Geometric Design of Roads and Junctions as per UK Standards

The Design Manual for Roads and Bridges (DMRB) in the UK provides detailed guidance for the geometric design of roads and junctions, focusing on safety, efficiency, and the optimization of traffic flow. Key elements covered in the DMRB include:

1. Design Speed

• The geometric design is primarily based on the design speed of the road, which influences all key features like curve radii, sight distances, and lane widths.
• Design speed is influenced by road classification and the anticipated function and capacity of the road.

2. Road Alignment

Horizontal Alignment: This involves designing the horizontal curvature, ensuring smooth transitions between straight and curved sections using appropriate radii and superelevation. The alignment should maintain safety at the design speed.

Horizontal alignment combines elements of straights and arcs connected by transition curves (normally spirals). Where required, the lengths of transition curves are calculated in accordance with the equation in paragraph 4.13 of CD 109.?

Vertical Alignment: This deals with the road’s gradients and changes in elevation, considering the comfort of drivers, vehicle capabilities, and sight distances.

Vertical alignment is composed of a series of straight-line gradients connected by vertical curves, normally parabolic in form (sags and hogs/crests). Curvature of vertical elements are expressed as “K values”.? The k value can be expressed as the length of curve required to achieve a 1% change in gradient, with the length of the vertical curve = change in gradient x k value. Approximate vertical curve Radius (in metres) = 100 x K. It is also worth noting that using the desirable minimum k values for crest curves usually ensures that visibility requirements are achieved across the vertical curve.

Superelevation: To counteract the centripetal force on bends, sometimes “superelevation” of the carriageway is required.? As described in clause 4.2 of CD 109 superelevation shall be provided in accordance with Equation 4.2 subject to maximum values for rural and urban roads.

Standard crossfall of 2.5% (to ensure drainage) is normally provided for larger horizontal curves and for tighter curves super elevation is applied which is normally developed over the length of the transition curves.? Designers should be careful to avoid creating flat areas when changing from crossfall in one direction to super elevation in the opposite direction and should also consider the design in conjunction with the longitudinal fall. A minimum longitudinal channel gradient of at least 0.5% should be maintained wherever superelevation is to be applied or reversed.

Sight Distance: Visibility is critical, so sight distance calculations ensure that drivers can stop or react to hazards in time. Stopping sight distance and overtaking sight distance are key parameters. The higher the design speed, the longer the distance to stop or make a manoeuvre. Sight distance is defined as the length of carriageway that the driver can see in both the horizontal and vertical planes. To achieve SSD on curved alignments, it may be necessary to widen verges or the central reservation where Road Restraint Systems (or retaining structures in the verge) could restrict forward visibility.

Two types of sight distance are detailed:

• stopping sight distance (SSD), defined as the minimum sight distance required by the driver in order to be able to stop the car before it hits an object on the highway, and
• full overtaking sight distance (FOSD), which only applies to single carriageways.

3. Cross-Section Design

Lane Widths: These are determined based on the type of road (e.g., motorways, rural roads) and the design speed. Standard lane widths are typically 3.65 meters for high-speed roads.

Shoulders and Verges: These are designed to accommodate errant vehicles, provide space for maintenance, and improve safety.

4. Junction Design

Junction design in DMRB is influenced by traffic volumes, types of vehicles, and safety considerations. All aim to provide drivers with a road layout that will minimise confusion. This is achieved through the selection of geometric parameters that control and regulate the vehicle paths through the junction. These determine priority so that all movements take place safely.

Good junction design incorporates:

• Consideration of operational safety – visibility, traffic signs, maintenance, drainage
• Economics and efficiency – capacity for queuing, journey time reliability, and land take
• Vehicle movements – origin and destinations, volumes of traffic, size/capacity of junction, swept paths for turning manoeuvres of large vehicles.

There are 4 main types of junctions in a hierarchy of priority, namely

1. Priority Junctions (or at-grade) – where one stream of traffic gives way to another. Examples are T-junctions, and Crossroads
2. Roundabouts – operate by a system of prioritisation also but aim to balance flows between connector roads based on geometry. The key feature of roundabouts is traffic entering the roundabout will give-way to traffic already on the circulatory carriageway. The exception to this is if the roundabout is controlled by traffic signals.
3. Grade-separated junctions – where traffic “slips on/off” a mainline road and some form of priority junction is located either above or below the mainline traffic. Usually provided where traffic flows are higher on one or more links or where speeds are high on one or more links.
4. Interchanges – an extension of grade-separation but where traffic streams are separated and re-join seamlessly either by a merge / diverge arrangements on to slip roads or an interchange link before merging onto another high-speed road.

Priority Junctions:

There are three basic types of priority junctions (excluding roundabouts which is dealt with separately):

1. Ghost Island Junctions (Road Markings)
2. Single Lane Dualling (Physical Island)
3. Simple Junction (No Channelising Islands)

DMRB Standard CD 123 ‘Geometric design of at-grade priority and signal-controlled junctions’ takes a comprehensive approach to priority junctions and some important considerations are given below.

• Justification for selection between different types of major/minor priority junctions is essential. This should consider, but is not limited to, the design year traffic flows at the junction, the swept path of the largest design vehicle (16.5 m long articulated vehicle), the design speed of major road, site geometry and constructability, traffic delay during construction, estimate of entry and exit traffic flow from the minor road and accident data. Tip: always check for abnormal loads.
• Safety considerations: visibility and stopping sight distance(s) should be ensured so that the scheme does not contain surprises or risks to drivers.?
• Geometric design of junction elements should be such that they consider all road users’ specific requirements, including walkers, cyclists and horse riders. The Active Travel team and Inclusive Mobility team can support on these elements (including undertaking a junction assessment and level of service review if needed).
• Adequate capacity of the proposed junction arrangement should be assessed for traffic demands, taking into account all road users.? Operational assessments can be undertaken using TRL software, PICADY for priority junctions and should be carried out based on opening year and design year (normally 15 years after opening) traffic flows.? Alternative software is available for traffic signal-controlled junctions including OSCADY and Linsig.?
• Any Departures and Relaxations should be identified and notified to the relevant highway organisation. Remember relaxations are not permitted on the immediate approach to junctions.
• The traffic signs and road marking design at major/minor priority junctions should follow the Traffic Signs Manual and Traffic Sign Regulations and General Directions, and along with any traffic signal control should be considered early in the design process. TSM chapter 6 worthy of a specific mention for traffic signal junctions, Traffic Signs Manual – Chapter 6 - Traffic Control as this chapter now deals with design and not just signs/markings. Reference should be made to LTN 1/20 as well.
• Consideration of any aesthetic landscape design should be given as early as possible, for spatial extents/constraints.? The design should ensure that landscape planting does not obstruct visibility splays at the junction.
• The smooth assembly of all the design elements of the priority junction should be assessed with a “driveability” check which is the visual assessment of the junction on all approaches from the driver’s eye view for its acceptability.

Roundabouts

CD 116 DMRB is the primary reference for roundabout design.

Operational assessments for roundabouts can be undertaken using TRL software, ARCADY and should be carried out based on opening year and design year (normally 15 years after opening) traffic flows.?

An important consideration in the application of standards is when CD 109 applies and when CD 116 applies.? Although there will be parameters that cross-over (e.g. visibility to the give way line) there is a notional separation between the two standards, as shown opposite.

Geometric parameters that impact capacity calculations:

Traffic flow against geometry is the over-riding factor in establishing the capacity of a roundabout. A number of geometric parameters have an impact on the capacity of a roundabout. These are:

• Entry width
• Effective flare length
• ICD
• Entry angle
• Approach width
• Entry radius
• Entry Path Deflection.

Of the above, improving an entry width and/or effective flare length of an arm is most likely to have a beneficial impact on that arm’s capacity.

Entry deflection and entry and exit radii and vehicle path:

Where speed limits are greater than 30mph, the aim of a roundabout’s design is to restrict normal roundabout vehicle entry speeds to 30mph or less.? This is achieved by assessing a vehicle’s entry path radius within 50m prior to a give way line using the methodology described in CD116 to ensure a length of vehicle entry path of between 20m to 25m has a radius that is between 20m and 100m (100m is a radius considered small enough to desirably restrict most vehicles to 30mph and so therefore allowing a relatively safe passage into the roundabout).? It should be noted that the standards do not require entry deflection to be measured as one single 2m wide 100m radius curve striking tangentially the approach centre line, the entry radius and the central island although, in reality, this is a common way for designers to calculate deflection.? Although simple in its application, such a method will invariably “over provide” for entry path curvature. CD 116 requires the designer to model the most realistic path a vehicle takes when travelling through a roundabout.? The path a vehicle takes is invariably a function of the number of steering adjustments a driver makes, and this will result in a compound curve consisting of a number of individual radii.? The production of such a curve will always be down to the judgement of the designer but the entry path should aim to represent a realistic reflection of a fast-moving vehicle approaching a roundabout at a time when there are no other vehicles to moderate driver behaviour. ?

Crossfall, longitudinal fall and crown lines:

CD 116 gives recommendations for crossfalls on the immediate approach, the circulatory carriageway and exit of medium and large Normal Roundabouts. Crossfall and longitudinal fall are significant factors in road safety at roundabouts and the designer must ensure that normal roundabout crossfalls reflect the likely general movements of traffic within the junction whilst ensuring that the level design allows adequate drainage within the junction. The recommended crossfall immediately before an entry and around a circulatory carriageway of a roundabout is 2%. Circulatory carriageway crossfall should not exceed 2.5%.? The maximum arithmetic difference in crossfall, either side of a crown line, on any given section of a roundabout should not be more than 5%.? CD116 also recommends that longitudinal fall on the immediate approach to a roundabout should be no more than 2% before entry.?

Grade Separated Junctions (including Interchanges):?

Junction arrangements with grade separation generally allow traffic to move freely and at higher overall speeds.

Interchanges are generally defined as: “Two or more road links separated in level to allow both roads to pass each other unhindered, with joining links or slip roads provided where traffic leaves/joins from one road to another”. Also, by removing the conflict between traffic movements at the point of intersection reduces the risk of accidents. The complexity and size of the layout increases due to traffic capacity and performance demands.?

The main point of reference for the design of Grade Separated Junctions is CD 122 in DMRB.? It is important to read and understand the ‘Terms and Definitions’ of this standard before commencing design. The design is an iterative process, affected by many influences and decisions, not least the degree of access to be provided and the traffic flows to be accommodated. The junction layout should give drivers and other road users a clear understanding of what is required of them. The design should provide:

• Conspicuous and obvious junction locations and layouts.
• Advance notification of the layout on the approach to any junctions or decision points.
• Understanding of all traffic movements.

Designers should ensure that as drivers approach a junction, they can easily perceive the junction form and layout so that they can select their path through the junction accordingly.? The CD 122 definition of visibility across loops for example has been subject to debate in the past, but the generally accepted guidance is that full visibility across the extents of the loop is required usually limiting landscaping in the middle of the loops significantly.? Earthworks and landscaping should be an integral part of junction design rather than an afterthought.? Ease of use should be checked for both daytime and night-time conditions.

The location of a grade separated junction can have a significant effect on the environment as well as its operational safety and performance.

The following considerations should be taken into account at the initial design stage to produce the optimum design for comparison with other junction types.

• Location
• Safety???????
• Performance (capacity to accommodate traffic)
• Consistency?????????
• Environment and Carbon impact
• Land take?
• Economic assessment
• Maintenance????????
• Capital cost

Notwithstanding other influences and constraints, the following are recommended layouts for consideration in order of increasing traffic flow demand:

1. Compact grade separation
2. Diamond – slip roads leading to / from two staggered priority junctions
3. Half-cloverleaf – two-way slip roads leading to/from two priority junctions on the minor road
4. Dumbbell roundabout – slip roads leading to/from two roundabouts connected by a central link road either under or over the mainline
5. Two bridge roundabout – a single large roundabout with the circulatory carriageway either under or over the mainline
6. Interchanges: 3-level roundabout – roundabout between two major roads of similar flow; The two mainlines are on the upper and lower levels with the roundabout on the central level
7. Interchanges: Complex – major roads with all movements catered for by free-flowing connector roads

5. Roadside Features

• Safety barriers, crash cushions, and clear zones are incorporated into the design to protect vehicles from hazards such as steep slopes or obstacles like poles or walls.
• Drainage design is also crucial for maintaining the structural integrity of the road and preventing water accumulation.

6. Non-Motorized User Considerations

• Provision for pedestrians, cyclists, and equestrians must be integrated into the geometric design, especially at junctions and crossing points.

The DMRB breaks these aspects into specific volumes, offering standards for road layout, alignment, and safety features. Each section of the road design, including junctions, must comply with these standards to ensure functionality and safety.

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