This article written by Wes Edwards was first published in the September 2009 issue of URBAN magazine
Great residential streets:
why don’t we have more of them?
As professionals involved in urban design part of our role is to make great streets, but what makes a street great and how can we ensure that residential streets are places that are good to live in? In this article I’ll explore the concepts around suburban residential streets.
I’m sure that if we all made a list of our favourite residential streets there would be a huge variety of streets nominated and for various and perhaps conflicting reasons as there are a range of elements that help to make a street great. Some are tangible physical attributes such as the width of the street, the speed of the traffic, or the number of trees. Others are more subjective, relating to character and other less measurable concepts.
Within the variety of attributes there are probably some that most of us would agree on, and more importantly have some ability to manage, control or influence, particularly at the planning or design stage. Most would probably say that a liveable residential street is one with low traffic volume and low vehicle speed. A good residential area needs to have sufficient parking in some form or other. A liveable residential street is friendly to pedestrians and cyclists, and a good street is also a place with character – a nice place to be not just a movement thoroughfare. How do we as planners and designers build these things into our streets?
One of the easiest ways to classify streets is on the basis of volume – busy streets and quiet streets. All other things being equal I expect most people in suburbia would prefer to live on a quiet street with low traffic volumes. High traffic volumes are associated with noise, and we tend to have less community interaction on a busy street than on a quiet one due to the difficulties in crossing the road. Walking and cycling is less enjoyable on a busy road.
When designing residential areas it would be a good goal to maximise the number of quiet streets as much as possible. Clearly not all streets can have low traffic volumes, but by careful attention to the pattern of streets we can distribute and concentrate traffic to work towards this goal. A common solution to this problem has been the creation of cul de sacs, but as anyone familiar with new urbanism principles knows cul de sacs have fallen out of favour primarily due to their low connectivity deterring walking and cycling.
It would also be inappropriate to place a street along a route that would attract too much traffic – a “rat-run” route that provides a short-cut between two busy roads needs to be designed to handle the higher volume of traffic or avoided all together in favour of streets that don’t provide for a shortcut.
As traffic volume increases the number of people in a hurry to go somewhere tends to increase and that brings us to the issue of speed.
As we all know high vehicle speeds don’t provide the best residential environments. Walking and cycling are more attractive when vehicles are moving more slowly, and should a crash occur the severity of the crash is substantially reduced when vehicles are travelling slowly. The chances of a pedestrian surviving a collision with a vehicle increase from 60% at 50km/hr to 95% at 30 km/hr. Keeping traffic speed to 30 km/hr or below on residential streets is therefore a worthy goal.
The problem of managing vehicle speed on streets has generated a wide variety of potential solutions. In some cases, such as addressing speed problems on existing streets, the available engineering and design options are limited to devices such as speed humps, but with new streets there are more effective options available. In every case activating the street with more pedestrian and other activity will help to reduce speed and moderate driver behaviour.
For new streets there are three key principles that provide lower speeds. Each one can influence speed but they work best in combination.
Road Network Pattern
The first principle is the pattern of street routing and linking. Producing slow speeds will always be more difficult on a busier street that acts as a short-cut than on a street that only provides access to the local area. As this aspect affects both volume and speed careful consideration of the form of the street network should be the first item addressed wherever possible.
In many cases the street pattern is constrained by the shape of the land and the opportunities to connect to other streets. Street location can also be strongly influenced by the stormwater and open-space network.
Street Leg Length
The second principle is to limit the “leg length” of streets. Vehicle speeds rise on streets with long uninterrupted straight or broadly curving sections. Limiting the length of these sections is one of the most effective ways of producing slower speeds. One way of doing this is to limit the overall length of streets. This can produce smaller blocks that increase the permeability in an area, but if not done well the legibility of an area – the ability to find your way without a map – can suffer. Other techniques such as introducing roundabouts or small squares can discretely interrupt the flow of traffic without sacrificing legibility.
Designing streets with appropriate leg-lengths is a task that needs to be done respecting a wide range of factors including the shape and size of the land; the resulting shape, size, and number of residential lots; permeability, connectivity and legibility; and intersection layout.
A range of studies show that cross-road intersections are less safe than T-intersections, particularly as traffic volumes increase. Some international design guidelines therefore prohibit cross-roads in residential areas and require that all streets meet at T-intersections. The legibility of an area and the connections through it can suffer if this is not well executed by creating continuous pedestrian routes even if vehicles have to negotiate a more circuitous route. In my experience crossroads can sometimes be appropriate in lower-order residential streets given the right circumstances and careful design. In other cases, particularly on busier roads where no alternatives exist, some form of intersection control such as a roundabout or traffic signals may be required to provide a suitable level of road safety.
Another aspect requiring some attention is limited driver tolerance for long periods of driving through slow-speed environments. Some design guidelines suggest that drivers should not be subjected to more than 200m of the slowest narrowest streets before they are able to access a wider more easily navigated route. This length conveniently matches the sort of maximum block length some guidelines suggest in order to provide sufficient permeability to promote walking and cycling.
There is good evidence to support the position that narrower street carriageways produce lower vehicle speeds. One element of this is that vehicles travelling down a narrower street have less lateral clearance to objects such as the kerb, trees, or parked cars and drivers slow down as a result. On the narrowest streets cars may sometimes need to slow down or stop to pass an oncoming vehicle.
As density of development increases lot sizes become smaller, but each lot still requires some form of road access. As a result the amount of road in an area tends to increase and can occupy a sizeable proportion of the land area. Wider than necessary road reserves can be very wasteful of land and this is important when trying to reduce sprawl. The width of the carriageway is only one element making up the width of a road reserve, but a key one.
Street carriageways can be too narrow if traffic volumes are too high or parking demand is high and not well provided for, and in some cases service or emergency vehicles may not be able to navigate a street choked with parked cars. The answer is not always to make the street carriageway wider.
Getting the width of the street right to adequately provide for the needs of all users is a multi-faceted task which would require a whole series of articles to address properly, and there is no one-size fits-all solution. Street width should vary according to traditional traffic engineering measures such as vehicle volume and speed, but also according to parking demand, housing density and lot size, housing type, front-yard setbacks and private landscaping requirements, pedestrian volumes and other matters.
If the street is likely to be on a bus route a wider carriageway may be required regardless of the class of the street.
In some cases stormwater features such as swales or rain gardens may have a strong influence on street design. In higher-density areas a swale on one side of a street will require a large number of small bridges to provide for property access unless alternate access via rear lanes is provided. Rear lanes can require significant width to provide suitable manoeuvring in and out of garages ad as a result this approach can use up a sizeable proportion of land for access, effectively doubling the number of “streets”. Rear lanes are also challenging to design to provide good outcomes with respect to crime-prevention, aesthetics, and traffic management, but some good examples exist.
Swales can also be located in a central median, but that can lead to other issues. A street with a central median increases the amount of road reserve by more than just the median width. In order to accommodate larger vehicles such as trucks, buses and fire appliances the total width of the two single-direction carriageways often ends up wider than a two-way carriageway to provide sufficient clearances, particularly when passing a cyclist. Manoeuvring in and out of parking spaces and driveways may also require a wider carriageway.
On standard low-volume streets it is often acceptable for larger trucks such as removals vehicles to swing wide and use the full width of a street at intersections, but a central median forces such vehicles to stay on the left side of the road and as a result intersections can occupy significantly more space. This not only reduces yield and density but creates a poorer environment for cyclists and pedestrians.
Parking is a topic worthy of an article or book in itself, so isn’t discussed in any real detail in this article, but the parking supply and demand in a street can play a large role in producing a good outcome. If the width of the street is the primary method for speed management the presence or absence of parked cars can make a large difference to the result.
There is a huge range of other things that make the difference between a poor or ho-hum street and a great street. Activity in the form of people using the street has got to be a critical success factor. The form and size of buildings have a great influence, and other aspects such as street-lighting and the number, type, and location of street trees and planting also play a role in developing a unique character and sense of place.
In New Zealand street design is controlled or influenced by a national standard, District Plans, and District Council Codes of Practice, and they are usually different. This reflects the difficulties inherent in trying to fit a multi-faceted design problem that has to be responsive to site-specific issues into a single document or even a single table. As a result a standard solution will usually not be the best solution for any particular circumstance, but will provide a lowest-common-denominator solution that will hopefully provide an acceptable design in most cases.
This situation is often exacerbated by Councils rigidly adhering to the standards when a different design solution could produce a better overall outcome. We now have legislation that allows us to develop shared surface streets where pedestrians, cyclists, and slow-moving vehicles can share the same space but these are yet to be “allowed” by any design codes and consequently many engineers shy away from them as they are not in the standard.
Bearing those limitations in mind, how do our guiding documents perform, and what could we do to improve them?
It’s usually a matter of never mind the quality, feel the width. The vast majority of documents only focus on the width of the various elements that make up a street. While this is a critical issue to get right it’s also one that is subject to so many variables. Many of these documents are being revised with a greater range of street types to choose from, but little in the way of guidance as to the reasons for adopting a particular width.
For example, many guidelines permit narrower carriageways in cul de sacs, but a street connected at both ends has greater resilience to blockages (for example by poorly parked cars) and can often be acceptable with a narrower carriageway.
Very few documents pay any consideration to speed management through control of leg length, and fewer promote consideration of road placement, intersection type and other key issues. This is an element that is reasonably easy to add into design guidelines and codes although clearly some element of flexibility and discretion is needed to deal with the inevitable situations where few alternatives exist.
Some councils are starting to produce street design guidelines and this may be a good way of avoiding some of the rigidity required by a standard or table in a District Plan or Development Code while still conveying the matters key to producing good streets.
Many District Plans are falling due for review over the next few years, and various council Codes of Practice are being revised to reflect evolving best-practice street design. The New Zealand Standard for Land Development and Subdivision Engineering (NZS 4404) is also being reviewed at present and a draft for comment is expected out shortly. I’d encourage all of us to have an input into these documents where ever possible; and any of us involved with the design of streets can perhaps incorporate a few of these principles into the next street we design.
Since this article was written, NZS 4404:2010 was published, with a different place-based approach to road design.
In closing I’d like to receive your comments on these issues and your nominations for the best and worst streets in New Zealand and why you like or don’t like about them. If we get a good response we could publish a list in a future issue of URBAN.