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T.P.D.M. V.2.3
(Al/2001)
3.9.3.29 A distance of at least 1000 mm from the traffic face of safety fence or 600 mm from the
back of post whichever is greater should be provided for any corrugated beam barrier fences
erected adjacent to the edge of a downhill slope or a retaining wall to allow the dynamic
deflection of the fences during the impact. It is stressed that if kerbs are present, it is
desirable to set the barrier face in the same line as the kerbline. Otherwise, additional
dynamic deflection should be allowed.
3.9.3.30 For an uphill slope, the height and slope angle are the factors determining whether barrier
should be provided in front of the slope. In the comparison of the severity of vehicular
impacts with slope to that with safety fences, Roadside Design Guide suggested that safety
fences should be provided for less severity when the slope angle is greater than 20° and the
slope height is greater than 3 m.
3.9.3.31 If no barrier fence is provided in front of an uphill slope, attention should be given to ensure
that the slope surface within a height of 1000 mm from the adjacent pavement should be
smooth.
3.9.3.32 Dynamic deflection varies according to the impact speed, impact angle and the
characteristics of the barrier fences. For an impact angle between 20° and 25°, Table 3.9.3.1
gives the minimum distance to the hazard.
Table 3.9.3.1
Dynamic Deflection for Various Barrier Types and Speeds
Barrier type/Speed
Distance between back of post and obstacle
80km/h
90km/h
lOOkm/h
HOkm/h
W-beam barrier with posts
spaced at 2 m centres
1.2m
1.3m
1.4m
1.6m
Standard/Modified
Thriebeam barrier with
posts spaced at 2 m centres
0.5m
0.7m
0.9m
l.lm
3.9.3.33 The effective safety fences should be extended far enough upstream and downstream to
prevent errant vehicles from hitting the hazardous object.
3.9.3.34 The safety fence may be flared so that they are installed away from the kerbline as it
approaches its terminal. The flaring of safety fence has several purposes as follows:
(i)
(ii)
The total length of safety fences can be reduced.
The safety fences can be located further away from the travel lanes.
3.9.3.35 The disadvantage of flared fences is that the greater the flare rate, the higher the angle at
which an errant vehicle can hit. Also, greater flare rate will increase the possibility of an
impacting vehicle being redirected back into or across the carriageway following an impact.
This situation is especially undesirable on two way carriageway where the impact vehicle
could be redirected into on-coming traffic.