The hazard presented by a roadside pole is related to both its location and type of construction as these factors contribute to the hazard the pole may pose and the consequences of an errant vehicle hitting the pole. Poles that are present in road reservations to reticulate electricity are problematic in that they are generally very expensive to remove and replace with an underground supply. However, this option should be considered in appropriate situations.
Poles are a common road furniture item used to support signs (regulatory, warning, guidance, informative), road lighting and various devices. In line with the preferred treatment for roadside hazards (i.e. removal), the practitioner’s aim should be to minimise the number of poles in the area of interest.
Appendix C of AS 1742.2 – 2009 discusses aspects of longitudinal and lateral placement, mounting height for signs, orientation, post type and selection. Signs should be erected so that sight distance is not compromised. Longitudinally, signs should be located to provide enough warning for a driver to be able to make a decision and respond as necessary. It is also important that signs are spaced far enough apart longitudinally that drivers are able to process the
information before encountering another sign. If these requirements are not satisfied, drivers may
Minimum lateral setback distances for signs and for road lighting poles are specified in AS 1742.2 – 2009 and AS/NZS 1158.1.3 – 1997 respectively. Where possible, poles should be located such that an errant vehicle is unlikely to hit them.
Pole removal
On tangents to curves where there is a crash history the removal of a pole may lead to crashes migrating to the next available pole. When considering removal of a pole with a crash history it is important to understand why vehicles are leaving the road and take action to keep vehicles on the road.
Undergrounding cables
Relocation of utility services to underground ducts and removal of the poles is the most effective option for the treatment of hazardous poles.
Rationalisation of pole functions
It may be possible to rationalise the number of poles along a road corridor by combining separate functions and services onto common poles. For example, traffic signals, road lighting and large signage may be supported by the same poles. Power cables, telecommunication services and spotlights can share common poles.
It may be possible to place all poles on the side of the road that has the better safety performance or least risk. This may involve changing the poles from side to side as the crash risk changes along a curved route.
Reducing pole numbers by increasing spacing
Increased pole spacing provides areas for errant vehicles to pass between poles as shown in Figure 5.3. The effective gaps for vehicles to pass through are dependent on the width of the vehicle and the exit angles.
If increased pole spacing is used to reduce the roadside risk then designers should check that the poles being removed to increase pole spacing are those that have been involved in crashes or have the higher risk. It would be counterproductive to remove poles which have not been a hazard but leave the high-risk poles in place.
Centre line Edge line
Edge line
Pole spacing.wmf
Close pole spacing
Wide spacefor vehicletrack
Narrowspace for vehicletrack
Wide pole spacing Edge line
Centre line Edge line
Centre line Edge line
Edge line
Pole spacing.wmf
Close pole spacing
Wide spacefor vehicletrack
Narrowspace for vehicletrack
Wide pole spacing Edge line
Centre line Edge line
Source: RTA (2008).
Figure 5.3: Pole spacing Relocation
Pole relocation needs to target areas where the run-off-road crashes are likely, for example on the approach to curves, the outside of curves, near lane merges, lane terminations, adjacent to exits from roundabouts and intersections. Research (Zegeer & Cynecki 1984; Zegeer & Parker 1984) has confirmed the belief that the number of crashes decreases as poles are moved further from the roadway.
The expected percentage reduction in pole crashes with increasing distance from the roadway is shown in Table 5.2. The data is shown graphically in Figure 5.4.
Table 5.2: Percentage reduction in pole crashes with increasing distance from the roadway Distance from roadway after relocation (m)
Distance from roadway
Reduce impact severity
The use of frangible poles may be effective in reducing the severity of pole-related crashes, if pole removal or relocation is not feasible. These types of poles are designed to collapse or break away on impact and thereby reduce the severity of injuries to the occupants of an impacting vehicle, compared to those that could occur if the pole was rigid.
Frangible poles
Rigid poles do not deform to a great extent, but are designed so that they remain upright after an impact. Alternatively, frangible poles are designed to deform upon vehicle impact and are usually used for road lighting poles as the lighting needs to be close to the road. Types of frangible poles include:
Slip-base poles that break away at the base upon impact, allowing the vehicle to pass beneath the pole in order to minimise or avoid injury to vehicle occupants.
Impact absorbing poles that collapse over the colliding vehicle and are designed to bring the vehicle to a controlled stop at the base of the pole. These deformable poles are designed to remain in the ground after being hit.
The following issues need to be considered when specifying frangible poles to reduce impact severity:
removing or relocating the pole should be considered before specifying frangible poles
the area behind the pole should be free of other hazards and in the case of break away poles a run-out area may be required
there should be limited pedestrian activity in the vicinity of the pole
the damaged pole and any elements that detach under impact should not pose a risk to other road users
Impact absorbing poles should be favoured over slip-base poles where there is closely abutting development, pedestrian and parking activity and a low traffic speed environment.
Signposts should be designed to be frangible in the event of impact by an errant vehicle (i.e. posts that are designed to fracture, break away, give way or bend), such that the damage to a colliding vehicle and risk of injury to vehicle occupants upon impact is minimised. Small signs are usually supported by posts that deform in a way that causes minimum damage to cars, whereas larger posts and supports (for larger signs) may be provided with mechanisms that are designed to yield in a controlled manner upon impact.
Aspects to be considered in the selection of pole type and setback from the roadway include:
surrounding land use
pedestrian activity
speed limit
whether the road is kerbed or un-kerbed
Slip-base poles
Slip-base poles consist of a standard pole stem, mounted on two base plates that are clamped together with bolts that release on impact thus allowing the pole stem to break away from its foundation. An example is shown in Figure 5.5. A disadvantage with slip-base poles is that the dislodged pole may create a secondary incident by falling on bystanders or adjacent vehicles.
The decision to use slip-base poles will depend on the space available and the resultant likelihood that a falling pole would cause injury to other users of the road or roadside area. For example, a slip-base pole will usually be inappropriate where pedestrian or cyclist traffic is common because a falling pole may pose an unacceptable risk to those road users.
Lack of maintenance is a significant problem with slip-base poles. They should be checked regularly to ensure they are free to slide and the bolt tension is correct. Wind vibration can cause poles to move the assembly and jam the bolts.
Impact absorbing poles
Impact absorbing poles remain attached to the base structure and absorb impact energy by progressively deforming and entrapping the impacting vehicle. The deformation of the pole is controlled by a designed weakening of the pole stem. Figure 5.6 illustrates that concept of an impact absorbing pole.
Impact absorbing poles have less maintenance issues than slip-base poles.
Source: Austroads (2004).
Figure 5.6: Examples of impact absorbing poles