Forward Visibility Splays
Forward Visibility is the distance along the street ahead which a driver of a vehicle can see. This determines if a driver is able to react and stop in time should an object enter its path (referred to as the Stopping Sight Distance). Prior to the publication of DMURS, SSDs previously used in Ireland were based on those contained within UK documents such as the NRA Design Manual for Roads and Bridges, Design Bulletin 32 (DB32) and Places Streets and Movements (PSM)1998.
In 2003 research commissioned by the UK Department for Transport established that there were widespread problems associated within the implementation of the geometric standards contained within DB321. Research found that the SSD standards contained within DB32 were overly conservative as:
Driver reaction times and deceleration rates used to derive the SSD standards far exceeded those contained within the Highway Code.2
The SSD standards did not take into account actual road design details.
Research was conducted across twenty sites within the UK with varying road conditions. Ten of these sites were located in established areas and ten were located in new build areas.
The research included a survey of speed, road geometry and visibility. A minimum of ten junctions were studied within each of the sites. With regard to forward visibility it was found that:
The average speed of vehicles decreased with more limited forward visibility.
The approach speed of vehicles toward junctions was generally lower than on the links between them.
Based on this evidence, the Manual for Streets, 2007 contains revised SSD standards for streets with a design speed of between 16 and 60kph. These standards have been adopted for use within DMURS. This is fully detailed in the Evidence and Research volume which occupancies the MfS. The Highway Code sets out the rules and regulations for road users in England, Scotland and Wales.
Further details in details in regard to Forward Visibility can be found in Section 4.4.4 of DMURS (and on Visibility Splays in Section 4.4.5) and on this website within under the Supplementary Details link.
Risk and Liability (the UK experience)
The issue of liability was examined in detail in the UK with regard to the application of more innovative standards that introduce elements of risk to calm traffic, such as those contained in the Manual for Streets (2007) and Manual for Streets 2 (2010).
It was found that the fears regarding liability were unfounded and grossly exaggerated (Manual for Streets 2, Designing Streets)
There have been very few cases in the UK relating to alleged defects in design. Most claims against highway authorities relate to alleged deficiencies in maintenance.
Despite clear evidence to the contrary, the fear of being sued over street design still exists in the UK. The Better Regulation Taskforce attributes the fear of being sued to:
A perceived compensation culture perpetuated through a combination of media coverage (sometimes inaccurate)
Claims management companies
People’s desire to find someone to blame for their accident
The constant repetition of the liability claims by media/public commentators.
It should also be noted that local authorities in Ireland are liable for the maintenance of roads, however this liability is less stringent in Ireland than in the UK, particularly with regard to maintenance.
Designers may also be mistaken if the view generally held that strict adherence to standards will always offer them protection.
The focus of the courts may be on the design process and the rationale for the decisions that were made.
In such cases designers will need to be able to demonstrate that all aspects of the design were fully analysed and that standards applied were considered with regard to suitability for purpose.
(Curve Radii, Gradient and Superelevation)
Changes in the direction of streets may disorientate pedestrians and increase walking distances between destinations.
The scope for changes in the alignment of Strategic Routes is limited, as these routes will generally need to be straight in order to efficiently link destinations.
There is scope to use changes in horizontal alignment on Local streets to calm traffic and promote place (where curvilinear or more organic grid networks are proposed) to do so changes in horizontal alignment should be combined with contextual measures and reductions in forward visibility.
The Manual for Streets 2 contains a set of standards for curve radii that are four steps below the desirable minimum contained with the UK DMRB. These standards have been adopted for use in DMUS.
A maximum of 5% is desirable where pedestrian are active. In hilly terrain, steeper gradients may be required but regard must be had to the maximum gradient for wheelchair users of 8.3%.
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 many routes that can be taken between destinations and may in fact have place making benefits.
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, is designed to assist drivers to maintain higher speeds around curves
Its use is inappropriate in urban areas where the design is intended to achieve a moderate or low speed environment. It is also difficult to implement in urban areas with frequent junctions and points of access.
ROAD SAFETY AUDITS
Road Safety Audits (RSA) are designed to identify potential hazards and how they could affect road users. Within Ireland it is mandatory to carry out a RSA on any permanent change to the road layout on National Roads. The standard is commended to roads authorities for use in preparation of their own road schemes on Regional or Local roads’ and it is common practice for local authorities to require an RSA for all road schemes. Circular RLR 16/2008, Road Safety Audits and Road User Audits issued by the Department of Transport also required that roads authorities carry out such audits on schemes funded or co-funded by the Department.
The RSA guidelines are primarily designed for use on National Road schemes. As such, they are primarily designed for use on large high capacity trunk roads that carry fast moving trafﬁc. Concerns are raised as if the same set of values that are applied on highly segregated (i.e higher speed) or isolated roads (i.e rural roads) were to be applied on an urban streets, the recommendation of the RSA could lead to solutions that detract from place, reduce levels of pedestrian amenity and may, in some cases, actually reduce safety levels, as “where the appearance is one of safety, individuals may drop their guard and accidents ensue”1.
For example, the core questions of the RSA are:
Does the design layout create confusion or ambiguity for road users that could lead to potential road trafﬁ c accidents?
Is there too much, or too little information for road users?
Is there too little, or too much visibility, or an obstruction to road users’ view?
Does the layout create hazards or obstacles to road users that could contribute to an increased risk of injuries?
The answers to these questions will be very different in on an urban street compared to a segregated or isolated road.
In many cases the RSA guidelines may direct designers toward solutions that are not compatible with typical urban conditions. For example, Section 2.3.3 of the RSA guidelines state.
Links that are well designed with few private access points traditionally have a good safety record.
There is a direct correlation between the number of access points between links and the accident numbers on any given road.
The majority of accidents take place at junctions, it is essential that junction spacing is maximised and consistent junction types are used.
Research undertaken for the Manual for Streets, 2007 (MfS)demonstrates that this approach is counterproductive in urban areas. MfS states:
There is no signiﬁcant difference in collision risk attributable to more permeable street layouts (Section 4.3.2)
Streets with direct frontage access to dwellings can operate safely with signiﬁcant levels of trafﬁc (section 5.6.2)
More frequent (and hence less busy) junctions need not lead to higher numbers of accidents.
To reduce the possibility of conﬂict with DMURS, Chapter 5 of the Manual states that the audit team responsible for carrying out a RSA:
Must take full cognisance of the principles, approaches and standards contained within DMURS.
Should not recommend any actions that will reduce ease of movement for pedestrians/cyclists in favour of motor vehicles or seek to add or remove measures that may result in the operating speed exceeding the intended design speed.
Should promote the creation of a self-regulating street environment.
Should have a clear understanding of the objectives of the design. The audit team should refer to the Road Safety Audit Brief Checklist.2
Within urban areas any RSA should include a process of Risk Assessment. A process for risk management is included in Chapter 5. of DMURS. This is adopted from the third edition of the UK IHT Road Safety Audit Guidelines, which included a Risk Assessment process as a response to the publication of the Manual for Streets. It is noted that an Auditor should:
“not assume that behaviour on roads will necessarily be displayed on streets”
“the emphasis within Audit should be on trying to assess what types of collisions may occur”
Shared surfaces are an effective way of promoting Place and expanding the pedestrian domain, whilst still allowing vehicle access. Where the main carriageway is shared conventual street design elements such as blacktop surfaces, kerbs and extensive line marking should not be used, as these are associated with segregated, or vehicle dominated spaces. A ‘place based’ or urban design led approach should be undertaken that uses a.variety of landscape treatments to calm trafﬁc and prioritise more vulnerable user
Designers may have safety concerns with regard to vehicles, cyclists and pedestrians (in particular the visually impaired) sharing the main carriageway. Research within the UK has found that these is no evidence of any increase in casualties associated with such schemes. On the contrary, there is evidence to suggest that well designed schemes in appropriate settings can bring beneﬁts in terms of visual amenity, economic performance and perceptions of personal safety.
The Institute of Highway Engineers Home Zone Design Guidelines recommended that vehicle movement on shared surface streets should be less than 100 movements per day for homezones2 (see right). Recently developed schemes in the UK have however challenged this, with more varied forms of shared surfaces that have trafﬁc movements well in excess of this threshold (such as the Ashford Ring Road
(see right) and Exhibition Road (see over).
The key condition is designers need to be conﬁdent that vehicular speeds can be kept below 20kph through the use of effective self-enforcing measures.
Shared surfaces are most effective when used in the city, town and village centres, where pedestrian activities are high and vehicle movements are only required for lower level access or minor circulatory purposes. Shared carriageways are also effective in the residential neighbourhoods on local streets. in such circumstances shared surface carriageways can be used extensively, particularly if a design speed of 20 km/h or less is desirable.
1 Refer to also Report to the UK Department of Transport: Stage 1 Appraisal of Shared Space (2009).
2 Refer also to Chapter 3 of the UK The Institute of Highway Engineers Home Zone Design Guidelines (2002)
Removal of Staggered Crossings
Removal of Staggered Crossings/Slip Lanes
Staggered crossings and left turning slips are common place in the urban environment. These require pedestrians to navigate several individual crossings when crossing a street. Staggered crossings are generally applied to enable additional vehicle moments through a junction (or across a crossing point) to reduce delays for vehicles. Such crossing however can signiﬁcantly delay journey times for pedestrians, thus discouraging journeys by more sustainable modes of transport - see also right. Left turning slips also result in larger corner radii, enable vehicles to turn at higher speeds. As such should an accident occur, the potential for serious injury is greatly increased.
DMURS seeks to signiﬁcantly restrict the use of staggered crossings and left turning slips in urban areas.
DMURS acknowledges that designers may have concerns regarding the omission of staggered crossings on wide streets (i.e. with four or more lanes and a median) on the grounds of safety. DMURS notes that these concerns can be overcome by:
Ensuring enough green time is provided for pedestrians to cross in a single movement1.
The use of pufﬁn crossings. Pelican crossings that include ﬂashing amber lights should not be used as vehicles may move forward not realising pedestrians are still on the median or far side of the crossing.2
Providing a wide refuge island (minimum of 2m) for those who are very slow to cross and therefore do not make it all the way across in a reasonable time.
Safety concerns regarding pedestrian crossings also needs to be viewed in the context of pedestrian behaviour. Signiﬁcant numbers of pedestrians fail to comply with the detour /delay created by staggered crossings and it will generally be more desirable from a safety point of view to provide a direct single-phase crossing (see right).