Design guide principles

This article was published in 2016, in Newsletter 127.

The ‘Urban Design Guidance for Transport Infrastructure Projects’ is meant to be the main design guide for all City Deal projects. This guide is not setting policy, but has to follow any existing policies that have been approved by the three different councils. For example, streets within the city have to follow the urban design guidance from the city council, but those outside the city have to follow the urban design guidance from South Cambridgeshire District Council, and main roads have to be engineered to standards already used by the county council. The guidance is meant to bring all this together in a single unifying document so that all the schemes follow the same design principles.

Design principles

From our point of view, the main principles should be those documented in our ‘Making Space for Cycling’ policy document. It appears that the three councils disagree with our aspiration for world-class infrastructure. The main design principles of directness, convenience and speed are all being considered as part of the set of City Deal projects, but our three-network principle is still missing.

The three-network principle states that cycling, walking and driving need different networks with different design requirements. This arrangement is needed on the main routes, such as Milton Road and Histon Road.

Widths

In addition to the three-networks approach described above, we also need to consider the minimum widths for other road-space uses. For example, buses are typically wider than your average car so need wider lanes, and trees need to have space for roots, although these roots can be placed under pavements and cycleways using something called structured planting. The table in the next column shows the ideal and minimum widths for the five different road-space uses, with an additional inferior column that gives a substandard width that could be acceptable over a very short distance.

Type Ideal Minimum Inferior
Footway 2.5m 2.0m 1.5m
Cycleway 2.5m 2.1m 1.8m
General traffic 3.2m 3.0m 2.8m
Bus lane 3.2m 3.0m <3.0m
Trees 3.0m 1.0m 0.5m

For example, a general traffic lane should ideally be 3.2m wide, as that is the width that has the best safety record, although a traffic lane that is 3.0m wide is still good. However, when you get below 2.8m then the situation turns ugly with larger vehicles unable to pass each other safely or efficiently.

The width for bus lanes is similar, except that when the width is reduced to less than 3.0m, it becomes inferior very quickly.

Cycleways ideally should be 2.5m wide, the standard used in countries with high levels of cycling, with a minimum width of 2.1m. If the width is reduced below this, it is very difficult to overtake people or be overtaken in a cycleway, causing significant capacity issues.

Layout

The layout of the street is also very important. Implementing a few simple principles can dramatically improve the urban realm as well as give more people the confidence to walk or cycle:

  • Footways on the outside
  • Cycleways next to footways
  • Trees to protect cyclists and pedestrians from moving motorised vehicles
  • Parked cars protect cyclists and pedestrians from moving motorised vehicles
  • Bus-stop islands should protect bus passengers from interactions with moving cyclists.

The following cross-sections show how the above layout and widths can be applied given various constraints.

Robin Heydon

Image as described adjacent

Image as described adjacent If space was not an issue, and the desire was for two general traffic lanes and two bus lanes, with side green spaces for trees, then the right of way would be 28.8m wide. The road is very wide, probably uncomfortably wide for many people.
Image as described adjacent By using the minimum widths for all of the elements, the width reduces to 22.2m wide.
Image as described adjacent Many of the radial routes, such as Milton Road, are up to 20m wide, and therefore two possible alternatives easily fit. The first has two general traffic lanes and desirable widths for people walking and cycling.
Image as described adjacent The second reduces the widths of the footways, trees, and general traffic lanes a little so that a bus lane can be fitted in.
Image as described adjacent Main roads that are just 18m wide do not easily accommodate a bus lane and trees. Without a bus lane, many elements have a reasonable width.
Image as described adjacent However, by requiring a bus lane, there is now no space for trees, the footways have become inferior, and there is no space for safe bus stops.
Image as described adjacent At 16m, there is no space to install a bus lane. The space for trees has become compromised but trees are still possible.
Image as described adjacent Only when the road has been reduced to 14m wide does everything become difficult. As shown, the cycle lanes now have an inferior width and therefore can carry significantly less cycle traffic than all the other examples.