1 the roadside hossein naraghi ce 590 special topics safety march 2003 time spent: 12 hrs

1

The roadside

Hossein Naraghi

CE 590 Special Topics

Safety

March 2003

Time spent: 12 hrs

2

The roadside The forgiving roadside

Collision with roadside objects account for 25-30 percent of fatal crashes• Significant in both urban and rural

environments Measures to address this problem

•Reducing crash frequency• Shoulder treatment• Speed control• Combating driver fatigue• Combating alcohol abuse

3

The forgiving roadside (continued)

• Reducing crash severities• Forgiving roadside

• Develop cost-effective ways to reduce severity of run-off-road crashes

There are two strategies1. Roadside clear zone

• If a vehicle leaves the roadway, it is less likely to strike a roadside object

• Impractical in many cases

2. Roadside hazard management program

4

Roadside clear zones Recovery area

The zone beside the road which let driver to be able to regain the control of the vehicle• If it has not struck a fixed object• If it has not rolled over

The distance vehicles travel along the roadside and the distance which it penetrates into the roadside

5

Roadside clear zones (continued)

Depends on• Vehicle traveling speed • The angle vehicle leaves the roadway

Early studies in US established that with flat sides slopes• 80-85% of vehicles traveling at highway

speeds could recover within 30 ft from the edge of the roadway

• This distance was considerably greater• On curves• On steeper slopes

6


Clear zone An area within the recovery area which is

ideally kept free of roadside hazard Roadside hazards

•Poles, trees, and shrubs with trunks greater than 4 inches

•Culvert end walls•Steep batters (greater than about 6:1 or

ideally 4:1)•drains

7


•Anything that pose a danger to errant vehicles

•Any obstacles within the zone should either be removed or shielded with guard fencing

An area which reflects the the probability of crashes occurring on that road •The cost-effectiveness of providing such a

zone

8

Roadside clear zones (continued) The clear zone dependent upon

•Traffic speed•Roadside geometry•Traffic volume

It is a compromise between safety, economic and in many cases environmental consideration

If it is not practicable to provide width of clear zone, alternative remedial treatment such as guard fencing can be considered

9


The clear zone is measured from the edge of the traffic lane.

“For curves with radius of less than 600 m (2,000 ft), the clear zone width on the outside of the curve should be doubled”

In low speed environment of urban area, a clear zone of not less than 1 m wide may be accepted to achieve the balance between safety and aesthetic

10

Roadside clear zones (continued) Cross section

Flatter roadside slope have a significant effect on single vehicle crashes

Crash rates fall steadily as side slopes are flattened from 3:1 to 7:1 or flatter

Little crash reduction is expected by flattening from a 2:1 to 3:1 slope

Side slopes of 5:1 or flatter are needed (Zegeer and Council, 1992)

11


Clear zones and side slopes are closely related•Clear zone must include a flat

traversable slope of 4:1 or flatter (Crillo 1993)

•Slopes steeper than 4:1 are too steep to allow recovery of control

•Vehicles encroaching on such slopes can be expected to travel all the way to the bottom if not rollover

12


Establishing and maintaining an obstacle free clear zone has definite safety benefits

It appears to be cost-effective on rural roads even at low traffic volumes

For volumes greater than 4000 vpd and speed around 62 mph a clear zone of 30 ft is required

Side slopes need to be flatter than 5:1 to significantly reduce the probability of rollovers

13

Roadside hazard management

Where it is not possible to provide roadside clear zone Impractical Too expensive

Roadside hazards present some measure of risk to Errant vehicles Pedestrians at risk from such vehicles

14

Roadside hazard management (continued)

The objective of roadside hazard management program is to keep this risk to a manageable level

With mass action programs, there is scope for a pro-active approach It is required to identify the features

associated with crashes of a given type, and then treat in priority order all sites which exhibit those features (Ogden and Howie 1990)

15


A hierarchy of treatments of fixed roadside hazards have proposed Eliminate all obstacles from the roadside,

either by good design and technology for new facilities, or removal or relocation of all existing obstacles

If it is not possible to eliminate all roadside objects• Identify those most likely to be struck,

establish priorities, and organize removal or relocating

16


•Make those objects most likely to be struck but impossible to remove harmless

Use a safety barrier or shield those obstacles impossible to remove or relocate

Typical treatments•Relocation of rigid utility poles, under

grounding of cables, or replacement with frangible poles

17


•Enhancing the safety of bridges and culverts by providing

• Guard fencing• Bridge railing• Improved delineation

•Flattening of batters or installation of guard fencing on steep or high embankments

•Shielding the trees with guard fence or removing them from the clear zone

•Removal of culvert headwalls or extension of culverts to increase recovery area

18


•Replacing the old guard fencing which does not meet the current standards

•Enhance delineation• Roadside devices• Pavement markings

The key component of any roadside management program is maintenance

Training of maintenance personnel is critical since proper installation and ongoing maintenance of many of these treatments is critical to their effectiveness

19

Utility poles Even though utility poles are present in a

significant number of crashes involving fixed roadside objects, not every pole represents a hazard

In general the hazard increase with• Traffic flow• Pole density (poles per length of the road)• Offset from the edge of the road• Greater for poles outside the horizontal curves• Greater at sites with low pavement friction

20

Utility poles (continued) Treatment of hazardous poles

• Relocation of the pole to a safer location• Locating light poles on the inside rather than

outside of a horizontal curve

• Shielding of poles with safety barriers• Guard fence• Impact attenuator

• Use of slip base poles which break away at the base when impacted

• Special electrical connections to ensure electrical safety

21

Utility poles (continued)• Do not retard the impacting vehicle and may

give rise to secondary crashes especially in area with high concentration of pedestrians or on narrow medians

• Use of impact absorbing poles• Especially effective for area with high

pedestrians activity

• High friction pavements where the pole is on a curve

• As a last and least satisfactory resort, attaching reflective delineators to the pole

22

Trees The most difficult aspect of roadside hazard

management concerns tree• Trees can be helpful in shielding headlights

and providing a visual barrier between the road and abutting property

• Trees add to the aesthetic of the road• Trees can also form an important subliminal

delineation function Substantial trees close to roadway result in

hazard

23

Trees (continued) Removing trees is highly effective

• On hazardous location in rural areas• High volume traffic with high speed• Close to the roadway• On the outside of horizontal curve• Low pavement friction

•Zegeer and Council (1992) quote a US study which suggest that

• Clearing trees to provide an additional 3 ft of recovery space could reduce crashes by 22%

• Additional 15 ft of recovery space reduce crashes by 71%

24

Road signs All road signs should have a frangible post

unless located behind a safety barrier Large signs such as advanced direction signs

should be of a frangible design• When struck the supporting post shears and the vehicle

passes beneath it• Installation of such posts is very important• Bolts must be correctly tensioned to ensure that they

will shear• The base section must not be too low

• Allowing debris to accumulate and impede break away

• The base section also must not be too high• It can snag an impacting vehicle

25

Road signs (continued) Frangible designs are not suitable for use in

low speed areas where vehicle does not have sufficient speed to satisfactory clear the falling pole• Roundabouts • Smaller, lighter signs mounted on lightweight

posts are more appropriate for low speed areas• Lightweight posts made of light gauge steel

pipe or channel section which easily deform on impact

• applicable to smaller road signs such as warning signs, regulatory signs and chevrons

26

Bridges and culverts Hazards associates with bridges can be

significant Narrow bridges

• increase the probability of vehicle colliding with bridge

• Reduce the opportunity for safe recovery Bridges are over-represented in crashes

relative to their length of road system Bridge crashes are more severe than

crashes as a whole

27

Bridges and culverts (continued)

Culverts can be hazard problem Culvert end wall or pipe end often located

quite close to the roadway Culvert end wall can become less

hazardous by being design to match the slop of embankment• Can be replaced with a grated inlet structure• Can be extended so the end wall is further

from the roadway

28


Ogden and Howie (1990) produced guidelines for treatment of bridge sites based on bridge width, traffic flow, and bridge length Install guideposts and reflective post-

mounted delineators when street lighting not present

Install bridge width markers on or adjacent to bridge end post, piers, or abutments on both the left and right side of roadway

29


Provide edge lining with raised reflective pavement markers where bridge width marker installed to facilitate good lane control

When bridge is on or adjacent to a curve with radius of less than 2000 ft install chevron signs on the outside of the curve

Install the guard fencing according to current standards if warranted

Upgrade the installation of guard fencing with current standards for bridges with existed guardrail

30

Other roadside hazards Devices aim to protect pedestrians and

bicycle facilities can potentially conflict with some of roadside safety principles•Where there are very large pedestrian

flows solid bollards may be erected at curb line to protect pedestrians from errant vehicles

• This is in conflict with forgiving roadside objectives

•There is clearly a tradeoff between the safety of vulnerable road users and the safety of motorists

31

Other roadside hazards When the roadside object is necessary to

protect vulnerable road users, those features should be as safe as possible• Horizontal rails which can spear an errant

vehicle should be avoided• Vertical elements should be used

• Less hazardous to motorists• More difficult for pedestrians to climb and walk on

the roadway

• Avoid to install hazardous features which have nothing to do with traffic

• Features for Landscaping purposes

32

Safety barriers Safety barriers include

Guard fences• Flexible

• Cable barriers

• Rigid• Concrete barriers

• Semi-rigid• Steel W-beam barriers• Box beam barriers

Crash cushions Bridge barriers

33

Safety barriers (continued) Warrant and guidelines for the

installation of guard fencing On embankments

• Slope and height exceed certain values• In US H/V of 3:1 and less and height of 5 m or more

Figure 12.12 page 288

Near roadside hazards• Any hazard within 30 ft of roadside may justify

shielding

34

Safety barriers (continued) On narrow medians

• To prevent head-on crashes • Where flow exceed 5000 vpd

Where road formation narrows• At some bridges and culverts

On the outside of sub-standard curves• Where differences between 85 percentile

speeds and advisory speed is greater than 10 mph

To protect structures and pedestrians

35

Steel guard railing Steel guard railing

commonly of a rolled W-section Rectangular hollow section Acts by resolving kinetic energy

possessed by an impacting vehicle into components in three dimensions• Vertical• Parallel to the rail• Perpendicular to the rail

36

Steel guard railing (continued)

The perpendicular and vertical components must be reduced or dissipated to redirect the vehicle effectively

The energy dissipation is accomplished through bending and crushing of various parts of vehicle and guard rail installation, including the soil

In order to be effective guard railing must installed to allow the energy dissipation to occur

37

Steel guard railing (continued)

The important point to stress is correct installation The final product must be safer than an

unprotected roadside object Incorrect installation impose a greater

hazard than the object it is shielding so the barrier can be ineffective or even counter-productive

Training program for people who make decision about guard rail program and people who install them is an important element of safety barrier program

38

Existing barriers Decision regarding acceptance, removal,

modification or replacement of barrier based Assessment of the performance of particular

installation concerned Significance of the departure from current

practice Other factors to consider in assessment

Potential hazard of the barrier in compare to hazard it shielded or compare that with a modern barrier

39

Existing barriers (continued)

Suitability of barriers post-spacing, terminals and transitions

Barrier length, alignment, clearance and location relative to adjacent lane

Barrier height Condition of the roadside between barrier

and traffic lane Alignment of the adjacent roadway

40


Examples of deficient guard rails Guard rail too low

• Vehicle may roll over the device Guard rail too high

• Vehicle may pass or trapped beneath the guard rail

Exposed, unanchored fish tail guard rail• Exposed end constitutes a hazard in itself• Result in the guard rail being demolished in a

collision rather than decelerating and redirecting the vehicle

41


Guard rail with inadequate taper• Correctly tapered guard rail should have its

end well away from the pavement Guard rail with inadequate post spacing

• On the approach to a bridge or rigid object• Post spacing should decrease near the bridge

end post• Will stiffen the transition between guard rail and

bridge

There is a need to inspect the existing guard rail and upgrading them to conform with requirements of current practice

42

Concrete barriers Concrete barriers

Commonly Known as New Jersey barrier primarily used for median barriers on

divided roads Warrants are in place to provide

directions for the use of this treatment Represent a significant hazard to errant

vehicles in all but very shallow angle collisions

43

Concrete barriers (continued)

Advantages Intend to prevent or minimize damage to

vehicles during low-angle impacts Does not defect to any appreciable

degree, even under sever impacts Negligible maintenance costs

Vehicles with low center of gravity are subject to roll over in collision with such a barrier Modify section with a constant side slope

may reduce the incidence of roll over

44

Cable barriers Cable barriers

Flexible barriers less common than rigid and semi-rigid barriers

Cost-effective •in low to moderate traffic flows•In situation where roadside or median

design can accommodate the large deflection inherent in these types of barrier

45

Median barriers Median barriers

Warranted on situation of high flow and narrow median width

Effective in reducing head-on crashes on divided roads but may increase other kinds of crashes

A study of their installation in UK found• 15% reduction in fatalities• Little change in injury crashes• PDO crashes increased by 14%

46

Bridge barriers Bridge barriers

Design to prevent a light vehicle running of the edge of a bridge

Railings designed to retain heavy vehicles are heavier and more expensive

• Warranted in exceptional situations with extreme crash consequences

Barriers on old bridges which do not meet the current standards may present a hazard

• Old bridges with timber rails can lead to spearing crashes• Replacing or modification of the railing may be an appropriate

treatment • Integrating the bridge railing and the approach guard railing

to provide a continuous rail

47

Crash cushions Crash cushions

Design to absorb some of the kinetic energy of errant vehicle before it impacts the object

Are effective at the end of longitudinal safety barriers in• Medians and freeway gore areas• Bridge piers in narrow medians• End of concrete barrier wall• Toll plazas

Use readily available materials• Sand or water filled barrels

48

Safety barriers and heavy vehicles

Heavier vehicle requires a stronger installation to restrain it Higher center of gravity is more susceptible

to rollover Barriers for heavy trucks only justified at

special locations• Encroachment of large truck could be catastrophic

• Bridges adjacent to a school or apartment building• Near a petrochemical plant

• Barriers to withstanding vehicles more than 9 ton are only warranted where traffic flow exceeds 100,000 vpd

49

Shoulder treatment Two major aspects of shoulders relate to

shoulder sealing Pavement age drops

Paved shoulders are much safer than unpaved shoulders on rural roads Reduce

• run-off-the –road crashes• Head-on crashes• Chance of losing control due to loose material of

unpaved shoulders By providing a grater recovery and maneuvering

space

50

Shoulder treatment (continued)

A number of studies examined the safety benefit of paved shoulders In an Australian study, Armour (1984), found that

road with paved shoulders• 60-70 percent lower fatal crashes than road with unpaved

shoulders• Benefit was greater on road section with curves or grades

Ogden (1993) in an study of Australian rural roads which had been treated with sealed shoulders as part of a maintenance program

• With paved shoulder of 2-4 ft he found such a treatment lowered crash rate by 43% on vehicle km basis

51

Shoulder treatment (continued)

Using cost data appropriate to this situation and Australian crash cost data• Ogden found the benefit cost ratio as follow

B/C = 2.6 x (AADT in thousands)

•This says that with benefit:cost ratio of 1.0 at an AADT of only about 385 vpd treatment of paving the shoulder 2-4 ft become cost effective

52

Pavement edge drops

Pavement edge drop results From resurfacing activity without desirable

shoulder improvements From wear and erosion of weak shoulder

materials A particularly susceptible location of edge drop is

inside of horizontal curves• Due to the off-tracking of trailing wheels of vehicles,

especially trucks Research emphasize the importance and

hazards of drop height and shape and found• Novice drivers are particularly at risk in such situations

1 the roadside hossein naraghi ce 590 special topics safety march 2003 time spent: 12 hrs

Documents

roadside slide

clear zone width

width of clear zone

clear zone dependent

forgiving roadside collision

roadside object impractical

roadside objects account

roadside hossein naraghi