DC, September 2002.
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Office of Safety FHWA SA-05-011