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T.RD.M.V.2.6
(Al/2002)
63 Expressway Design Standards
63,1 Design Speed
6.3.1.1 The concept and assessment of design speed is set out in Section 3.3.2 of Chapter 3 of this
Volume. On Expressways the elimination of access other than at interchanges, the
prohibition of use by pedestrians, together with the more generous alignment, gives
confidence to the driver and higher operating speeds will result. It is essential therefore
that the design speed adopted should be in the highest band possible for dual carriageways.
6.3.1.2 Table 6.3.1.1 provides details of appropriate design speeds for Expressways in Rural and
Urban situations, for the main route, link roads, and slip roads.
Table 6.3.1.1
Expressway Design Speeds fkm/h)
Main Routes Link Roads Slip Roads
Rural Conditions 100 100 80 or above
Urban Conditions 80 or above 80 50 or above
6.3.1.3 With regard to Table 6.3.1.1 the aim should be to adopt the highest design speed possible,
commensurate with economic design. It is not appropriate to vary design speeds along a
route because of localised difficulties, but in these situations consideration should be given
to varying particular design elements such as horizontal curvature or sight distance so that
instead of desirable minimum values, lesser values and if necessary, absolute minimum
values are provided. However it is stressed that at junctions along Expressways, design
elements should not be less than desirable minimum values except where an Expressway
passes from rural to urban conditions, and to avoid excessive costs it may be appropriate to
adopt standards less than the desirable minimum to maintain the same design speed
throughout the route. If for any reason it is proposed to depart from the standards given in
Table 6.3.1.1 full justification must be provided and approval from the Transport
Department must be obtained.
6.3.1.4 In order to provide a surface commensurate with the higher design speeds adopted,
Expressways should normally, but see paragraph 6.3.1.7, have a friction course surfacing,
which is an open graded porous bituminous material described and set out in the Highways
Department Road Note 5.
6.3.1.5 The use of friction course surfacing is generally recommended for Expressways as this
material has amongst others the following properties compared with continuously graded
bituminous materials:
(i)
(ii)
(iii)
(iv)
minimisation of road spray from vehicles during and after raining;
greatly increased texture depth;
reduction of the potential for vehicles to aquaplane;
improved skid resistance at high speeds.