TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 27
At this point, the key geometric parameters of the junction design should be assessed and a geometric
layout developed in accordance with this standard. The swept path of vehicles likely to use the junction
shall then be checked using a computer based programme to ensure safe movements through the
intersection.
4.2.4
Stage 4
–
Assemble and Model/Check Proposed Design
Having established the various components of the junction design, the Designer shall check that the
capacity of the junction is still adequate. This includes examining if the junction is located on a route
with a wide variation in flow and turning movements, particularly those having prolonged daily peak
periods. The check should be undertaken prior to assembling the component parts to form a complete
junction.
At this stage a Swept Path analysis shall be undertaken to confirm driveability using an appropriate
software programme. The swept path of vehicles likely to use the junction shall be checked using a
computer based swept path analysis programme by the Designer to ensure that all vehicle movements
can be accommodated. The swept path analysis shall be appropriate for the vehicle using the junction.
See Appendix A for details of approved Design Vehicles.
Heavy Goods Vehicles (HGVs) may be selected as the Design Vehicle, in which case they should
enter and depart from the junction in the correct lane(s). However, where these vehicles and other
vehicles operating under restricted access conditions only use the junction occasionally, it may be
acceptable for the design to be based on them encroaching into other traffic lanes. This may cause
some inconvenience to other road users, but may be acceptable where there is a low frequency of
occurrence.
Both the tracking width and swept path width shall be considered in the design
of the road for use by
the Design Vehicles. Tracking width lines delineate the path of the vehicle
tires as the vehicle moves
through the turn. Swept path width lines delineate the path of the vehicle body as the vehicle moves
through the turn and will therefore always exceed the tracking width. Refer to Figure 4.2.
Before proceeding to final design, a driveability check should be performed, to assess firstly the
smooth assembly of the components of the junction design. This should include a computer generated
visual assessment of the junction on all approaches from t
he road user’s eye view. Secondly, the
junction should be considered within the context of its adjacent links and
those adjacent junctions on
the particular route. As a whole, the layout should be designed to suit the anticipated traffic pattern,
with the principal movements following smooth vehicular paths.
4.2.5
Stage 5
–
Final Design
A design shall be developed for the junction layout in accordance with the relevant sections of this
standard.
4.3
Design Vehicle
The use of computer software to predict the swept path of large vehicles is mandatory in the design
of all junction types. The vehicle type used shall be appropriate to the predicted use of the junction or
direct access. The vehicle tracking software should be capable of allowing vehicle speed to be
measured. The Designer shall ensure that the junction design can accommodate a driver negotiating
the junction at a minimum speed of 5km/hr for direct accesses and 15km/hr for priority junctions. At
all other junctions, Designers shall demonstrate using tracking software that the design vehicles
included in Appendix A can travel at the design speed set out in this standard.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 28
All of the geometric parameters outlined in this standard for the design of priority junctions have been
developed to cater for a 16.5m long articulated vehicle, whose turning width is greater than the vehicle
dimensions permitted in the existing Road Traffic (Construction, Equipment and Use of Vehicles)
Regulations, or likely to be permitted in the near future. In cases where hard strips are present, the
design vehicle may encroach on these while turning, and at some simple junctions with local roads,
the design vehicle may encroach into opposing traffic lanes.
Figure 4.2: Tracking Width and Swept Path of Vehicle
4.4
Improvement of Existing Junctions
Junctions are improved to increase traffic flow or to provide safety improvements.
For junctions where traffic flow patterns have changed since design, a traffic assessment of the
existing and anticipated traffic flows should be conducted as per the current TII Project Appraisal
Guidelines. Improvements could
include additional lanes, change in traffic control.
For junctions where flow patterns have changed since design but geometric improvements are not
justified based on the traffic assessment, additional road markings may be appropriate. Road
markings may help to:
a)
improve throughput at high levels of traffic flow;
b)
cater for particularly high turning movements;