Introduction
A highway intersection is required to control conflicting and merging streams of traffic so that delay is minimised. This is achieved through choice of geometric parameters that control and regulate the vehicle paths through the intersection. These determine priority so that all movements take place with safety.
The three main types of junction dealt with in this chapter are:
· Priority intersections, either simple T-junctions, staggered T-junctions or crossroads
· Signalised intersections
· Roundabouts.
All aim to provide vehicle drivers with a road layout that will minimise confusion. The need for flexibility dictates the choice of most suitable junction type. The selection process requires the economic, environmental and operational effects of each proposed option to be evaluated.
The assessment process requires the determination of the projected traffic flow at the location in question, termed the design reference flow. The range within which this figure falls will indicate a junction design which is both economically and operationally efficient rather than one where there is either gross over or under provision. Different combinations of turning movements should be tested in order to check the performance characteristics of each junction option under consideration.
The starting point for any junction design is thus the determination of the volume of traffic incident on it together with the various turning, merging and conflicting movements involved. The basis for the design will be the flow estimate for some point in the future – the design reference flow (DRF). It is an hourly flow rate. Anything from the highest annual hourly flow to the fiftieth highest hourly flow can be used. For urban roads, use of the thirtieth highest flow is usual, with the fiftieth highest used on interurban routes.
Deriving design reference flows from baseline traffic figures
Existing junctions
At existing junctions, it will be possible to directly estimate peak hour and daily traffic flows together with all turning movements. In order for the measurements to be as representative as possible of general peak flow levels, it is desirable to take them during a normal weekday (Monday to Thursday) within a neutral month (April, May, June, September or October). Factoring up the observed morning and evening peak hour flows using indices given in the National Road Traffic Forecasts can lead to the derivation of DRFs for the design year (normally 15 years after opening). Flow patterns and turning proportions observed in the base year can be extrapolated in order to predict future patterns of movement.
New junctions
In a situation where a junction is being designed for a new road or where flow patterns through an existing junction are predicted to change significantly because of changes to the general network, flows must be derived by use of a traffic modelling process which will generate estimates of 12, 16 or 24 hour link flows for a future chosen design year. AADT flows are then obtained by factoring the 12, 16 or 24 hour flows. The AAHT is then calculated (AADT ∏ 24) and then factored to represent the appropriate highest hourly flow using derived factors. Tidal flow is then taken into consideration; generally a 60/40 split in favour of the peak hour direction is assumed. Turning proportions are also guesstimated so that the junction can be designed.
Short-term variations in flow
Traffic does not usually arrive at a junction at a uniform or constant rate. During certain periods, traffic may arrive at a rate higher than the DRF, at other periods lower. If the junction analysis for a priority junction/roundabout is 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 peaktime 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.
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 hundredth 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 viability. 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 capacity at all times.
The values originally given in Table 5A of Appendix D14 of the Traffic Appraisal Manual range 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.
