Design manual
Design Manual for Urban Roads and Streets
The design of roads and streets within urban speed limits.
Department of Transport, Tourism and Sport and Department of Housing, Planning and Local Government · 2019 (version 1.1) · 165 pages
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- 109May 2019 (Version 1.1) The Y distance along the visibility splay should correspond to the SSD for the design speed of the major arm, taken from Table 4.2 while also making adjustments for those streets which are frequented by larger vehicles. For example, within Industrial Estates and/or on Arterial and Link streets with higher frequency bus routes. In general, junction visibility splays should be kept clear of obstructions, however, objects that would not be large enough to wholly obscure a vehicle, pedestrian or cyclist may be acceptable providing their impact on the overall visibility envelope is not significant. Slim objects such as signs, public lighting columns and street trees may be provided, but designers should be aware of their cumulative impact. • Street furniture, such as seats and bicycle stands may also be acceptable, subject to being sufficiently spaced. • Splays should generally be kept free of on-street parking, but flexibility can be shown on lower speed streets with regard to minor encroachments. • Pedestrian guardrails can cause severe obstruction of visibility envelopes, and the use of guardrails should be avoided (see Section 4.2.5 Street Furniture). Designers should also check the visibility envelop in the vertical plane on approach to junctions (see Section 4.4.6 Alignment and Curvature, Figure 4.67) Designers may have concerns about reducing visibility splays at junctions that carry higher volumes of traffic at more moderate speeds. This issue was addressed further in respect of research carried for the UK Manual for Streets 2 (2010). This included ‘busy radial roads’, many of which included bus routes within a variety of 20, 30 and 40 mph environments.34 The research concluded that there is no evidence that reduced SSDs are directly associated with increased collision risk, as shown on a variety of street types at a variety of speeds. The Manual for Streets 2 (2010) also refers to research where it was found that higher cycle collision rates occurred at T-Junctions with greater visibility.35 The research concluded that this was because drivers were less cautious where greater visibility was provided. Designers must also take a holistic view of the application of reduced forward visibility splays. As illustrated in the Adamstown Street Design Guide (2010), there are other place making and traffic calming benefits that can be implemented by reducing forward visibility splays at junctions (see Figure 4.64). 34 Refer to 10.4 of UK Manual for Streets 2 (2010) and the report High Risk Collision Sites and Y Distance Visibility (2010). 35 Refer to Layout and Design Factors Affecting Cycle Safety at T-Junctions (1992). Figure 4.64: Flow diagram showing the interlinked traffic calming and place making benefits of reduced visibility splays. INCREASED RETAIL VIABILITY INCREASED ON-STREET PARKING INCREASED PEDESTRIAN ACTIVITY REDUCED BUILDING SETBACKS REDUCED VISIBILITY SPLAYS INCREASED LEVELS OF TRAFFIC CALMING
- Design Manual for Urban Roads and Streets110 4.4.6 Alignment and Curvature Changes in the alignment of roads and streets are generally referred to in the horizontal and vertical sense. When these changes occur, the FSD, is reduced and, as noted above, this is one of the most effective measures used to increase driver caution and calm traffic (see Figure 4.65). Horizontal Alignment The horizontal alignment of a street consists of straight sections and curves. Whilst changes to the horizontal alignment calms traffic, this needs to be balanced with safety concerns. To prevent abrupt changes in direction minimum FSD required along a street should correspond to the minimum SSD appropriate to the design speed. FSD is checked at horizontal curves by measuring between points on the curve along the centreline of the inner lane (see Figure 4.66). Frequent changes to the horizontal alignment should also be balanced with permeability and legibility. Overuse of changes in the direction of streets may disorientate pedestrians and increase walking distances between destinations. In this regard: • Designers should avoid major changes in the alignment of Arterial and Link streets as these routes will generally need to be directional in order to efficiently link destinations. • Major changes in horizontal alignment of Arterial and Link streets should be restricted to where required in response to the topography or constraints of a site. • There is greater scope to use changes in horizontal alignment on Local streets to promote lower speeds and a more intimate sense of place (see Section 4.4.7 Horizontal and Vertical Deflections) • Designers should not rely on curvature alone to reduce vehicle speeds. Changes in horizontal alignment should be combined with contextual measures that reduce forward visibility, such as building lines and on-street parking. Envelope of Visibility Figure 4.66: Forward visibility at horizontal curves need to take account of SSD values Figure 4.65: Example from Clongriffin, Co. Dublin where a change in the alignment of the street calms traffic as drivers proceed cautiously due to the uncertainly of what lay ahead (image source: Google Street View). Stopping Sight Distance
- 111May 2019 (Version 1.1) Horizontal Curvature At a horizontal curve, the centrifugal force a vehicle travelling around that curve is generally counteracted by a combination of 2 factors: friction between the tyres and the road surface, and superelevation of the carriageway, where the carriageway is constructed such that the outside carriageway edge is higher than the inside carriageway edge. Traditionally, the design approach has been to combine these factors to ensure that a vehicle can travel around a bend without reducing speed or without causing significant discomfort to the occupants of the vehicle. Where a horizontal alignment along a street requires changes in direction, the curves between straight sections should have radii in accordance with Table 4.3. Crossfall Designers should also consider superelevation where one side of the road is designed to be higher than the other in order to resist the centrifugal effect of turning a corner. As the aim of superelevation is to assist drivers to maintain higher speeds around curves, its use is inappropriate where the design is intended to achieve a moderate or low speed environment. As also noted in the Manual for Streets 2 (2010), superelevation is also difficult to implement in urban areas with frequent junctions and points of access.36 36 Refer to Section 8.3 of the UK Manual for Streets 2 (2010). However a crossfall of 2.5% is generally provided on carriageways to assist in drainage, which would tend to result in adverse camber at horizontal curves. Consequently, in order to assist in achieving lower vehicle speeds through a more restrictive horizontal alignment, there is a need to provide sharper horizontal curves that do not have the benefit of high levels of superelevation to counteract the centrifugal force. Designers should refer to Table 4.3 for minimum radius with adverse camber of 2.5%. Where the introduction of radii less than those for minimum radius with adverse camber of 2.5% is unavoidable, a reasonable level of superelevation may be introduced to eliminate adverse camber and introduce a favourable crossfall. Minimum curve radii for a superelevation rate of 2.5% are also presented in Table 4.3, and may be used in such circumstances. Design Speed (km/h) 10 20 30 40 50 60 Crest Curve K Value N/A N/A N/A 2.6 4.7 8.2 Sag Curve K Value N/A N/A 2.3 4.1 6.4 9.2 Table 4.3: Carriageway geometry parameters for horizontal and vertical curvature. Design Speed (km/h) 10 20 30 40 50 60 Minimum Radius with adverse camber of 2.5% - 11 26 56 104 178 Minimum Radius with superelevation of 2.5 % - - - 46 82 136 VERTICAL CURVATURE HORIZONTAL CURVATURE
- Design Manual for Urban Roads and Streets112 Vertical Alignment A vertical alignment consists of a series of straight-line gradients that are connected by curves, usually parabolic curves. Vertical alignment is less of an issue on urban streets that carry traffic at moderate design speeds and changes in vertical alignment should be considered at the network level as a response to the topography of a site. The required envelope of forward visibility in the vertical plane is illustrated in Figure 4.67 below. The envelope should encompass the area between a driver eye height in the range of 1.05 metres to 2.00 metres, and an object height in the range of 0.6 metres to 2.00 metres Vertical Curvature Where changes in gradient are required along an alignment, vertical curves are introduced, such that the appropriate SSDs are achieved, and an adequate level of driver comfort is provided. Ordinarily in urban areas where it can be expected that vehicle speeds will reduce in response to changes in alignment, it will be sufficient to design vertical curves such that the minimum SSD is provided. Vertical curves can take the form of Crest or Sag curves, the length of a vertical curve, L, is the critical design parameter, and is determined by multiplying the K Values set out in Table 4.3 by the algebraic change of gradient expressed as a percentage, that is: L = Ka Where: K = The constant of curvature a = The algebraic change in gradient. Vertical Crest Curve Design At crest curves visibility can be obstructed by the road surface itself. Crest curve, accordingly, should be designed such that the curvature is sufficient to maintain an adequate FSD and SSD for a driver. In urban areas, where vehicle speeds are low and gradients are generally modest, the design of vertical crest curves can be simplified as follows: • For very low design speeds (i.e. less than 40 km/h), and where the algebraic difference in gradient between straight sections is less than 12%, it will generally not be necessary to specifically design a vertical crest curve; however the carriageway should be shaped to avoid an abrupt change in vertical alignment. • For design speeds of 40 km/h and above, and again where the algebraic difference in gradient is modest, up to a maximum of 12 %, it will normally be sufficient to provide a vertical curve with a length determined using the K-values presented in Table 4.3. In exceptional circumstances where the algebraic difference in gradient exceeds these limits, it will be necessary for the designer to determine a crest curve length suitable for the circumstances from first principles. Figure 4.67: Visibility envelope in vertical plane. Envelope of Visibility Stopping Sight Distance 2m 0.6m 1.05m 2m Direction of Travel
- 113May 2019 (Version 1.1) Vertical Sag Curve Design When designing vertical sag curves, there are three potential design parameters that need to be considered: • Driver Comfort. • Clearance from Structures. • Night-time Conditions. In urban areas, the obstruction of visibility due to structures (overbridges, gantries etc.) is likely to be an uncommon occurrence, and night time visibility only becomes an issue on unlit roads. Therefore the sag curve K values presented in Table 4.3 are based on the driver comfort parameter, and have been derived using a comfort criterion of 0.3m/s2 maximum vertical acceleration. Maximum and Minimum Gradients In urban areas, it is likely that the comfort of vulnerable road users will be the determining factor for desirable maximum longitudinal gradients on streets. Part M of the building regulations advises that access routes with a gradient of 1:20 or less are preferred. Therefore a maximum gradient of 5% is desirable on streets where pedestrians are active. In hilly terrain, steeper gradients may be required but regard must be had to the maximum gradient that most wheelchair users can negotiate of 8.3%, although this should be limited to shorter distances A designer may need to consider mitigation measures, such as intermediate landings, to ensure that pedestrian routes are accessible. This also needs to be considered at the network level and as a response to place making. The inclusion of streets that exceed these gradients may not be significant within a network where there are alternative routes that can be taken between destinations and where steeper gradients may in fact have placemaking benefits. A minimum longitudinal gradient of 0.5% is desirable to maintain effective drainage on streets. Care needs to be taken at vertical curves, and in particular at sag curves, to ensure that there is provision at level points of curves to allow surface water to run off the carriageway.
- Design Manual for Urban Roads and Streets114 Figure 4.69: Example from Dorset Street, Dublin, where the carriageway has been raised and paved to slow turning vehicles and enhance the pedestrian crossing. Figure 4.68: illustration of how off-setting junctions can create a change in alignment (without reducing permeability or legibility) and reduce forward visibility. 4.4.7 Horizontal and Vertical Deflections Horizontal or vertical deflections are changes that occur within the alignment of the carriageway to slow vehicles by requiring drivers to slow and navigate obstacles. Deflections include chicanes (horizontal) or ramps (vertical). The use of such physically intrusive measures is not necessary within a self-regulating street environment. Less aggressive features, such as junction offsets (see Figure 4.68), raised tables and changes to kerb lines, can be used strategically as supplementary measures which calm traffic and assist pedestrian movement by allowing them to cross at grade (see Section 4.3.2 Pedestrian Crossings). Raised tables, or platforms, may be placed strategically throughout a network to promote lower design speeds, slow turning vehicles at junctions and enable pedestrians to cross the street at grade. Key locations where these should be considered include: • On longer straights where there is more than 70m between junctions.37 • At equal priority junctions. • At entrance treatments where Local streets meet Arterial and Link streets (see Figure 4.69). • Outside Focal Points and areas of civic importance (such as schools). 37 Refer to Section 7.4.3 of the UK Manual for Street (2007). • At pedestrian crossings. • To reinforce a change between design speeds (such as at entrance treatments). As raised tables are primarily designed to reinforce lower speed environments, their use should generally be limited to Local streets and/or the Centres. The use of raised tables more broadly across Arterial and Link streets (excluding those within Centres) should be limited to sections where speeds are to be lowered for a particular purpose (i.e. adjacent to Focal Points and/or key pedestrian crossings). The principal aim of the designer should be to slow vehicles without causing undue discomfort. In this regard: • An entry slope of 1:20 will allow most vehicles to cross at moderate speeds • An entry slope of 1:15 is more appropriate for lower speeds. • The minimum length of level section of the table should be 2m (to allow a pedestrians to cross). • The height of a raised table should generally correspond with that of the adjoining kerb. Where buses operate the maximum height should be 75mm to reduce passenger discomfort.
- 115May 2019 (Version 1.1) Horizontal deflections are particularly effective when considered at the network level and used in combination with restrictions in forward visibility (see Section 4.4.6 Alignment and Curvature and Figure 4.70). When deployed throughout a network on Local streets they can also be used to discourage through traffic (see Section 3.4.1 Vehicle Permeability). Deflections can be created by varying the kerb line/street alignment causing the carriageway to broaden and narrow and/or creating a series of directional adjustments. Car parking may also be used to similar effect (see Section 4.4.9 On-Street Parking and Loading). Other methods that may be considered at the network level include off-setting junctions to create a 3 Way Off Set Network (See Section 3.4.1 Vehicle Permeability). Singular treatments include pinch-points that narrow the width of the carriageway over a short section of the street. These can be used in combination with raised tables at key locations on Local streets and/or within the Centres (see Figure 4.71). To be visually effective a pinch point should seek to reduce the width of the carriageway by a minimum of 0.5m for a minimum length of 6m.38 38 A minimum of 3.7m (3.1m at ‘gateways’) is required for fire vehicle access as per Table 5.2 of the Building Regulations 2006 (Technical Guidance Document B – Fire Safety). Figure 4.70: Examples from Poundbury, Dorchester, UK, where changes in the kerb line and carriageway alignment calm traffic by limiting forward visibility, creating pinch points and requiring multiple changes in direction. Figure 4.71: An example from Ingress Park, Kent, UK, of how the path and speed of a vehicle is altered within a low speed environment through the use of vertical and horizontal deflections (and material changes).
- Design Manual for Urban Roads and Streets116 4.4.8 Kerbs Kerbs traditionally provided a street drainage function but have more recently come to define the pedestrian domain from the vehicular carriageway. In so doing kerbs are key to establishing the level of segregation or integration which is to occur within a street. Lower kerbs, or lack thereof, can therefore create a greater sense of shared space and can be used to calm traffic. Lower kerb heights are also easier for pedestrians to negotiate, particularly for the mobility impaired. With regard to the height of kerbs: • The standard height for kerbs is 125mm and this provides a clear definition of a segregated street environment. These should be used on all streets where design speeds and pedestrian activity are more moderate, such as on Arterial and Link streets. • Lower kerbs of 60mm are more appropriate in areas of higher pedestrian activity and where lower design speeds are applied, such as on all streets within Centres, around Focal Points and on Local streets (see Figures 4.72 and 4.73). • Where a shared surface is proposed a kerb should not be used. Designers may consider embedding a kerb line or drainage channel (see Figure 4.74) into the carriageway to indicate an area of pedestrian refuge. This is particularly important for visually-impaired users who feel less comfortable on shared surfaces and also require a kerb line for navigation (see Section 4.3.4 Pedestrianised and Shared Streets). Changes to kerb lines can also be used to slow drivers at critical points by changing the alignment of the carriageway to create pinchpoints, build-outs and horizontal deflections (see Section 4.4.7 Horizontal and Vertical Deflections). Build-outs should be used on approaches to junctions and pedestrian crossings in order to tighten corner radii, reinforce visibility splays and reduce crossing distances (see Sections 4.3.2 Pedestrian Crossings and 4.4.5 Visibility Splays). Figure 4.73: Example from Clongriffin Co. Dublin, where the footpath, kerb line and vehicular carriageway are at the same level. Whilst pedestrian and vehicular space are still clearly defined, a greater sense of shared space is still created. Figure 4.74: Example of an drainage channel on Exhibition Road, London. The kerb line indicates an area of pedestrian refuge and is used to guide the visually impaired. Figure 4.72: Example of a low kerb from Drogheda, Co. Louth, which is used to reinforce lower design speeds and create a greater sense of shared space.
- 117May 2019 (Version 1.1) 4.4.9 On-Street Parking and Loading One of the principal objectives of this Manual is to promote the use of more sustainable forms of transport. Whilst a place-based approach to street design will reduce car dependency, as noted in the Urban Design Manual,39 people may wish to own and park a car, even if it is not used on a regular basis. On-street parking and loading refers to spaces that are directly adjacent to and accessible from the main vehicular carriageway. Onstreet parking, when well designed can: • Calm traffic by increasing driver caution, visually narrow the carriageway and reduce forward visibility. • Add to the vitality of communities by supporting retail/commercial activities that front on to streets through the generation of pedestrian activity as people come and go from their vehicles. • Contribute to pedestrian/cyclist comfort by providing a buffer between the vehicular carriageway and foot/cycle path. • Reduce the need or temptation for drivers to kerb mount and block foot/cycle paths. • Provide good levels of passive security as spaces are overlooked by buildings. The quantity of on-street parking that is needed in a given area depends on a number of factors, but is most closely related to proximity to Centres, the availability of public transport and the density, type and intensity of land use. Notwithstanding these factors, on-street parking has a finite capacity, depending on the per unit parking requirements. For example in residential areas: • On-street parking alone can generally cater for densities up to 35-40 dwellings per ha (net). • Once densities reach 40-50 dwellings per ha (net) the street will become saturated with parking and reduced parking rates (a max of 1.5 per dwelling) and/or supplementary off-street parking will be required. 39 Refer to Section 11 of the Urban Design Manual (2010). • For densities over 50 dwelling per hectare, large areas of off-street parking, such as basements, will generally be required. Getting the balance right presents a challenge to designers. If parking is over provided it will conflict with sustainability objectives and can be visually dominant. Conversely, if parking does not cater for user needs or is under provided it may encourage poor parking practices (including illegal ones) such as kerb mounting, parking on footpaths and within areas of open space. Whilst off-street parking may form part of a design response, the first priority of a designer should be to locate parking on-street as follows: • On Arterial and Link streets on-street parking spaces should be provided in a series of bays that are parallel to the vehicular carriageway. • Perpendicular or angled spaces may be provided in lower speed environments such as Local streets. They may be applied more generally in Centres to cater for increased demands around shopping areas. • On-street parking on public streets should not be allocated to individual dwellings. This allows for a more efficient turnover of spaces and, as such, fewer spaces are needed overall. • Loading facilities should preferably, be provided off street. However, this is not always possible or desirable within older centres and/or where it would lead to an excessive number of access points to driveways. There are a number of measures that should be used by designers to ensure that parking and loading areas are well designed (see Figures 4.75 and 4.76): • To reduce the visual impact of parking the number of parking spaces per bay should generally be limited to three parallel spaces (including loading areas) and six perpendicular spaces.
- Design Manual for Urban Roads and Streets118 • Perpendicular parking should generally be restricted to one side of the street to encourage a greater sense of enclosure and ensure that parking does not dominate the streetscape. • To reinforce narrower carriageways (particularly when spaces are empty) each parking/loading bay should be finished so that it is clearly distinguishable from the main carriageway. • Kerb build-outs, or similar treatment, should be provided to separate each bank of parking/loading. These will enable space for the planting of street trees and other street facilities (such as lighting or bike racks).40 • Kerb build-outs should also be provided on the approach to junctions to facilitate visibility splays (see Section 4.4.5 - Visibility Splays), reduce corner radii (see Section 4.3.3 Corner Radii) and ensure a clear line of sight between vehicles and pedestrian crossings. 40 Refer to page 186 of the National Cycle Manual (2011).1-2m 2m 2.4m 5.5m 4.8-5m 1-2m2m Figure 4.76: Extract from the Newcastle LAP (South Dublin County Council) illustrating the layout of a Local street with a uniform mix of parallel and perpendicular parking. Figure 4.75. Example from Ballymun, Co. Dublin (top) and Leixlip, Co. Kildare (bottom) where kerb build-outs and contrasting materials are used to separate and define bays of parking from the vehicular carriageway, reduce corner radii and facilitate planting or landscape treatments.
Source Department of Transport, Tourism and Sport and Department of Housing, Planning and Local Government: Design Manual for Urban Roads and Streets (2019 (version 1.1)) · Licence the publisher's termsText read from the publisher’s PDF; tables, maps and drawings do not survive as text, and page numbers are the PDF’s (its first page is 1). Check the original before relying on a passage.