TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 131
7.10
Traffic Flows
7.10.1
Hourly Design Flow
Hourly Design Flows shall be calculated using an appraisal method agreed with TII. For Inter-Urban
road types, the 50th and 200th highest hourly flows respectively shall be used. The highest value of
the total design flow, corrected for HGVs and gradient as described later in
this Section, projected to
the 15th year after opening, shall be taken as the basis of design for merges, diverges, and weaving
sections.
7.10.2
Mainline Traffic Capacity
For the purpose of designing junctions and interchanges, the maximum lane capacity for Type 1 Dual
Carriageway national roads should be taken as 1,600 vehicles per hour (vph) and for Motorways as
1,800 vph. These values have been used in Figures 7.3 and 7.5 in this Standard. If higher values have
been used in the design of the mainline carriageways, then the equivalent number of lanes should be
used, instead of design flows, for that part of Figures 7.3 and 7.5 that relate to the mainline.
7.10.3
Design Flows and Connector Road Cross Sections
Connector road cross sections corresponding to design traffic flow ranges are given in Table 7.4.
Further details of the cross sections are given in Section 7.11.
Table 7.4: Connector Road Cross Sections for Design Flows
For Mainline Road Type
Connector Road Cross Section for:
Motorway
Type 1 Dual
Carriageway
National road
Slip Road
Interchange
Link/Loop
Merge
Diverge
Merge/Diverge
Peak Corrected Design
Flow on Connector Road:
Vehicles per hour
0-900
0-800
Single Lane - 4.00m
Carriageway
Single
Lane -
4.00m
Carriageway
900-1350
800-1200
Single Lane -
4.00m
Carriageway
Two Lane -
6m
Carriageway
1350-2700
1200-2400
Two Lane -
7.3m
carriageway
Two Lane -
7.3m
carriageway
2700-3600
2400-3200
Two Lane -
7.3m
carriageway
Notes:
Cross-sectional details for Connector Road verges and hard strips are given in the relevant tables of DN-GEO-03036.
7.10.4
Flow Corrections for Uphill Gradients and for LCVs
Corrections for uphill gradients and for the presence of HGVs as set out in Table 7.5 shall be made to
the predicted hourly flows before corresponding values are read off from Table 7.4 and from Figures
7.3 and 7.5.
To establish the mainline gradient a one kilometre section shall be used, 0.5 km either side of the
merge or diverge nose tip, and the average gradient determined. The merge connector road gradient
shall be based on the average of the 0.5 km before the nose tip.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 132
Table 7.5: Percentage Correction Factors for Gradients and the presence of HGVs
%HGV
Mainline Gradient
Merge Connector Gradient
<2%
>2%
<2%
2%-4%
>4%
5
-
+10
-
+15
+30
10
-
+15
-
+20
+35
15
-
+20
+5
+25
+40
20
+5
+25
+10
+30
+45
7.11
Geometric Standards
7.11.1
Cross Sections
For the purpose of designing grade separated junctions, Table 7.4 gives the required number of lanes
and widths for the running carriageway for a range of design flows and road types. Details of other
components such as hard shoulders, hard strips and verges that make up the cross section for slip
roads and interchange links are detailed in DN-GEO-03036. Values for the appropriate widths of such
components can be found in the relevant tables of DN-GEO-03036.
7.11.2
Maximum Lengths of Slip Roads and Interchange Links
A Slip Road longer than 0.75 km must be designed as an Interchange Link.
Single Lane Interchange Links must only be provided:
a)
when their length does not exceed 1 km and they are on an average uphill grade
of up to 3%, are level or on a downhill grade; or
b)
where their length does not exceed 0.5 km and they are on an average uphill
grade of 3% or steeper.
Where two lane interchange links are proposed, care will be needed to ensure that any subsequent
merge can be designed in accordance with this standard. Layout A and Layout B merges are not
permitted for two lane slip roads.
7.11.3
Design Speed
Design speeds for the mainline must be determined from DN-GEO-03031. The design speeds of
connector roads must be as given in Table 7.6. The design speed for link roads should normally be
one design speed step below that of the mainline, as shown in Table 7.6 and this reduced design
speed will need to be made clear to the vehicle driver. To help achieve this, link roads should be
subject to an appropriate speed limit, either mandatory or advisory. Where the proposed link road
design speed is one design speed step below that of the mainline and this cannot be made obvious
to the driver, the higher design speed should be used. Where the link road is a connection to a
motorway, motorway merge parameters apply, regardless of the design speed.
