15 metres

This week, I have been lucky enough to have had time to think about how I would redesign a secondary type of street and it's adjacent residential areas.

It's a real project which I have no idea where it will lead to (if anything), but it is great to have some thinking and designing time.

In terms of street classification, we can use the follow;

• Major roads - dual carriageway trunk roads or motorways
• Primary streets - single (sometimes dual) carriageway A-roads
• Secondary streets - single carriageway B-roads or unclassified roads
• Local streets - residential streets

The definitions are a bit more complicated than I have set out, but in essence I am interested in the secondary street. It is a pretty bog-standard suburban street which mainly residential frontage (houses and flats). It has a parade of shops, a few individual businesses and a primary school The street carries two bus routes and it conveys traffic from one local area to another. At each end of the street, it connects to busy A-roads.

In terms of its geometry, it is generally straight and it is an almost a consistent 15 metres between the highway boundaries - 15m being the minimum width. The carriageway is 8m in width and nominally marked with 1.2m wide advisory cycle lanes and 2.8m general lanes along part of the street and general lanes with a hatched median elsewhere. 

The street carries some 14,500 vehicles per day which is very high and so it is not surprising that traffic congestion is bad twice a day. 

The footways are wide with old trees towards the kerbside - I have looked at a couple of historic photos and the street has never had a verge. The street has plenty of dropped kerbs for vehicle access and a number of side streets leading off it, all with nice tight radiuses and into filtered neighbourhoods. On street parking is restricted during the day and there are some footway parking bays (all four wheels up). 

Existing layout

Luckily the streets either side are filtered, but this does mean residents have to use this street to access their homes by car and there are cases of people using parallel routes through these areas to jump the traffic queues a little bit (leaving the secondary street at one point and rejoining a bit further up) - dealing with that is important, but I'll stick to our secondary street here.

So what is the project brief? In essence, it's about trying to make the place more liveable. This means getting some people out of their cars (especially those making short trips), making it easier to cross the road, providing protection to get people cycling, reduce casualties and collision risk and perhaps to discourage a bit of through-traffic. Simple aims, but quite difficult to achieve in terms of the usual issues of cost, resident acceptability and politics (but that's all another story).

From a technical point of view, it is in theory complete possible to strip everything out and rebuild the street layout from the ground up. This approach would allow some pretty good layouts;

One-way stepped cycle tracks with buffers from traffic

Two-way cycle track with buffer to traffic

Making the street one-way would free up loads of space, but being a bus route, it does mean looping buses onto other primary or secondary streets created a much long walk for local residents who we want to be able to get the bus for some trips;

The big issue from a technical point is that a layout such as V2 means changing an existing area of footway into carriageway which is costly in construction terms and the potential need to move utilities. It is easier to convert carriageway to cycle track.

We also have the existing trees to contend with. They are mainly mature and so any proposal to remove them will undoubtedly meet with objection (although removing them and planting new ones would be so much easier). The outcome of this means we are probably looking to work with them and having the cycle track pass on the outside of them - this does mean we lose the buffer we could have had;

It does mean that we are probably left with narrower cycle tracks as we pass each tree or groups of trees, but perhaps it's good enough for what we want to achieve. As we reach the side road junctions or driveways, we can tweak the layout to give a buffer which allows the footway and cycle track to stay at the same level and be continuous with drivers having to drive over them via ramped kerb units or little asphalt ramps within the buffer;

Junction width continuous footway/ cycle track

Section

You will have noticed that I have selected 3m wide traffic lanes. In my experience, the bus operator will always push for wider lanes. Buses tend to be around 2.5 - 2.55m in width from what I can gather (London's New Bus for London is 2.52m for example) and so this says to me that we actually need a 20mph speed limit. As well as buses becoming rolling road blocks, the speed limit is reinforced by the lane width, no centre line, plus if we hump crossings on the street, it helps with keeping driver speed down as well as creating level crossings for pedestrians.

Humped zebra crossing and a bit of a squeeze

in the cycle track to pass the tree

As usual, there are compromises, especially in trying to squeeze a cycle track around the existing trees, but for 15 metres of highway width, I think you can get a lot of improvements with something for all modes if we assume a secondary street will always carry a level of traffic where people cycling need protection, where people walking need crossings and where we facilitate bus routes.