A summary of the more common types of hazards that may exist adjacent to roads is provided in Table 4.3. A variety of fixed hazards may occur in roadsides and hazards may also be regarded as ‘point’ hazards or ‘continuous’ hazards as shown in Commentary 5.
For the general purpose of this guide the following objects are not considered to be hazardous fixed objects for vehicle occupants:
small size steel and timber sign support posts that comply with AS 1742.2 – 2009, Table D2 and Table D3
tubular thin-walled traffic signal posts at urban intersections.
Whilst these objects may not be hazardous to vehicle occupants they are likely to be hazardous for errant motorcyclists and therefore the number of poles adjacent to roads should be minimised and their design made as forgiving as practicable. In addition, the presence of these objects may cause an errant driver to take evasive action in order to avoid them and this may lead to a more serious crash.
In practice many traffic signal poles are not frangible and are not protected. The reason for this is that these poles require adequate strength to support the necessary traffic signal and road lighting hardware, particularly under wind loading, and the provision of barriers to shield the poles is usually impracticable or would lead to other disbenefits. Most importantly, traffic signal systems provide significant net road safety benefits.
Table 4.3: Summary of roadside hazards
Hazard Comment
Other vehicles Where the area of interest (as defined by clear zone widths) includes one or more opposing lanes of traffic, the danger of errant vehicles crossing the median and colliding with oncoming or stationary vehicles is high. This issue also applies to special facilities that are accommodated in a median or a separate reservation adjacent to the road. Such facilities may include:
a high occupancy vehicle lane
local traffic (e.g. frontage roads)
traffic adjacent to the through traffic where there is a speed differential equal to or greater than 20 km/h
transit corridors (e.g. busways, railways, light rail etc.)
freight railways.
These facilities may require a safety barrier to separate their operations from an adjacent road carriageway. For example, transit corridors or freight railways within or adjacent to intermediate or high speed roads are typically protected with an appropriate safety barrier unless a comprehensive risk assessment demonstrates that protection is not required. Consideration needs to be given to not only the risk to motorists but also to users of the transit corridor or the freight railway.
Fill batter Fill batters may be hazardous due to the combination of height and slope and surface condition, as well as what may be on the slope or at the base of the embankment. They become critical when the slope exceeds 3:1 as vehicles are likely to overturn.
A warrant for treatment of fill batters on high-speed roads is shown in Figure 4.6 and an embankment assessment process is described in Section 4.3.4.
Cut batter Cut batters may be hazardous due to the combination of height, slope and surface. Slopes steeper than 4:1 may cause an errant vehicle to become unstable and rough surfaces (e.g. jagged rock) may also cause vehicle instability and excessive damage to the vehicle.
Non-frangible objects are hazardous when:
they are too close to the travelled path
piers are unshielded in the median reserve
snagging on an exposed face does not allow the vehicle to slide along the structure.
Retaining walls may be hazardous depending on the type, height and lateral location with respect to traffic.
Surface texture of the walls and treatment of the end of walls can also be hazardous to errant vehicle occupants.
Rock cuttings are hazardous when:
a steep-sided slope is more than 4:1
there is a steep-sided or a deep ditch at the foot of the slope
the height is less than 1.5 m above travelled path level
they are close to the travelled path
there are unshielded rock excavations and exposed rock cuts
there is a rough surface with irregularities of more than 65 mm, even though the face may be in an even plane (refer to Section 5.4.8).
Steel or concrete bridge parapets are hazardous when:
the upper railing is not designed for absorption of car crashes
terminations are not protected.
Non-frangible objects (such as bridge piers, bridge end posts, concrete barrier end-on impact, rock face cuttings, large items of built environment infrastructure, etc.)
Buildings and walls are hazardous when:
they are too close to the travelled path
the exposed angle of the property wall or the wall itself blocks the errant vehicle.
Hazard Comment Trees, poles and vegetation are hazardous when they are large and too close to the travelled path. Trees are
particularly dangerous when:
the diameter is more than 70 to 100 mm (depending on species)
fallen branches are left aside the travelled path
tree stumps are more than 100 mm over ground level.
The problems associated with forests and groups of close-spaced trees depend on the spacing of the trees and thus influence the type of safety measures required.
Many items of road furniture rely on break away or energy-absorbing structures to protect errant vehicles. Road furniture items considered to be potential hazards are:
utility poles (power, telephone overhead cables) and high-voltage electricity columns
sign supports, including vertical sign supports, sign gantry legs, posts of large signs and overhead sign supports, traffic sign supports
steel and high-mast lighting columns, lighting poles and luminaire supports
rural mailboxes and structures
any non-yielding pole.
Trees, poles and vegetation
Vertical sign supports, traffic signs, posts of large signs, sign gantry legs and lighting columns are dangerous when:
the structure is not yielding when hit
lighting columns are close to the travel lanes
lighting columns are on medians.
Culverts Cross drainage of road reserves is achieved by the provision of culverts that may vary in size from a single small pipe (e.g. 375 mm) to large multiple pipes or box culverts.
Culverts that do not have their inlet and outlet matched to a traversable foreslope are a hazard.
If not treated single culverts and end treatments wider than 1.0 m are a hazard for passenger size vehicles.
Untreated large culverts (i.e. ends not matched to foreslope and no grates over openings; single pipe > 900 mm diameter, multiple pipes > 750 mm diameter) within the clear zone are a hazard and should be assessed taking into account factors such as the:
volume of traffic
height of drop associated with the culvert
culvert size
distance and pavement slope between the headwall and the edge of traffic lanes.
Untreated culvert ends at right angles to the direction of traffic are a hazard, for example, culverts under driveways or median crossings.
Culvert headwalls Culvert headwalls are hazardous when they are:
too close to the travelled path
not matched to embankment foreslope
higher than 100 mm
mounted into the drain beside the travelled path.
Drains and kerb Acceptable longitudinal open drain profiles are described in Section 4.6 of the Guide to Road Design – Part 3:
Geometric Design (Austroads 2009b). However, some crash testing of drain shapes indicates that, depending on the angle, shapes outside of those described in Part 3 are traversable (Thomson & Valtonen 2002); refer also to Commentary 6 for more information.
As a general guide, longitudinal drains and kerbs are hazardous where:
foreslopes have gradients 3:1 or steeper
if the foreslope and the backslope (2:1) form a V-shape, as it is possible to crash into the back slope
Hazard Comment
Road safety barriers should be regarded as hazards in that they are roadside objects which may be impacted by errant vehicles. They should only be used where they constitute a lesser hazard to road users than the hazard being shielded.
There are increased risks associated with barriers when:
the vehicle crashes against an inappropriate barrier (e.g. barrier not suited to site constraints, improper dimensions, poor positioning or untreated terminations)
the vehicle crashes against improperly maintained road safety barriers
vehicles can move behind the road safety barrier
the distance between the barrier and the hazard is less than the working width of the barrier when impacted, allowing the vehicle to deform the barrier and contact the hazardous features
the height of the barrier is too low
the length of need is not adequate
the barrier is too short
the length of anchorage is too short
there is a short gap between two barriers
they are too high and limit sight distance
a gating end treatment is installed without a hazard-free run-out area
the barrier effectiveness is reduced due to terrain effects, kerbing, drains etc.
penetration of a lower test level barrier occurs when it is impacted by a vehicle that is larger than the test vehicle.
Motorcyclists are likely to be injured when hitting road safety barriers and barrier delineators.
Road safety barriers
Road safety barrier terminals can be dangerous when:
the termination of the barrier is not properly anchored
the distance between the obstruction and the barrier terminal is too short
the transition between deformable and rigid barriers causes high deceleration
they do not meet performance class requirements.
Bodies of water Bodies of water should be evaluated with respect to the degree of potential hazard they pose (NYS DOT 2003).
This will be a combination of the amount of water and its accessibility. The depth of water may be ranked according to whether:
a vehicle can completely submerge, resulting in the drowning of uninjured non-swimmers, disabled or elderly persons, or infants (depth of water > 0.6 m)
water could fill an upright car to a point where an unconscious or injured driver or passenger would drown (typically assumed to be a depth of 0.6 m)
an upside down car would be in water deep enough that an unconscious person would drown (a depth of 0.3 m).
Fast-moving bodies of water are considered to be more hazardous than still water. In general, designers should carefully consider the risk associated with bodies of water over 0.6 m deep, or water courses with a normal base flow depth greater than 0.6 m, as these could cause a stunned, trapped, or injured occupant to drown.
Other factors to consider include the:
slope of the vehicle path to the water
total distance available in which to stop a vehicle
likelihood of a vehicle being upside down upon reaching the water
persistent or intermittent presence (flooding potential) of the water hazard
presence of intervening obstructions that would reduce the likelihood of an errant vehicle reaching the water.