Designing for 20mph
The purpose of this paper by Urban Design London is to summarise a range of traffic
calming options available to street designers, by pulling together ideas and examples
that have already been implemented in various parts of London and the UK. This
paper is meant to be used as an aid memoir and is not meant to replace any
literature published on this subject (DfT LTN 1/17, TRL report, etc). The paper will
also provide some guidance regarding ‘issues to consider’ for each category of
options listed.
Research carried out by TRL suggests that 20 mph zones are an effective way to
decrease the frequency and severity of road accidents, largely by reducing traffic
speeds. The Mayor has set targets to substantially improve road safety and a
number of 20mph zones are being rolled out beyond the typical residential setting, to
major roads across London.
20 mph speed limits without self-enforcing features can be relatively inexpensive to
implement. However, it has been suggested that where 85% of vehicles are
travelling above 24 mph on a street before interventions, a 20 mph speed limit on its
own would be insufficient without additional traffic calming measures provided (DfT
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Traffic Advisory Leaflet 9/99). After an extensive area-wide 20mph zone was
implemented in Portsmouth in 2007, it was found that average speed reduction on all
roads was only 1.3%, though the number of recorded casualties fell by 22% after the
implementation (compared to a 14% drop in the rest of the UK in comparable areas).
Traffic calming measures can also help improve the overall character and feel of an
area. This in turn can affect how pedestrians and cyclists use streets, therefore
helping make a safer and more pleasant environment for all users.
The paper is open to any comments, suggestions and examples on the options
listed.
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1. Carriageway widths - real and apparent
Introduction
Drivers may slow down when they feel the
space they are travelling in is narrow or its
width changes. This could be because they
feel less sure of the space available to them
and less confident of what they will encounter
ahead. This could be due to the fact that
pedestrians and other activity next to the
carriageway is closer, more visible and more
likely to encroach onto the carriageway or to
the fact that the driver has to negotiate the
road width with on coming traffic.
Advice given by TfL and others suggest that
carriageway widths should be below 3m or
over 4.5m as widths in between can be very
difficult for cyclists using the carriageway who
can be squeezed by passing traffic. As
widths over 4.5m may support faster driving
speeds, some of the ideas described below
can be very useful in meeting both speed
reduction and cycling integration aims.
1.1 Gateway narrowing
This can require cars to stop and give
way to oncoming vehicles, helping mark a
change in driving behaviour. Space
should be left for cyclists on the kerbside
and it should be ensured that they can
rejoin the traffic flow safely.
Example – Warrington (internet)
1.2 Pavement build-outs
Space is reallocated from carriageway to
pavement rebalancing the allocation of space
between buildings. New pavement areas are often
good spots for trees and planters as they are less
likely to have services beneath them.
Example – Camden High Street
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1.3 Central median strip
Carriageways are narrowed with the
removed space used to create a
central strip. Pedestrians standing on
the strip, and crossing the narrower
carriageways, may help drivers feel
that they should be driving slower. The
median strip is sometimes fully flush
or flat in places to allow overrun as
necessary and also to reduce
maintenance costs.
Example 1 – Oxford Street
1.4 Kerb/Gutter detailing
The carriageway next to the kerb, the
traditional gutter area, can be made
of different materials, or coloured in a
way that although flush and drivable,
appears as part of the pavement or
kerb so the carriageway looks
narrower than it actually is.
Example – Coventry City Centre
Issues to consider:
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Can support informal pedestrian crossing
Can channelise carriageways, giving less space for cyclists. It can also cause issues for
buses manoeuvres around stops, which could cause delay to bus time reliability.
Override areas can be provided for buses and loading vehicles (see Oxford Street)
Can offer smooth flow of traffic if lane number is kept consistent
Has a potential lack of resilience if a vehicle breaks down or there is an accident
Can provide additional space for lighting, trees, bike parking etc.
Kerb/gutter detailing (example 1.4) could provide more breathing space for cyclists and
large vehicles
Pavement build-outs can give more space for shops and pedestrians and less of a
vehicle dominance in the area
Gateway narrowing requires drivers to slow down in order to negotiate safety, but its
effectiveness along a street may be short
Can be problematic for parking and vehicle capacity
Lack of separated space for cyclists
Not suitable for distributor routes
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2. Reduced delineation between carriageway and pavement areas
Introduction
Over the last few years there has been a lot of interest in shared space schemes.
However most projects do not propose that all space is totally shared between
vehicles, pedestrians, cyclists and people sitting, socialising or playing in an area.
Designated areas for each use are still encouraged, but they encourage pedestrians
and cyclists to feel safer, by slowing traffic speeds. Pedestrians are encouraged to
negotiate carriageway area with vehicles in order to cross the carriageway. Shared
space schemes are generally trying to make drivers feel that they are sharing space
with more vulnerable users and modify their behaviour accordingly. The designs
therefore rely on there being a significant amount on non vehicle activity to achieve
this character.
2.1 Guardrail removal
Removing guardrails can make drivers feel
more connected with pavements and those
using them which may encourage slower
speeds. Un railed streets also feel less
dominated by highway kit, again making
drivers aware of other activities. An
assessment should be made first in order to
establish whether the guardrail can be
removed safely.
Example – Walworth Road
2.2 Carriageway Patterns and Materials
Example 1 - Exhibition Road
Example 2 – Mare Street, Hackney
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Changes in materials, colours or finishes can be used to disrupt the linear
appearance of a carriageway. These may be permanent or temporary. They can be
carried across both vehicle and pedestrian areas to encourage shared behaviour.
Consultation should be carried out to ensure the surface could be used safely by
disabled people.
2.3 Dominant Footways
Footway widening, decluttering, visual
improvements (including trees) and
visible pavement activities can change
the feel of a street, making it less vehicle
dominated. Therefore as well as
narrowing carriageways, footway
widening and encouraging pedestrian
activity can help enforce the impression
of what would be an appropriate slower
speed to travel at.
Example – Hornchurch (trees, planters and seating have been added to the pavement to emphasise
and encourage pedestrian use)
2.4 Level Surfaces
Kerb lines can be flush between
pavement and carriageway. This can
give a visual indication that people will
walk or cycle across or within the
carriageway. Care should be taken that
safe and easy to use routes are
available for those with sight
impairments.
Example - Venn Street
Issues to consider:
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Increases usable pavement width
Can dramatically change the appearance of the street and influence user behaviour
Removing guardrails can improve visual impact and reduce clutter
Allows for informal crossing and deliveries along pavement length
Level surface can be difficult for those with sight impairments
Level surface can be costly to lay and maintain, drainage needs careful consideration
Level surface can be most appropriate in streets with high ped and low traffic flows
Shared space can create a more adaptable space/surface to meet different demands
(see 4.0 Elastic Streets)
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3. Prominence and priority of non motor vehicles
Introduction
There are a variety of ways to indicate to road users that the priority lies with cyclists
and/or pedestrian users and that they therefore may have to reduce their speed
accordingly. Segregating or separating such users from vehicles may dilute their
influence on driver behaviour. Therefore when thinking about designing for sub
20mph behaviours integration may be most relevant as discussed below. However
we should keep in mind that when designing with cyclists in mind, we should
consider their needs fully and work to ensure they are not obstructed or put at risk.
3.1 Creating cycle lanes
Space reallocated to a highly visible cycle
lane can make the street feel less vehicle
dominated, as well as narrowing the
carriageway for vehicles. However the
way in which this is done may effect how
it influences driver behaviour. If hard
forms of segregation are used drivers
may feel able to go faster without
worrying about the cyclists.
Example 1 Royal College Street, Camden. Light segregation and floating parking has been used so
the whole carriageway, and its users, can be percieved as one
Example 2 – Baylis Road, Southwark;
the cycle lane is integrated into the
carriageway and will therefore alert
drivers that they need to be aware of
cyclists around them. However cycling
pushed to the side gives an apparent
clear run for vehicles and so may not be
an effective way of reducing speeds.
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3.2 Closing routes to vehicles
Preventing access and rat runs for vehicles
while keeping routes open for pedestrians
and cyclists can help reduce speeds on
side streets. However care is needed to
ensure drivers are not confused and end up
driving down more streets trying to find a
route through. Traffic flow studies and
temporary closures can be implemented to
check the effect the closure will have to
general traffic in the area.
Example – Hackney
3.3 Contraflow Cycling
Allowing cycling in both directions on
one way streets can slow driver
behaviour. These usually have an
‘except cycling’ sign under the no entry
signs and sometimes a cycle symbol on
the tarmac in the opposite direction to
vehicle use. They also sometimes have
separated entry space for the cycles.
Example – Holland Street, Kensington and
Chelsea
3.4 Integrating cycling
Integrating cyclists into narrower
carriageways can encourage all road
users to engage better with each other.
This can also help with keeping a
constant, but slower, traffic flow. This is
shown with a bicycle sign painted on
the carriageway. Care is needed at
turings though to ensure cyclists are
not invisible and ‘squeezed’ by tuening
vehicles.
Example – Hornchurch
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3.4 Integrating Pedestrians
Integrating pedestrians into new road
layout schemes can help encourage
more informal crossing and can
therefore help reduce speeds. This
can be achieved by providing larger
footway areas for pedestrians,
providing median strips and informal
crossing opportunities, as well as
more formal crossings. This can also
help improve the general character
and feel of an area/junction.
Example – Piccadilly Circus
Issues to consider:
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Visible presence and heightened importance given to cyclists when they appear
to be primary and important users of carriageway
When cycle lane is clearly separated from vehicle lane may not help to slow or
moderate vehicle behaviour
Junctions must be designed appropriately, taking cyclists and pedestrians into
consideration
Pedestrians should be incorporated into schemes and informal crossings should
be encouraged where possible
Consideration should be made for uses of the space/street during different times
of day/year
4. Elastic Streets
Introduction
A term used in the Roads Task Force, elastic streets are low cost, temporary
projects that change the character of a street. These changes, though temporary
or low cost, can often impact upon the way vehicles use the street. Depending
on whether the temporary changes are successful or not, they can then be
implemented permanently.
Cities around the world have implemented their own versions of Elastic streets,
with Paris offering a ‘beach’ along the river Seine every summer since 2002 and
New York promoting schemes such as ‘play streets’ and ‘Summer streets’ (For
three consecutive Saturdays every summer, nearly seven miles of New York
City’s streets are opened up for everyone to play, run, walk and bike).
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4.1 Street Furniture
Temporarily placing street
furniture on strategic parts of a
street can change its character
and use and therefore reduce
traffic speeds. Particular care
has to be taken to make sure it
does not negatively impact on
mobility and visually impaired
pedestrians.
Example – Flat Iron Square
4.2 Freestanding carriageway
planters
Freestanding carriageway planters
are a quick and cost effective way
to change the layout of the road
and also add some more
greenery/character to the street
(though they can be expensive to
maintain). This will also reduce
carriageway space for vehicles and
therefore encourage the reduction
of speed.
Example – Narrow Way, Hackney
4.3 Temporary change of use
This can give back part or the entire
carriageway to the community and
reduce or stop traffic running through a
street at certain times. The change of
use can be for markets on weekends
or bank holidays, for Play Street
schemes or other events. Vehicle
users are then likely to slow down,
when the street is open again, as there
may be more ongoing pedestrian use
of the street and drivers may associate
pedestrian activities with the street.
Example – Hackney Play Streets
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4.4 Market Stalls
The presence of market stalls on the side of
the street or the street itself can alert drivers
to reduce their speed significantly and take
care of the pedestrians, shoppers and stall
holders. Even in occasions when the market
is not in use, if there are road markings
showing the position for the market stalls,
this can still inform drivers of the nature of
the street.
Example – Whitecross Street Market
Issues to consider:
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Can create a close community feel and bring character/sense of place back to an
area
Can cause vehicular congestion at the times of use/closure
Measures are temporary and can therefore be removed if necesarry
Can be cheap to put in place
Free standing carriageway planters can be expensive to maintain
5. Vertical Deflections
Introduction
Vertical deflections are often used to alert drivers of a change in road conditions and
therefore assist with speed reduction. These are often used at junctions, crossings or
road entry points, as these are areas where accidents are more common. They are
also used along streets as cushions or humps to repeatedly slow vehicles. However
this type of deflection are not a favoured option and as mentioned in the Local
Implementation Plan 2014/2015 to 2016/2017 Delivery plan (p.24); ‘all other options
should be exhausted before consideration is given by boroughs to the potential use
of vertical deflections such as road humps and speed cushions.’.
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5.1 Raised Entry Treatments
The carriageway at a side road
is raised to pavement level
across its mouth. This can give
an indication to drivers of
change of condition and also
emphasise that pedestrians may
be crossing at that point.
Example – Brixton
5.2 Raised crossings
Raising the crossing to pavement
height at formal or informal crossings
can help alert vehicles to reduce their
speed, even when there are no
pedestrians waiting to cross.
Example - Surbiton
5.3 Speed Cushions
Speed cushions are placed in many
residential streets to reduce vehicle speeds.
These are not very popular and are little
used now. DfT guidance includes details of
hump proportions and H and S forms which
help buses go over them. They can go
across the whole width or sit alongside each
other across the street as shown here.
5.4 Virtual humps
Humps and other features can be painted
onto the road to psychologically deter
drivers from speeding, even though they
are actually flat.
Example – Philadelphia, USA.
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5.5 Raised Junctions
Raising a junction at the level of the
pavement. This can raise people’s
awareness of an impending change in
condition and movement from different
directions and should therefore reduce
speed.
Example – Camberwell
Issues to consider:
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Can give informal indication that vehicles should slow down
Visually highlights that different road conditions are coming up (pedestrians crossing/
different traffic movement)
Can be a problem for buses if not correctly constructed
Can give more importance for movement across main traffic route
May give pedestrians a false sense of security/entitlement
Raised entry treatment can widen area vehicles can run on, allowing some to take
wider, faster corners (use of upstanding features to contain corner radi helps with
this)
Can potentially be expensive to build and maintain, with ramps often failing in areas
of heavy use
Wide areas of flat kerb can be confusing for those with site impairments, and/or lead
to large areas of blister tactile
Speed cushions can be used over large areas, but drivers may speed between them
Speed cushions do not normally specifically relate to, or support pedestrian crossings
Speed cushions can lead to drivers having to take certain wheel tracks so less ability
to get out of the way in the case of an incident.
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6. Geometries, junctions and turns
Introduction
Tight corners signal free junctions and decisions on which routes should have
priority at junctions/roundabouts can make drivers more cautious and help
reduce speeds. Priority between vehicles, cyclists and pedestrians has to then
be negotiated between users.
6.1 Unconventional
geometries
Kerb lines and surfacing can
create unusual and diverting
shapes which may make drivers
feel they are within a shared
environment and help reduce
speeds.
Example - Coventry City Centre
6.2 Altering Large Gyratories or
Roundabouts
Altering the appearance and
priority at large roundabouts can
make drivers approach more
cautiously and reduce their
speeds. Vehicles, cyclists and
pedestrians have to negotiate
access which means that the
speed is reduced throughout the
area not just at specific points. It
also allows for slower, but more
consistent traffic flows.
Example – Poynton
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6.3 Turning Angles
Small corner radii (tighter turning
angles) make vehicles approach a
junction with more caution and to slow
down significantly to check it is safe to
proceed. Small corner radii also help
pedestrians cross at their desire lines.
Wider, rounded corners can cause
people with visual impairments
problems, as the rounded sections of
kerbing can disorientate them when it
comes to crossing the street.
Example – Elephant and Castle, South
Roundabout
Issues to consider:
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May alter the character of the area, due to the lower vehicle speeds
Can make cycling safer on gyratory, roundabouts and junctions
May confuse road users in the first instance
May have impacts on traffic capacity
Can be expensive to construct and maintain
Can help all road users (pedestrians and vehicles) interact better, to establish priority
and right of way
Visually appealing, as has fewer road markings and less clutter
Can be a radical option to plan and implement
7. Crossings
Introduction
There are a variety of different crossing types
that prioritise pedestrians and cyclists over
vehicle users. These are; zebra crossings
(pedestrians), pelican crossings (pedestrians),
puffin crossings (pedestrians) and toucan
crossings( pedestrians and cyclists), as well as
informal crossings. The positioning, number of
crossings on a road and width may have an
effect on vehicular speed.
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7.1 Zebra Crossings
Zebra crossings are not signalled and vehicles should stop as soon as a
pedestrian is waiting. Vehicles approaching Zebra crossings often approach with
caution and do slow down, in case they have to stop.
7.2 Pelican Crossings
Pelican is the older type of crossing and will slowly be replaced by the puffin
crossing. Pedestrians can push a button to cross (though if they move away, the
traffic light will still change). Crossing times are not variable, though even when
the light has gone red for pedestrians there is still time for pedestrians already on
the crossing, as the vehicle traffic light flashes amber for several seconds.
Vehicles approaching such crossings will not usually slow down, unless the light
changes.
7.3 Puffin Crossings
Puffin crossings are the latest pedestrian crossings controlled by signals. They
have movement detectors, so they can allow for variable crossing times for
pedestrians – depending on their needs. They can also detect if the pedestrian
has walked away, after pushing the button and cancel the action automatically.
7.4 Toucan Crossings
Toucan crossing signals operate in the same way that puffin crossings operate,
but are used by both pedestrians and cyclists. This is the only crossing currently
in use where cyclists are allowed to ride across on.
7.5 Informal Crossings
Informal crossings can be provided by implementing some of the tools mentioned
in this paper (unconventional road geometries, raised carriageway, central
median strip, shared surface materials etc). These measures then allow for
informal crossing points, as all road users are encouraged to interact and
vehicles will reduce their speed.
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Issues to consider:
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Vehicles have to prioritise pedestrians and cyclists
Vehicles tend to slow down on approaching crossings
The automated functions on Puffin and Toucan crossings can help reduce
user’s frustration
Pedestrians and cyclists may not use crossings, if these are not provided
on the desired line of crossing
Vehicular traffic can build up if the crossings are used too frequently
Pelican crossings do not have movement detectors and can therefore stop
traffic even when the pedestrian has moved away/crossed already
Informal crossings can make pedestrian and vehicle users interact more,
as longer stretches of road can be used to cross (and not just the
designated crossing areas)
Informal crossings can encourage of steady flow of slower moving traffic
Pedestrians, cyclists and vehicles all have to concentrate more, when at
informal crossings, in order to interact better and avoid any accidents
8. Signage and lines
Introduction
Minimal signage and road markings can often make vehicle users feel uneasy
and therefore help reduce their speed. Fewer lines and markings also makes a
space feel like it less designed for the vehicle and is therefore more balanced
between users.
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8.1 Minimalist streets
Signs and markings are usually put
in place to ensure safety. Their
removal would need to be done with
care and consideration for each
specific scheme and with exceptions
from the DfT. However, experience
has shown that these can be very
successful.
Example – New Road, Brighton
8.2 Interactive Signs
Variable Message Signs or other interactive signage
can inform drivers of upcoming change of conditions
or existing speed limits and ask them to reduce their
speed. Variable message signs are often found on
larger roads, but interactive signs are quite common
in 20mph and 30mph zones
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8.3 Informal Signs
Informal road signs (unofficial road
signs that are not regulated by the
Department for Transport) can inform
drivers to slow down, for example if
near a school. They can be prepared
by an active local community or local
schools, but as they are informal,
they may be ignored by drivers and
not have the desired effect.
Example – Devon
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8.4 Traffic Signals - Split Cycle
Offset Optimisation Technique
(SCOOT)
SCOOT is a traffic control system
used in urban areas that changes
traffic signal timings automatically,
based upon traffic demands at
each time. It works in such a way
as to manage traffic flow, stops,
delay and improve journey time
reliability. It can be used on busy
stretches of road to help reduce
speed but help with overall journey
times.
Example – Camden High Street
Issues to consider:
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Can help reduce speeds in approaching vehicles
SCOOT is automated and can change and adapt for each traffic situation
SCOOT may frustrate road users if they have to keep stopping at consecutive signals
SCOOT can be expensive to implement
Informal signs and naked streets can give a street more character and encourage
more interaction and eye contact between road users.
Informal signs can cause road clutter if not placed carefully and also may be ignored
by road users
Interactive signs can convey different messages at different times
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9. Parking and loading
Introduction
Allowing parking and loading on a street can increase drivers’ awareness that
vehicles will be pulling in and out of parking bays. This, in turn, can help reduce
vehicle speeds, though in some situations it can cause an unpleasant
environment for pedestrians and cyclists using the road.
9.1 Diagonal or Central Parking
Providing diagonal or central
parking on a road can decrease the
width of the carriageway and in turn
force road users to reduce their
speed.
Example – Oxford. A neighbourhood in
Oxford decided to implement a number of
traffic calming measures in order to slow
traffic. Parking cars diagonally reduces the
available carriageway to passing traffic.
9.2 Pavement Level Pads
Pavement level pads can be used
for loading. The surface treatment
is paved (though different
treatment to the pavement), but
the road markings make it clear
that the space can be used by
vehicles for loading, but also by
pedestrians when it is empty.
Example – Camden High Street
9.3 Parking on Outside of Cycle
Lane
Parking of the outside of the cycle
lane can help narrow the carriage
way, without putting cyclists at risk,
or taking away space from them,
though it does segregate users. A
buffer door opening space is
required between the parking and
the cycling space. Example - Brighton
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Issues to consider:
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Can help narrow the carriageway, though can also cause vehicle congestions in
the instance of an accident/ difficulty for emergency vehicles to get through
Can help local high streets recover and thrive, as more parking space may create
more business for them
Can cause a problem for cyclists, as vehicles enter and exit parking spaces
Pavement level pads allow for loading vehicles, without prioritising them over
pedestrians and give a consistent road width
Parking on outside of cycle lane ensures that provision for cyclists in not
compromised and makes for a more ‘human’ scale street
10. Character, appearance and use of area
Introduction
The character appearance and use of an area can often influence drivers’,
cyclists’ and pedestrians’ behaviour on the streets. It is therefore important to
design places and streets with an identifiable character, so that users can
respond to the space accordingly.
10.1 Building Frontages
The shape, position and upkeep of
buildings containing the street, their
facades, doors, windows signs and
architectural details can have a great
impact on the character of an area.
Depending on the scale and
condition of a building, a street may
seem more appropriate for slower or
faster vehicular speeds.
Example 1 – Bexleyheath – prominent
church at junction gives character and
highlights town centre
Example 2 – Chancery Lane – strong and
consistent building lines contain the street
and impose upon the driver.
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10.2 Use of Retail forecourts
A forecourt is a privately owned bit of
space outside a shop and can be
used for selling, seating,
entertainment, displaying of goods
etc. Such uses can also take place on
highway land under licence. Active
and well managed use of such areas
can help make a street feel lively and
can therefore give vehicle drivers a
sign to slow down. Example – Exmouth
Market
10.4 Street Art
Placing street art on parts of the public realm can draw people’s attention to it
and possibly help reduce the speed of vehicles.
Example 1 – South East London
Example 2 – Copenhagen
10.5 Lighting
Placing interesting lighting on
pavements, under bridges or by the
street may make a street more
identifiable and may encourage users
to slow down. This is mostly effective
at night, but a good lighting scheme
can also encourage better pedestrian
use, which can further help reduce the
speed of traffic.
Example - Southwark
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Issues to consider:
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Helping create/add character to a street or area can support a community feel
Can make pedestrians and cyclists feel more welcome and safer
Can help with the economic development of an area or high street
Retail forecourts and trees can take away footway space from pedestrians and
can also cause problems in terms of cleaning, maintenance, deliveries,
obstructions, lack of visibility etc
Street art and lighting can create a focus for pedestrian users, but can also
distract drivers’ attention away from the road
Street lighting may encourage drivers to go faster, of they feel they can see
better (depending on the scheme)
11. Enforcement
Introduction
Enforcement has no design input and can often require ongoing measures to stay
in place. Enforcement can catch drivers out, but it does not necessarily alter their
driving speed and behaviour outside of the enforcement areas, which are often
limited.
11.1 Speed Limits
Speed limit signs inform drivers what the maximum speed on that
street is. They can only be enforceable if the police (or
neighbourhood watch) is present.
11.2 Cameras
Speed cameras are a reliable way of enforcing speed limits. Standard speed
cameras can be effective on most streets, whereas average speed cameras are
most appropriate over long stretches of carriageway without junctions.
11.3 Neighbourhood Watch
Involving the local community and having neighbourhood watch in place in order
to enforce speed limits locally. This requires local support and resources, which
may not easily be available to local communities. It also means that a third party
is assisting police with enforcements, which may not be seen as an ideal option.
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Issues to consider:
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Makes drivers aware/reminds drivers what the speed limit is
The presence of a speed camera/ active neighbourhood watch will make most
drivers reduce their speed to the speed limit
Hard to enforce outside the speed camera / neighbourhood watch catchment
Speed cameras can be expensive to install and run
Drivers frequently ignore speed limit signs, especially if they cannot see the
reason to slow down
Neighbourhood watch schemes are hard to keep running with limited community
resources and also means that there is a third party assisting with police
enforcement of speed limits
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