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G4 Street Geometry

CHAPTER AIMS

• Advise how the requirements of different users can be accommodated in street design.
• Summarise research which shows that increased visibility encourages higher vehicle speeds.
• Describe how street space can be allocated based on pedestrian need, using swept path analysis to ensure that minimum access requirements for vehicles are met.
• Describe the rationale behind using shorter vehicle stopping distances to determine visibility requirements on links and at junctions.
• Recommend that the design of streets should determine vehicle speed.
• Recommend a maximum design speed of 20 mph for residential streets.

G4.1 INTRODUCTION

G4.1.1 Several issues need to be considered in order to satisfy the various user requirements detailed in Chapter G3, namely:

• street widths and components;
• junctions;
• features for controlling vehicle speeds;
• forward visibility on links; and
• visibility splays at junctions.

Figure G4.1 Illustrates what various carriageway widths can accommodate. They are not necessarily recommendations.

G4.2 STREET DIMENSIONS

G4.2.1 The design of new streets or the improvement of existing ones should take into account the functions of the street, and the type, density and character of the development.

G4.2.2 Carriageway widths should be appropriate for the particular context and uses of the street. Key factors to take into account include:

• the volume of vehicular traffic and pedestrian activity;
• the traffic composition;
• the demarcation, if any, between carriageway and footway (e.g. kerb, street furniture or trees and planting);
• whether parking is to take place in the carriageway and, if so, its distribution, arrangement, the frequency of occupation, and the likely level of parking enforcement(if any);
• the design speed (recommended to be 20 mph or less in residential areas); the curvature of the street (bends require greater width to accommodate the swept path of larger vehicles); and
• any intention to include one-way streets, or short stretches of single lane working in two-way streets.

G4.2.3 In lightly-trafficked streets, carriageways may be narrowed over short lengths to a single lane as a traffic-calming feature. In such sections the width between constraining features such as bollards should be no more than 3.5 m. In particular circumstances this may be reduced to a minimum value of 2.75 m, which will still allow for occasional large vehicles. However, widths between 2.75 m and 3.25 m should be avoided in most cases, since they could result in drivers trying to squeeze past cyclists. The local Fire Safety Officer should be consulted where a carriageway width of less than 3.7 m is proposed (see paragraph G3.7.3). Where access by larger vehicles is not permitted this could be reduced to 2.3m.

G4.2.4 Each street in the network is allocated a particular street character type, depending on where it sits within the place/movement hierarchy (see Chapter 2) and the requirements of its users (see Chapter G3). Individual streets can then be designed in detail using the relevant typical arrangement as a starting point. For example, one street might have a fairly high movement status combined with a medium place status, whilst another might have very little movement status but a high place status. The typical arrangement for each street character represented using a plan and cross-section as illustrated in Figure G4.1.

G4.2.5 These street types can be defined in a design code, as demonstrated in the case study box on the next page.

Figure G4.1 Typical representation of a street character type. This example shows the detail for minor side street junctions. Key plan (a) shows the locations, (b) is a cross-section and (c) the plan.

Figure G4.2 On street parking and shallow gradient junction table suitable for accommodating buses.

SWEPT PATH ANALYSIS

G4.2.6 Swept path analysis, or tracking, is used to determine the space required for various vehicles and is a key tool for designing carriageways for vehicular movement within the overall layout of the street. The potential layouts of buildings and spaces do not have to be dictated by carriageway alignment - they should generally be considered first, with the carriageway alignment being designed to fit within the remaining space (Fig. G4.3). Forward visibility should be checked immediately after or as part of the same process. This may result in adjustments to the house layout in some areas.

Figure G4.3 Left to right: (a) the buildings and urban edge of a street help to form the place; (b) the kerb line can be used to reinforce this; and (c) the remaining carriageway space is tracked for movement and for the provision of places where people may park their vehicles.

G4.2.7 The use of computer-aided design ( CAD) tracking models and similar techniques often proves to be beneficial in determining how the street will operate and how vehicles will move within it. Layouts designed using this approach enable buildings to be laid out to suit the character of the street, with footways and kerbs helping to define and emphasise spaces. Designers have the freedom to vary the space between kerbs or buildings. However, consideration should be given to the potential impact of on road parking. The kerb line does not need to follow the line of vehicle tracking if careful attention is given to the combination of sightlines, parking and pedestrian movements.

SHARED SURFACE STREETS AND SQUARES

G4.2.8 In traditional street layouts, footways and carriageways are separated by a kerb. In a street with a shared surface, this demarcation is absent and pedestrians and vehicles share the same surface. The key aims are to:

• encourage low vehicle speeds;
• create an environment in which pedestrians can walk, or stop and chat, without feeling intimidated by motor traffic;
• make it easier for people to move around; and
• promote social interaction.

Figure G4.4 A shared surface in a residential area

G4.2.9 In the absence of a formal carriageway, experience shows that motorists entering the area will tend to drive more cautiously and negotiate the right of way with pedestrians on a more conciliatory level (Fig. G4.4).

G4.2.10 However, shared surfaces can cause problems for some disabled people. People with cognitive difficulties may find the environment difficult to interpret. In addition, the absence of a conventional kerb poses problems for blind or partially-sighted people, who often rely on this feature to find their way around. It is therefore important that shared surface schemes include an alternative means for visually-impaired people to navigate by.

G4.2.11 Research published by the Guide Dogs for the Blind Association in September 2006 ¹ illustrated the problems that shared surfaces cause for blind or partially-sighted and other disabled people. Further research has been commissioned by the Department for Transport to address the issues raised, and this work is currently underway.

G4.2.12 Consultation with the community and users, particularly with disability groups and access officers, is essential when any shared surface scheme is developed. Early indications are that, in many instances, a protected space, with appropriate physical demarcation, will need to be provided, so that those pedestrians who may be unable to negotiate priority with vehicles can use the street safely and comfortably.

G4.2.13 When designing shared surface schemes, careful attention to detail is required to avoid other problems, such as:

• undifferentiated surfaces leading to poor parking behaviour;
• vulnerable road users feeling threatened by having no space protected from vehicles; and
• the positioning and quantity of planting, street furniture and other features creating visual clutter.

G4.2.14 Subject to making suitable provision for disabled people, shared surface streets are likely to work well:

• in short lengths, or where they form cul-de-sacs (Fig. G4.7);
• where the volume of motor traffic is below 100 vehicles per hour (vph) (peak); and
• where parking is controlled or it takes place in designated areas.

Figure G4.5 (a) and (b) A shared-surface square in Poundbury, Dorset
TO BE REPLACED BY SCOTTISH IMAGES

G4.2.15 Shared surface streets are often constructed from paviours or other materials rather than asphalt, which helps emphasise their difference from conventional streets. Research for MfS has shown that block paving reduces traffic speeds by between 2.5 mph and 4.5 mph, compared with speeds on asphalt surfaces (Fig. G4.6).

Figure G4.6 A shared surface scheme. Beaulieu Park, Chelmsford.
TO BE REPLACED BY SCOTTISH IMAGE

Figure G4.7 Home Zone, The Drum, Bo'ness (Phil Jones, Phil Jones Associates)

HOME ZONES

G4.2.16 Home Zones are residential areas designed with streets to be places for people, instead of just for motor traffic. By creating a high-quality street environment, Home Zones strike a better balance between the needs of the local community and drivers (Fig. G4.7). Involving the local community is the key to a successful scheme. Good and effective consultation with all sectors of the community, including young people, can help ensure that the design of individual Home Zones meets the needs of the local residents.

G4.2.17 Home Zones often include shared surfaces as part of the scheme design and in doing so they too can create difficulties for disabled people. Research commissioned by the Disabled Persons Transport Advisory Committee ( DPTAC) on the implications of Home Zones for disabled people, published in 2007, demonstrates those concerns. Design guidance relating to this research has been published. ²

G4.2.18 Home Zones are encouraged in both the planning and transport policies for new developments and existing streets. They are distinguished from other streets by having signed entry and exit points, which indicate the special nature of the street.

G4.2.19 The first Home Zones emerged in Scotland in 2000 when the Scottish Executive invited local authorities to put in bids to take part in a pilot study to evaluate the success of Home Zones. As a result the following four areas were selected as pilot home zones: Aberdeen (Tillydrone Area), Dundee (redevelopment of the Royal Infirmary site), Edinburgh (Caledonian Area), and Thurso, Ormlie (see case study page 106). A report evaluating the success of these home zones, and two additional home zones was published by the Scottish Executive in 2007 ³.

G4.2.20 Local authorities in Scotland were given the powers to create and designate roads as Home Zones in section 74 of the Transport (Scotland) Act 2001⁴. The legal procedure for creating a Home Zone in Scotland is set out in The Home Zones (Scotland) Regulations (2002)⁵ and guidance is provided in Home Zones Guidance Consultation ⁶.

CASE STUDY

ORMILE, THURSO

Ormlie is a small housing estate on the south western side of Thurso in the northern Highlands. It comprises of a mixture of one, two and three bedroom houses, served by walkways and residential roads, including a number of cul de sacs. The Home Zone was developed in a response to widespread concern over road safety and many aspects of the layout of the estate and the fabric of the housing. In particular there were major concerns about alleyways leading on to roads; increasing numbers of cars; and high walls that obscured sightlines.

Features of the Ormlie Home Zone

• tree and shrub planting in a formerly expansive open environment
• horizontal deflections provided by long radius curves along the streets
• vertical deflections provided by a raised table and raised pedestrian crossing point (fig G4.8 a)
• incorporation of bus and delivery vehicle access in the Home Zone design
• public art developed with involvement of the local community (fig G4.8 b)

Fig G4.8 (a) Raised speed table Lord Thurso Court, helping to reduce vehicle speeds and facilitate pedestrian movement (b) wavy wall created using local Caithness Stone aiding the sense of context and place identity (Scottish Executive⁷).

The Home Zones in Scotland Evaluation Report⁸ concluded that the Home Zone is making a difference to the way the community interacts, has increased the use of outdoor space and the sense of ownership, connection, care and enjoyment of their place.

Research on shared space streets

A study of public transport in London Borough Pedestrian Priority Areas ( PPAs) undertaken by TRL for the Bus Priority Team at Transport for London concluded that there is a self- limiting factor on pedestrians sharing space with motorists, of around 100 vph. Above this, pedestrians treat the general path taken by motor vehicles as a 'road' to be crossed rather than as a space to occupy. The speed of vehicles also had a strong influence on how pedestrians used the shared area. Although this research project concentrated on PPAs, it is reasonable to assume that these factors are relevant to other shared space schemes. This does not mean that shared surface schemes are unsafe above this level of traffic, but does indicate that at higher flows pedestrians are unlikely to share the space.

G4.2.21 Developers sometimes implement 'Home Zone style' schemes without formal designation. However, it is preferable for the proper steps to be followed, which where possible should involve the new community in deciding how the street will be used.

G4.2.22 In existing streets, it is essential that the design of the Home Zone involves significant participation by local residents and local access groups. In new-build situations, a partnership between the developer and the relevant authorities will enable prospective residents to be made aware of the proposed designation of the street as a Home Zone. This will pave the way for the formal consultation procedure once the street becomes adopted.

G4.2.23 Further guidance on the design of Home Zones is given in Home Zones; Challenging the Future of Our Streets⁹, the Institute of Highway Incorporated Engineers' ( IHIE) Home Zone Design Guidelines¹⁰ and at the website www.homezones.org.uk.

Figure G4.10 Quadrant kerbstones used instead of large radii at junctions reduce the dominance of the carriageway. This is reinforced by the placement and form of the adjacent buildings. However, note the lack of dropped kerbs and tactile paving ( WSP, Hopeman).

G4.3 JUNCTIONS

G4.3.1 Junctions that are commonly used in residential areas include:

• crossroads and staggered junctions;
• T and Y junctions; and
• roundabouts.

Fig. G4.9 Illustrative junction layouts

Figure G4.9 illustrates a broader range of junction geometries to show how these basic types can be developed to create distinctive places. Mini-roundabouts and shared surface squares can be incorporated within some of the depicted arrangements.

G4.3.2 Junctions are generally places of high accessibility and good natural surveillance. They are therefore ideal places for locating public buildings, shops and public transport stops, etc. Junctions are places of interaction among street users. Their design is therefore critical to achieving a proper balance between their place and movement functions.

G4.3.3 The basic junction forms should be determined at the masterplanning stage. At the street design stage, they will have to be considered in more detail in order to determine how they are going to work in practice. Masterplanning and detailed design will cover issues such as traffic priority arrangements, the need, or otherwise, for signs, markings and kerbs, and how property and building lines are related.

G4.3.4 The resulting spaces and townscape should ideally be represented in three dimensions - see G4.11.

G4.3.5 Often, the key to a well-designed junction is the way in which buildings are placed around it and how they enclose the space in which the junction sits. Building placement should therefore be decided upon first, with the junction then designed to suit the available space.

G4.3.6 Junction design should facilitate direct pedestrian desire lines, and this will often mean using small corner radii. The use of swept path analysis will ensure that the junctions are negotiable by vehicles (Fig. G4.10). However consideration should be given to the robustness of the design to withstand any occasional vehicle overrun.

Figure G4.11 Example of three-dimensional presentations.

G4.3.7 Junctions may be marked to indicate which arms have priority, but on quieter streets and at speeds of 20mph or less it would generally be acceptable to leave them unmarked. A lack of marked priority may encourage motorists to slow down to negotiate their way through, making the junction more comfortable for use by pedestrians. However, this approach requires careful consideration (see Chapter G7).

G4.3.8 Crossroads are convenient for pedestrians, as they minimise diversion from desire lines when crossing the street. They also make it easier to create permeable and legible street networks.

G4.3.9 Permeable layouts can also be achieved using T and Y junctions. Y junctions can increase flexibility in layout design.

G4.3.10 Staggered junctions can reduce vehicle conflict compared with crossroads, but may reduce directness for pedestrians. If it is necessary to maintain a view point or vista, and if there is sufficient room between buildings, staggered junctions can be provided within continuous building lines. (Fig. G4.12).

Figure G4.12 - Using staggered junctions to maintain a view point or vista.

G4.3.11 Where designers are concerned about potential user conflict, they may consider placing the junction on a speed table. Another option might be to close one of the arms to motor traffic (while leaving it open for pedestrians and cyclists).

G4.3.12 Conventional roundabouts are not generally appropriate for residential developments. Their capacity advantages are not usually relevant, they can have a negative impact on vulnerable road users, and they often do little for the street scene. They are also inefficient in terms of land-use.

G4.3.13 Larger roundabouts are inconvenient for pedestrians because they are deflected from their desire lines, and people waiting to cross one of the arms may not be able to anticipate easily the movement of motor vehicles on the roundabout, or entering or leaving it.

G4.3.14 Roundabouts can be hazardous for cyclists. Drivers entering at relatively high speed may not notice cyclists on the circulatory carriageway, and cyclists travelling past an arm are vulnerable to being hit by vehicles entering or leaving the junction.

G4.3.15 Mini-roundabouts share some of these disadvantages. However they may have some application in residential areas, as they cause less deviation for pedestrians and are easier for cyclists to use. In addition, they do not occupy as much land. Practitioners should refer to Mini-roundabouts: Good Practice Guidelines.¹¹

G4.3.16 Compact-style roundabouts (sometimes referred to as Continental Roundabouts) may also be suitable for consideration. They sit between conventional roundabouts and mini-roundabouts in terms of land take. They retain a conventional central island, but differ in other respects - there is minimal flare at entry and exit, and they have a single-lane circulatory carriageway. In addition, the circulatory carriageway has negative camber, so water drains away from the centre, which simplifies drainage arrangements. Their geometry is effective in reducing entry, circulatory and exit speeds. ¹² They are safer for cyclists because of the reduced speeds, together with the fact that drivers cannot overtake on the circulatory carriageway. Their use is described in TD 16 Geometric Design of Roundabouts. ¹³ An example of a compact (continental) - style roundabout can be found on the Ardler case study on in Section G9.

SPACING OF JUNCTIONS

G4.3.17 The spacing of junctions should be determined by the type and size of urban blocks appropriate for the development. Block size should be based on the need for permeability, and generally tends to become smaller as density and pedestrian activity increases.

G4.3.18 Smaller blocks create the need for more frequent junctions. This improves permeability for pedestrians and cyclists, and the impact of motor traffic is dispersed over a wider area. Research in the preparation of Manual for Streets¹⁴ demonstrated that more frequent (and hence less busy) junctions need not lead to higher numbers of accidents.

G4.3.19 Junctions do not always need to cater for all types of traffic. Some of the arms of a junction may be limited to pedestrian and cycle movement only.

Figure G4.13 This street avoids the use of vertical traffic-calming features, but the irregular alignment is unsightly and unlikely to have much speed-reducing effect, because of the width of the carriageway. It also results in irregular grassed areas that create a maintenance burden while contributing little to street quality.

Figure G4.14 Trees planted in the highway at Newhall, Harlow, help to reduce vehicle speeds ( EDAW).

G4.4 ACHIEVING APPROPRIATE TRAFFIC SPEEDS

G4.4.1 Conflict among various user groups can be minimised or avoided by reducing the speed and flow of motor vehicles. Ideally, designers should aim to create streets that control vehicle speeds naturally rather than having to rely on unsympathetic traffic-calming measures (Fig. G4.13). In general, providing a separate pedestrian and/or cycle route away from motor traffic should only be considered as a last resort (see the hierarchy of provision in Chapter G1).

G4.4.2 For residential streets, a maximum design speed of 20 mph should normally be an objective. The severity of injuries and the likelihood of death resulting from a collision at 20 mph are considerably less than can be expected at 30 mph. In addition, vehicle noise and the intimidation of pedestrians and cyclists are likely to be significantly lower.

G4.4.3 Evidence from traffic-calming schemes suggests that speed-controlling features are required at intervals of no more than 70 m in order to achieve speeds of 20 mph or less. ¹⁵ Straight and uninterrupted links should therefore be limited to a maximum of around 70 m to help ensure that the arrangement has a natural traffic-calming effect.

G4.4.4 A continuous link can be broken up by introducing features along it to slow traffic. The range of traffic-calming measures available act in different ways, with varying degrees of effectiveness:

• Street dimensions - can have a significant influence on speeds. Keeping lengths of street between junctions short is particularly effective. Street width also has an effect on speed (see box).
• Reduced visibility - research carried out in preparation of Manual for Streets found that reductions in forward visibility are associated with reduced driving speeds (see box).
• Changes in priority - at roundabouts and other junctions. This can be used to disrupt flow and therefore bring overall speeds down.
• Physical features - involving vertical or horizontal deflection can be very effective in reducing speed. It is preferable to use other means of controlling speeds, if practicable, but there will be situations (e.g retro-fit) where physical features represent the optimum solution. Additional sources of advice on traffic calming can be found in Traffic Advisory Leaflet 2/05. ¹⁶
• Psychology and perception - street features and human activity can have an influence on the speed at which people choose to drive. Research ¹⁷ suggests that features likely to be effective include the following:
  • edge markings that visually narrow the road - speed reduction is likely to be greatest where the edging is textured to appear unsuitable for driving on;
  • the close proximity of buildings to the road;
  • reduced carriageway width;
  • obstructions in the carriageway (Fig. G4.14);
  • features associated with potential activity in, or close to, the carriageway, such as pedestrian refuges;
  • on-street parking, particularly when the vehicles are parked in echelon formation or perpendicular to the carriageway;
  • the types of land use associated with greater numbers of people, for example shops; and
  • pedestrian activity.

Influence of geometry on speed

Research carried out in the preparation of MfS considered the influence of geometry on vehicle speed and casualties in 20 residential and mixed-use areas in the UK. Two highway geometric factors stand out as influencing driving speed, all other things being equal. They are:

• Forward visibility; and
• Carriageway width

Improved visibility and/or increased carriageway width were found to correlate with increased vehicle speeds. Increased width for a given visibility, or vice versa, were found to increase speed. These data are summarised in Fig. G4.15

The relationship between visibility, highway width and driver speed identified on links was also found to apply at junctions. A full description of the research findings is available in TRL report 661. ¹⁸

Figure G4.15 Correlation between visibility and carriageway width and vehicle speeds (a) average speeds and (b) 85th percentile speeds. These graphs can be used to give an indication of the speed at which traffic will travel for a given carriageway width/forward visibility combination.

G4.4.5 Speed limits for residential areas are normally 30 mph, but 20 mph limits are becoming more common. If the road is lit, a 30 mph limit is signed only where it begins - repeater signs are not used here. All other speed limits have to be signed where they start and be accompanied by repeater signs.

G4.4.6 A street with a 20 mph limit is not the same as a 20 mph zone. To create a 20 mph zone, it is a legal requirement that traffic-calming measures are installed to ensure that low speeds are maintained throughout. In such cases, the limit is signed only on entering the zone, and no repeater signs are necessary.

G4.4.7 Speed limits below 30 mph, other than 20 mph limits or 20 mph zones, require individual consent from the Scottish Ministers. Designers should note that such approval is unlikely to be given.

G4.4.8 A speed limit is not an indication of the appropriate speed to drive at. It is the responsibility of drivers to travel within the speed limit at a speed suited to the conditions. However, for new streets, or where existing streets are being modified, and the design speed is below the speed limit, it will be necessary to include measures that reduce traffic speeds accordingly.

G4.4.9 Difficulties may be encountered where a new development connects to an existing road. If the junction geometry cannot be made to conform to the requirements for prevailing traffic speeds, the installation of traffic-calming measures on the approach will allow the use of a lower design speed to be used for the new junction.

G4.5 STOPPING SIGHT DISTANCE

G4.5.1 The stopping sight distance ( SSD) is the distance within which drivers need to be able to see ahead and stop from a given speed. It is calculated from the speed of the vehicle, the time required for a driver to identify a hazard and then begin to brake (the perception-reaction time), and the vehicle's rate of deceleration. For new streets, the design speed is set by the designer. For existing streets, the 85th percentile wet-weather speed is used.

G4.5.2 The basic formula for calculating SSD (in metres) is:

SSD = vt + v ²/2d

where:
v = speed (m/s)
t = driver perception-reaction time (seconds)
d = deceleration (m/s ²)

G4.5.3 The desirable minimum SSDs used in the Design Manual for Roads and Bridges¹⁹ are based on a driver perception-reaction time of 2 seconds and a deceleration rate of 2.45 m/s ² (equivalent to 0.25g where g is acceleration due to gravity (9.81 m/s ² )).

G4.5.4 Drivers are normally able to stop much more quickly than this in response to an emergency. The stopping distances given in the Highway Code assume a driver reaction time of 0.67 seconds, and a deceleration rate of 6.57 m/s ² (0.67g).

G4.5.5 While it is not appropriate to design street geometry based on braking in an emergency, there is scope for using lower SSDs than those used in DMRB. This is based upon the following:

• a review of practice in other countries has shown that DMRB values are much more conservative than those used elsewhere; ²⁰
• research which shows that the 90th percentile reaction time for drivers confronted with a side-road hazard in a driving simulator is 0.9 seconds (see TRL Report 332²¹);
• carriageway surfaces are normally able to develop a skidding resistance of at least 0.45g in wet weather conditions. Deceleration rates of 0.25g (the previously assumed value) are more typically associated with snow-covered roads; and
• of the sites studied in the preparation of this manual, no relationship was found between SSDs and casualties, regardless of whether the sites complied with DMRB or not.

G4.5.6 The SSD values used in Designing Streets are based on a perception-reaction time of 1.5 seconds and a deceleration rate of 0.45g (4.41 m/s ²). Table G4.1 uses these values to show the effect of speed on SSD. These values are independent of traffic flow or type of highway. It is recommended that they are used on all routes with 85 ^th percentile wet weather speeds up to 60kph.

Table G4.1 Derived SSDs for streets (figures rounded).

G4.5.7 Below around 20 m, shorter SSDs themselves will not achieve low vehicle speeds: speed-reducing features will be needed. For higher speed roads, i.e. with an 85th percentile speed over 60 km/h, it may be appropriate to use longer SSDs, e.g. a driver-perception reaction time of 2 seconds and a deceleration rate of 0.45g unless the route in question is a trunk road, in which case the values given in DMRB should be applied.

G4.5.8 Gradients affect stopping distances. The deceleration rate of 0.45g used to calculate the figures in Table G4.1 is for a level road. A 10% gradient will increase (or decrease) the rate by around 0.1g.

G4.6 VISIBILITY REQUIREMENTS

G4.6.1 Visibility should be checked at junctions and along the street. Visibility is measured horizontally and vertically.

G4.6.2 Using plan views of proposed layouts, checks for visibility in the horizontal plane and ensures that views are not obscured by vertical obstructions.

G4.6.3 Checking visibility in the vertical plane is then carried out to ensure that views in the horizontal plane are not compromised by obstructions such as the crest of a hill, or a bridge at a dip in the road ahead. It also takes into account the variation in driver eye height and the height range of obstructions. Eye height is assumed to range from 1.05 m (for car drivers) to 2 m (for lorry drivers). Drivers need to be able to see obstructions 2 m high down to a point 600 mm above the carriageway. The latter dimension is used to ensure small children can be seen (Fig. G4.16).

G4.6.4 The SSD figure relates to the position of the driver. However, the distance between the driver and the front of the vehicle is typically up to 2.4 m, which is a significant proportion of shorter stopping distances. It is therefore recommended that an allowance is made by adding 2.4 m to the SSD.

Figure G4.16 Vertical visibility envelope.

G4.7 VISIBILITY SPLAYS AT JUNCTIONS

G4.7.1 The visibility splay at a junction ensures there is adequate inter-visibility between vehicles on the major and minor arms (Fig. G4.17).

G4.7.2 The distance back along the minor arm from which visibility is measured is known as the X distance. It is generally measured back from the 'give way' line (or an imaginary 'give way' line if no such markings are provided). This distance is normally measured along the centreline of the minor arm for simplicity, but in some circumstances (for example where there is a wide splitter island on the minor arm) it will be more appropriate to measure it from the actual position of the driver.

G4.7.3 The Y distance represents the distance that a driver who is about to exit from the minor arm can see to his left and right along the main alignment. For simplicity it is often measured along the nearside kerb line of the main arm, although vehicles will normally be travelling a distance from the kerb line and therefore a more realistic approach would be to measure to the nearside wheel track. The measurement is taken from the point where this line intersects the centreline of the minor arm (unless, as above, there is good reason to measure from the actual position of the driver).

G4.7.4 When the main alignment is curved and the minor arm joins on the outside of a bend, another check is necessary to make sure that an approaching vehicle on the main arm is visible over the whole of the Y distance. This is done by drawing an additional sight line which meets the kerb line at a tangent.

G4.7.5 Some circumstances make it unlikely that vehicles approaching from the left on the main arm will cross the centreline of the main arm - opposing flows may be physically segregated at that point, for example. If so, the visibility splay to the left can be measured to the centreline of the main arm.

Figure G4.17 Measurement of junction visibility splays (a) on a straight road, (b) and (c) on bends.

X DISTANCE

G4.7.6 An X distance of 2.4 m should normally be used in most built-up situations, as this represents a reasonable maximum distance between the front of the car and the driver's eye.

G4.7.7 A minimum figure of 2 m may be considered in some very lightly-trafficked and slow-speed situations, but using this value will mean that the front of some vehicles will protrude slightly into the running carriageway of the major arm. The ability of drivers and cyclists to see this overhang from a reasonable distance, and to manoeuvre around it without undue difficulty, should be considered.

G4.7.8 Using an X distance in excess of 2.4 m is not generally required in built-up areas.

G4.7.9 Longer X distances enable drivers to look for gaps as they approach the junction. This increases junction capacity for the minor arm, and so may be justified in some circumstances, but it also increases the possibility that drivers on the minor approach will fail to take account of other road users, particularly pedestrians and cyclists. Longer X distances may also result in more shunt accidents on the minor arm. TRL Report No. 184²² found that accident risk increased with greater minor-road sight distance.

Y DISTANCE

G4.7.10 The Y distance should be based on values for SSD (Table G4.1).

G4.8 FORWARD VISIBILITY

G4.8.1 Forward visibility is the distance a driver needs to see ahead to stop safely for obstructions in the road. The minimum forward visibility required is equal to the minimum SSD. It is checked by measuring between points on a curve along the centreline of the inner traffic lane (see Fig. G4.19). Consideration should be given to vertical geometry and any other obstructions.

G4.8.2 There will be situations where it is desirable to reduce forward visibility in conjunction with other methods to control traffic speeds - the influence of geometry on speed box describes how forward visibility influences speed. An example is shown in Fig G4.18.

Figure G4.18 Limiting forward visibility helps keep speeds down in Poundbury, Dorset.
TO BE REPLACED WITH SCOTTISH IMAGE

Visibility along the street edge

G4.8.3 Vehicle exits at the back edge of the footway mean that emerging drivers will have to take account of people on the footway. The absence of wide visibility splays at private driveways will encourage drivers to emerge more cautiously. Consideration should be given to whether this will be appropriate, taking into account the following:

• the frequency of vehicle movements;
• the amount of pedestrian activity; and
• the width of the footway.

Figure G4.19 Measurement of forward visibility

G4.8.4 When it is judged that footway visibility splays are to be provided, consideration should be given to the best means of achieving this in a manner sympathetic to the visual appearance of the street (Fig. G4.20). This may include:

• the use of boundary railings rather than walls (Fig. G4.21); and
• the omission of boundary walls or fences at the exit location.

OBSTACLES TO VISIBILITY

G4.8.5 Parking in visibility splays in built-up areas is quite common, yet it does not appear to create significant problems in practice. Ideally, defined parking bays should be provided outside the visibility splay. However, in some circumstances, where speeds are low, some encroachment may be acceptable.

G4.8.6 The impact of other obstacles, such as street trees and street lighting columns, should be assessed in terms of their impact on the overall envelope of visibility. In general, occasional obstacles to visibility that are not large enough to fully obscure a whole vehicle or a pedestrian, including a child or wheelchair user, will not have a significant impact on road safety.

Figure G4.20 Beaulieu Park, Chelmsford - low vegetation provides subtle provision of visibility at private driveway.
TO BE REPLACED WITH SCOTTISH IMAGE

Figure G4.21 Beaulieu Park, Chelmsford: the visibility splays are provided by railings rather than boundary walls, although the railings could have followed the property boundary.
TO BE REPLACED WITH SCOTTISH IMAGE

G4.9 FRONTAGE ACCESS

G4.9.1 One of the key differences between streets and roads is that streets normally provide direct access to buildings and public spaces. This helps to generate activity and a positive relationship between the street and its surroundings. Providing direct access to buildings is also efficient in land-use terms.

G4.9.2 The provision of frontage vehicle access onto a street should be considered from the viewpoint of the people passing along the street, as well as those requiring access (Fig. G4.22). Factors to consider include:

• the speed and volume of traffic on the street;
• the presence of gathered accesses - a single access point can serve a number of properties or a communal parking area, for example. This may be acceptable where a series of individual accesses would not be; and
• the distance between the property boundary and the carriageway - to provide adequate visibility for the emerging driver.

Figure G4.22 Frontage access for individual dwellings onto a main street into Edinburgh.

G4.9.3 In the past, a relatively low limit on traffic flow (300 vehicles per peak hour or some 3,000 vehicles per day) has generally been used when deciding whether direct access was appropriate. This is equivalent to the traffic generated by around 400 houses. Above this level, many local-authority residential road guidelines required the provision of a 'local distributor road'.

G4.9.4 Such roads are usually very unsuccessful in terms of placemaking and providing for pedestrians and cyclists. In many cases, buildings turn their backs onto local distributors, creating dead frontages and sterile environments. Separate service roads are another possible design response, but these are wasteful of land and reduce visual enclosure and quality.

G4.9.5 It is recommended that the limit for providing direct access on roads with a 30 mph speed restriction is raised to at least 10,000 vehicles per day (see box).

Traffic flow and road safety for streets with direct frontage access

The relationship between traffic flow and road safety for streets with direct frontage access was researched for MfS. Data on recorded accidents and traffic flow for a total of 20 sites were obtained. All of the sites were similar in terms of land use (continuous houses with driveways), speed limit (30 mph) and geometry (single-carriageway roads with limited side road junctions). Traffic flows at the sites varied from some 600 vehicles per day to some 23,000 vehicles per day, with an average traffic flow of some 4,000 vehicles per day.

It was found that very few accidents occurred involving vehicles turning into and out of driveways, even on heavily-trafficked roads.

Links with direct frontage access can be designed for significantly higher traffic flows than have been used in the past, and there is good evidence to raise this figure to 10,000 vehicles per day. It could be increased further, and it is suggested that local authorities review their standards with reference to their own traffic flows and personal injury accident records. The research indicated that a link carrying this volume of traffic, with characteristics similar to those studied, would experience around one driveway-related accident every five years per kilometre. Fewer accidents would be expected on links where the speed of traffic is limited to 20 mph or less, which should be the aim in residential areas.

G4.10 TURNING AREAS

G4.10.1 Connected street networks will generally eliminate the need for drivers to make three-point turns.

G4.10.2 Where it is necessary to provide for three-point turns (e.g. in a cul-de-sac), a tracking assessment should be made to indicate the types of vehicles that may be making this manoeuvre and how they can be accommodated. The turning space provided should relate to its environment, not specifically to vehicle movement (see Fig. G4.23), as this can result in a space with no use other than for turning vehicles. To be effective and usable, the turning head must be kept clear of parked vehicles. Therefore it is essential that adequate parking is provided for residents in suitable locations.

Figure G4.23 Different turning spaces and unusable turning heads

G4.10.3 Routeing for waste vehicles should be determined at the concept masterplan or scheme design stage (see paragraph G3.8.4). Wherever possible, routing should be configured so that the refuse collection can be made without the need for the vehicle having to reverse, as turning heads may be obstructed by parked vehicles and reversing refuse vehicles create a risk to other street users.

G4.11 OVERRUN AREAS

G4.11.1 Overrun areas are used at bends and junctions (including roundabouts). They are areas of carriageway with a surface texture and/or appearance intended to deter overrunning by cars and other light vehicles. Their purpose is to allow the passage of large vehicles, such as buses and refuse vehicles, while maintaining 'tight' carriageway dimensions that deter smaller vehicles from speeding.

G4.11.2 Overrun areas should generally be avoided in residential and mixed-use streets. They can:

• be visually intrusive;
• interfere with pedestrian desire lines (Fig. G4.24); and
• pose a hazard for cyclists.

However, they can help to overcome problems with access for larger vehicles and so may represent the best solution in some circumstances.

Figure G4.24 The overrun area at this junction is hazardous for pedestrians and/or requires them to divert from their desire line. Notice also the unsightly placing of inspection covers. The layout is particularly hazardous for blind and partially-sighted pedestrians.

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