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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- 49May 2019 (Version 1.1) Transition Zones A Transition Zone refers to an area that may be needed for slowing vehicles when entering an urban area from a faster moving road, such as from a rural road into a city, town or village or from a motorway into an integrated street network (see Figure 3.19). Designers should emphasise Transitions Zones by: • Introducing measures that provide enclosure, such as large trees. • Applying transitional geometric measures, such as the narrowing of carriageways. • Applying changes to carriageway surfacing materials. The length of a Transition Zone will largely be influenced by the required reduction in speed. Designers should also take into account how visible/prominent any subsequent Gateway is. If a Gateway is highly visible from a distance, a Transition Zone may not be necessary as drivers will instinctively be inclined to slow. Figure 3.19: Illustration of a Gateway and Transition Zone that reinforces a large speed reduction when entering an integrated street network. Figure 3.18: Image from Traffic in Villages (2011) showing a various number of gateway treatments designed to enhance the character of the village and calm traffic. GATEWAY TRANSITION ZONE Integrated Street Network Motorway
- Design Manual for Urban Roads and Streets50 3.4 Management 3.4.1 Vehicle Permeability Integrated networks do not require the same degree of restrictions to be placed on the movement of vehicles as is applied to more conventional/segregated networks. A network of integrated/self-regulating streets provides the framework for higher levels of accessibility for slow modes (including motor vehicles at slow speed) and strategic continuity for cross-network modes at more moderate speeds (such as public transport) as (see Figure 3.20): • The slower nature of Local streets (i.e. 10-30 km/h) will result in them being less attractive to through traffic. Although trips through Local streets may be more direct (and therefore legible), the traffic-calmed nature of these streets may not necessarily result in significant advantages in overall journey times. • Through traffic will be attracted to Arterial/ Link streets where trips are more direct and are designed to cater for more moderate speeds (i.e. up to 50km/h). • Public transport along Arterial/Link streets can be prioritised by measures such as Quality Bus Corridors and Bus Lanes (see Section 3.4.3 Bus Services). There are a number of advantages to more permeable networks in regard to the management of traffic and vehicle speeds such as: • Drivers are more likely to maintain lower speeds over shorter distances than over longer ones. As drivers are able to access individual properties more directly from Access/Link streets (where speeds are more moderate) they are more likely to comply with lower speed limits on Local streets (see Figure 3.21). • Permeable layouts provide more frequent junctions which have a traffic-calming effect as drivers slow and show greater levels of caution.7 7 Refer to Whose Street is it Anyway? Redefining Residential Street Design (2006). Figure 3.20: Examples from Adamstown, Co. Dublin. Through routes (top) are designed to cater for more moderate speeds and to prioritise public transport movement. Local streets (bottom) are slower moving, thus discouraging use by through traffic. Figure 3.21: Drivers are more likely to comply with posted speed limits where less time is spent on streets with a low design speed (as per the bottom example) A B A B
- 51May 2019 (Version 1.1) • Increasing access to neighbourhood cells can result in the more equitable distribution of traffic and the impacts of congestion as it is no longer concentrated on a few select junctions or local access streets (see Section 3.4.2 Traffic Congestion) and noise and air pollution (see Section 3.4.5 Noise and Air Pollution). • The value of place can also be improved as slower moving traffic has less impact on the surrounding environment (see Section 3.4.5 Noise and Air Pollution). • Frequent entrances to a neighbourhood cell can reduce the size of individual junctions and streets. This will reduce the potential for severance between communities and increase pedestrian/ cyclist mobility as streets/junctions are more compact and easier to navigate. Designers may be concerned that more permeable street layouts will result in a higher rate of collisions. However, research has shown that there is no significant difference in the collision risk attributable to more permeable street layouts in urban areas and that more frequent and less busy junctions need not lead to higher numbers of accidents.8 The degree to which permeability is provided for different transport modes can be categorised into four types (see Figure 3.22): • Dendritic Networks which place significant restrictions on movement for all users. • Open Networks which allow full permeability for all users. • 3 way Off-Set Networks which contain a large proportion of 3 way junctions. • Filtered Permeability networks which allow full permeability to some users whilst placing greater restrictions on others. 8 Refer to Whose Street is it Anyway? Redefining Residential Street Design (2006) and ‘Safenet’ analysis Manual for Streets: Evidence and research (2007). Figure 3.22: Types of Street Network identified within Road Safety Planning: New tools for Sustainable Road Safety and Community Development (2007). DENDRITIC NETWORK 3 WAY OFF-SET NETWORK FILTERED PERMEABILITY NETWORK OPEN NETWORK
- Design Manual for Urban Roads and Streets52 Designers should avoid the creation of Dendritic networks which place heavy restrictions on movement. The use of cul-desacs that do not allow through access for users should be restricted throughout any network (see Section 3.3.2 Block Sizes). Open Networks place few restrictions on the permeability of users. They are best suited to contexts where maximum accessibility is desirable for all users such as within Centres. Open Networks may also be desirable in Business Parks/Industrial Areas to allow more efficient access for commercial vehicles. 3 Way Off-Set Networks allow through movement for all modes, however, they discourage faster modes by requiring vehicles to slow, stop and/or change direction repeatedly when travelling along Local streets. Such networks are suitable to all contexts, but there are limitations to their overall effectiveness. The use of multiple junctions offsets can reduce legibility. This can discourage walking/cycling as the network is difficult to navigate and the route unclear (as well as increasing journey times). It can also result in driver frustration, as noted above. Filtered Permeability Networks, which restrict universal permeability, may be applied where designers are seeking to prioritise the movement of more sustainable modes (i.e. pedestrians, cyclists and public transport) over private vehicles. For example bus gates and other measures, may also be used to prioritise bus movements, particularly in Centres (see Section 3.4.3 Bus Services). The limited use of vehicular cul-de-sacs may be considered in Neighbourhoods and Suburbs where there is a particular concern regarding through traffic. The use of vehicular cul-de-sacs to enforce Filtered Permeability networks should be approached with caution. Their overuse can result in many of the negatives associated with Dendritic networks being replicated. Additional design measures should be applied to ensure that pedestrian and cycle links are not perceived as ‘anti social spaces’. Links should maintain clear sight lines and be overlooked by development (see Figure 3.23). Figure 3.23: Examples of vehicular cul-de-sacs in Adamstown, Co. Dublin, which allows for through pedestrian and cyclist access only and has incorporated design measures to ensure that it is safe (i.e. clear sightlines and passive surveillance).
- 53May 2019 (Version 1.1) Figure 3.24: Example from Drogheda, Co. Louth, of narrow street that was converted from a twoway system to a one way system to facilitate a series of improvements within the town centre that calm traffic, expand the pedestrian domain and strengthen the sense of place. Within existing networks, pressure is often applied from local communities to create vehicular cul-de-sacs. Designers should approach such requests with caution, as street closures will often simply shift the problem elsewhere. One-way streets have also been widely implemented, retrospectively, in order to filter vehicle permeability and relieve traffic congestion. The use of one-way systems for traffic management should also be approached with caution by designers as they: • Promote faster speeds as drivers are likely to drive faster when no risk is perceived from oncoming traffic. • Will result in longer vehicular journeys, including those for cyclists and public transport. • Can be confusing for users when they deflect people away from destinations. • Require additional signage. Conversion to one-way systems may be beneficial on narrow carriageways where the street reserve is limited in order to provide additional space for pedestrians, cyclists and other public realm improvements. Counter flow cycle lanes should also be considered in order to maintain permeability for cyclists Examples include Centres where the implementation of a one-way system has direct placemaking benefits as it allows for additional footpath width and/or on-street parking (see Figure 3.24). The key to network design is balance. An optimal approach to network design is to start from the position of an Open Network. This will provide for the development of a robust network that can evolve over time to meet the changing needs of a place. Parts of the network may then be refined by incorporating elements of Filtered Permeability Networks and 3 Way-Off Set Networks according to local conditions and where there are clear benefits in terms of prioritising more sustainable modes of transport, improving safety and reducing energy consumption.
- Design Manual for Urban Roads and Streets54 Figure 3.26: Extract from the Traffic Management Guidelines (2003) showing Traffic flow capacity increasing as speed reduces. 3.4.2 Traffic Congestion A primary function of all transport policies has been to reduce the waste of resources caused by congestion. National and regional transport policies and plans have recognised that it is not feasible or sustainable to accommodate continued demand for car use. In contrast, sustainable modes (walking, cycling and public transport) can cater for very high volumes of movement in a far more efficient manner (see Figure 3.25). Policies and plans, therefore, promote sustainable modes of travel and acknowledge that, in the absence of demand management, a certain level of car congestion is inevitable. One of the outcomes of a more connected, traffic-calmed network will be reduced car dependency and increased use of more sustainable modes of transport. This is the most balanced way of addressing traffic congestion. Higher levels of connectivity for all users will also enable greater vehicular permeability, albeit at slower speeds. The benefits of this approach include: • Slower vehicle speeds are often perceived to be a cause of congestion but can lead to increased traffic capacity (see Figure 3.26). Figure 3.25: Illustration of the amount of space required to transport the same number of people via different modes of transport (image source: Munster Planning Office, Germany) SPACE REQUIRED TO TRANSPORT 60 PEOPLE WITHIN A STREET • More frequent minor junctions with fewer vehicle movements calm traffic and are easier for pedestrians and cyclists to navigate. Within urban networks, delay and congestion overwhelmingly occur at junctions. Segregated networks channel traffic towards fewer junctions and this can locally concentrate the negative impacts of traffic, resulting in large junctions where bottlenecks occur (see Figure 3.27). The design of junctions has traditionally prioritised the minimisation of vehicular queuing and delay. As a result pedestrians can face significant delays. This is also evident in the various computer programs used to analyse junction design, which have the calculation and minimisation of vehicular queuing and delay as their primary outputs. Designers will often seek to provide junctions that operate below 90% capacity as measured by the ratio of flow to capacity (RFC). CAR BUS BICYCLE
- 55May 2019 (Version 1.1) Smarter Travel (2009) requires greater priority to be given to the movement of pedestrians in order to facilitate more sustainable travel patterns. This includes the reprioritisation of traffic signal timings (both new and existing) to favour pedestrians and cyclist instead of vehicles and to reduce pedestrian crossing distances9 (see Section 4.3.2 Pedestrian Crossings and 4.4.3 Junction Design). The creation of more compact junctions that minimise pedestrian and cyclist waiting times, will place additional pressures on junction performance. In areas where pedestrian activity is high (such as in Neighbourhoods and Centres) junctions may have to operate at saturation levels for short periods (i.e. above 93% during peak periods). Where junctions operate at or near saturation levels and they are frequented by bus services, priority measures should ensure services are not unduly delayed (see Section 3.4.3 Bus Services). Where longer periods of saturation occur, pedestrian cycle times may be extended. This should be done in preference to the implementation of staged/staggered crossings (see Section 4.3.2 Pedestrian Crossings). 9 Refer to Action 16 of Smarter Travel (2009). Figure 3.27: Highly segregated ‘cell and distributor’ networks channel faster moving traffic to large junctions where bottlenecks may occur (left). More permeable networks result in more frequent minor junctions with fewer vehicle movements (right) which calm traffic and are easier for pedestrians and cyclists to navigate.
- Design Manual for Urban Roads and Streets56 3.4.3 Bus Services Street networks underpin the efficiency and sustainability of public-transport and, consequently, the ability to facilitate higher development densities along public transport corridors in accordance with the objectives of Smarter Travel (2009). This includes an objective that all houses within urban areas are located within 800m of a bus route/ stop.10 Permeable networks which maximise connectivity will assist in achieving this objective. Smarter Travel (2009) also requires the implementation of bus priority measures,11 such as Quality Bus Corridors and Bus Lanes. These ensure that buses can move through congested networks with minimal delays. Designers must have regard to the location of bus services as a strategic network issue. In general: • Bus services should primarily be directed along Arterial and Link streets as these will be the most direct routes with between destinations with the greatest number of connections. • QBCs or Green Routes should be provided on streets which cater for higher frequency services12 over longer distances (see Figure 3.28). 10 Refer also to Action 13 of Smarter Travel (2009). 11 Refer to Action 12 of Smarter Travel (2009). 12 Refer to Bus Connects (NTA) for further information on service frequency. • On lower frequency routes, or in less congested networks, bus lanes that allow buses to move towards the front of queuing traffic at junctions may suffice. This approach may also be preferred on existing streets where the street reserve is constrained. Designers should consult with bus operators regarding the need for dedicated lanes. Under-used or unnecessary lanes can serve only to increase the width of carriageways (encouraging greater vehicle speeds) and consume space that could otherwise be dedicated to placemaking/traffic-calming measures such as planted verges, wider footpaths, cycle tracks or lanes and on-street parking. Designers should also consider the use of bus gates (see Figure 3.29) and selective bus detection technology that prioritise buses to improve journey times by restricting other motorised vehicles. These should be strategically placed throughout a network, and in particular within Centres, to filter permeability and ensure more rapid movement for buses. Figure 3.28: QBCs and Bus lanes should be considered on all Strategic Routes where the high frequency services occur or where their future need has been established. Figure 3.29: Example of a ‘bus gate’ in Tallaght, Co. Dublin, which filters permeability to allow for the free passage of buses whilst excluding other vehicles.
- 57May 2019 (Version 1.1) 3.4.4 Relief Roads The focus of this Manual is the creation of place-based/sustainable street networks, which balance pedestrian and vehicle movement. However, it is recognised that there are some roads which are required to cater for the efficient movement of larger volumes of motorised traffic at faster speeds over longer distances. These are generally referred as Inner Relief Roads and Urban Relief Roads.13 Inner Relief Roads are generally used to divert traffic within an urban area, away from a Centre or Node. The design of these routes needs careful consideration. Chapter 2 highlights the issues associated with the provision of higher speed/highly segregated routes though cities, towns and villages. Authorities in many urban areas have attempted to overcome issues of severance by vertically separating these routes into a series of tunnels, cuttings or elevated carriageways. Such solutions, however, tend to be reserved for major national projects and can have significant negative impacts on place (see Figure 3.30). It is more likely that Inner Relief Roads through urban areas will need to occur at moderate speeds (50 km/h). The route should be integrated within the urban fabric so that a sense of place is maintained and to prevent severance between adjoining areas. There are many examples in Ireland of streets that carry significant volumes of through traffic at moderate speeds and retain a high place value/levels of connectivity (see Figure 3.31). Successful solutions tend to be designed as boulevards with well planted medians and verges that provide a buffer between the heavily-trafficked carriageway and the surrounding pedestrian environment. Boulevards may also be designed as a ‘multiway’ boulevard with a central carriageway for through traffic and access carriageways at the side (see Section 3.4.5 Noise and Air Pollution). 13 See DN-GEO-03031 Rural Road Link Design (2017). Figure 3.31: Dorset Street, Dublin, an example of a street that carries large volumes of traffic and where recent improvements have ensured it maintains an important place function. Figure 3.30: Examples of major urban roads that move large volumes of traffic via vertical segregation. These require significant investment in infrastructure. As illustrated in the middle and bottom examples they can have negative impacts in terms of place and/or connectivity (image sources: Google Street View). Gran Via Les Corts Catalanes, Barcelona Dublin Port Tunnel M4 London
- Design Manual for Urban Roads and Streets58 Urban Relief Roads are generally routed around urban areas and are commonly referred to as By-Passes or Outer Ring Roads. Designers may use these routes to direct longer distance traffic, and in particular Heavy Goods Vehicles (HGVs), away from cities, towns and villages provided they are clearly separated from the urban fabric (see Figure 3.32). Urban development should not extend to the edge of these routes without full integration into the surrounding street network. This is a strategic issue that should be resolved via a County Development Plan/ Local Area Plan (see Figure 3.33) and may also require close consultation with TII, where the road is part of the national road network.14 In the case of a motorway or national grade separated dual carriageway the future integration of the road would not be an option. 14 Refer to Spatial Planning and National Roads: Guidelines for Planning Authorities (2012). Figure 3.32 Outer Relief Roads can be used to direct long distance traffic away from cities, towns and villages Figure 3.33: As urban expands toward an Urban Relief Road (top) a strategic decision will need to be made as to either maintain segregation and ‘leapfrog’ leaving a green belt (middle), or moderate speed, retrofit and integrate route (bottom). A B
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