being done with the aid of one of the Transport Research Laboratory’s
computer programs (PICADY/ARCADY), such variations can be allowed
for using a ‘flow profile’. A typical profile could involve the inputting of peak-
time flows in 15-minute intervals. When calculations are being completed
by hand for a priority junction/roundabout, such short-term variations may
be taken into consideration by utilising an hourly flow equal to 1.125 times the
DRF.
In the case of a priority junction, this adjustment should be applied to the
design flows on both the minor and major arms. In the case of a roundabout
intersection, this factored flow will impact not only on the entry flows to the
roundabout but also the circulating flows within the intersection.
5.2.4 Conversion of AADT to highest hourly flows
Appendix D14 of the Traffic Appraisal Manual(DoT, 1996) originally detailed
factors linking AADT to the tenth, thirtieth, fiftieth, hundredth and two hun-
dredth highest annual hourly peak flow for three classes of roads:
Main urban
Interurban
Recreational interurban.
Particularly on urban highways, where peaks are less marked, the thirtieth
highest flow may be most appropriate. On an interurban route, the fiftieth
highest might apply. On recreational routes where peaks occur infrequently, the
two hundredth highest may be the value most consistent with economic viabil-
ity. The general implication is that where the design flow is exceeded some degree
of congestion will result, but this is preferable and economically more justifiable
to the situation where congestion will never occur and the road is under capac-
ity at all times.
The values originally given in Table 5A of Appendix D14 of the Traffic
Appraisal Manualrange between approximately 2.4 and 4.4. The use of these
national expansion figures for converting AADT to peak hour flows is no longer
recommended. Rather it is advised that local traffic data be used in order to
compile such factors.
5.3 Major/minor priority intersections,
5.3.1 Introduction
A priority intersection occurs between two roads, one termed the ‘major’ road
and the other the ‘minor’ road. The major road is the one assigned a permanent
priority of traffic movement over that of the minor road. The minor road must
The Design of Highway Intersections 105
