For the purposes of assigning capacity of different urban road types, five dif-
ferent highway types are considered:
Motorways (UM)
Urban all-purpose road Type 1 (UAP1)
Urban all-purpose road Type 2 (UAP2)
Urban all-purpose road Type 3 (UAP3)
Urban all-purpose road Type 4 (UAP4).For motorways, the prime determinant of capacity is the carriageway width as
access is severely restricted, with minimal impediments to through traffic. In the
case of all-purpose roads, however, flow is affected by the speed limit, the
frequency of side roads, the degree of parking and loading, the frequency of
at-grade pedestrian crossings and bus stops/accesses. In particular, UAP3 and
UAP4 may carry high percentages of local traffic, resulting in an escalation in
the levels of turning movements both at the junctions and accesses.
The design of rural roads is as detailed in Tables 4.15, 4.16, 4.17 and 4.18.
Tables 4.15 and 4.16 relate carriageway widths to peak-hour flows for 2-way
roads. Table 4.17 details capacities for 1-way urban roads. Table 4.18 details
adjustments required to be made to the peak-hour flows where the proportion
of heavy goods vehicles in the traffic stream exceeds 15%.
The flows supplied in the tables are the maximum that typical urban high-
ways can carry consistently in one hour. It should be noted that, in most situa-
tions, the design flows on existing non-dual carriageway routes will be governed
by the capacity of the terminal junction. The design flows themselves are only
appropriate when the highway in question is used solely as a traffic link. Caution
should therefore be exercised to ensure that the carriageway widths chosen do
not correspond to maximum design flows which exceed the capacity of the junc-
tions at peak times.
Flows for single carriageways as indicated in Table 4.15 are based on a 60/40
directional split in the flow. The flows for highway type UM as given in Table
4.16 apply to motorways where junctions are relatively closely spaced.4.6.2 Forecast flows on urban roads
The flow ranges supplied in Tables 4.15 to 4.17 provide a guide for the assess-
ment of appropriate carriageway standards, applicable both to new urban high-
ways and to the upgrading of existing facilities. The purpose of these standards
is to aid the highway planner in deciding on a carriageway width which will
deliver an appropriate level of service to the motorists. The capacities referred
to in these tables apply to highway links. The effects of junctions are dealt with
in the next chapter.
If an existing route is being improved/upgraded, existing traffic flows should
be measured by manual counts, with allowances made for hourly, daily or96 Highway Engineering