Cycling network

Cyclists make use of the road network in order to complete their journey. They can use roads both with and without marked cycle lanes, but are excluded from motorways. In addition to the use of the road network, cyclists also make use of off-road cycle paths and alleyways which have the potential to shorten trip distances and therefore travel times. The road network is used as a starting point for the creation of the cycling network.

Aerial photography and maps of off-road cycle paths were used to assist in the digitisation of the locations of off-road cycle paths in order to add them to the cycling network. Once the off-road cycling links were manually digitised, the cycling network model underwent a sensibility review, to ensure all known off-road cycle paths had been correctly added.

The speed for cycling along flat sections of the cycling network was set to 20km/h, to represent a cyclist of reasonable fitness. This speed was then adjusted according to the average gradient of each of the edges in the cycling network. The speeds were varied up to 30km/h when travelling downhill, and reduced to 10km/h when travelling uphill. These speeds are slightly higher than the cycling speeds in the UK, and hills in New Zealand also have a lower impact on speeds than in the UK. The UK analysis has recognised that this is an area of work requiring further research.

Table 14.2 describes the relationship used to relate slope gradient to the modelled speed in the cycling network.

Table 14.2    Look-up table to relate edge traversal speeds in the cycling network to the edge’s gradient (vertical distance/horizontal distance)

Minimum gradient

Maximum gradient

Modelled speed (km/h)

< -0.05

-0.05

10

-0.05

-0.03

15

-0.03

0.03

20

0.03

0.05

25

0.05

> 0.05

30

 

Given the off-road cycling links had been added to a copy of the road network, the appropriate fields for edge length and edge traversal time already existed.

The GIS was again used to calculate the edge length in metres. The traversal time and edge traversal speed fields were removed and the following fields were added: TF_gradient, TF_speed, FT_speed, TF_seconds, FT_seconds (note: FT = from-to, TF = to-from, this relates to the direction in which the network edge was digitised).

First, the elevation of the endpoints of each edge on the network was calculated using a digital elevation model (DEM). From this, the gradient of each edge was calculated in the direction in which the edge was digitised and the result stored in the TF_gradient field.

Then using the relationship described in table 14.2, the speeds along the network edges were updated.

Once the speeds had been calculated, the edge traversal speeds in the direction of digitisation (TF) and the direction reverse to digitisation (FT) were calculated, using the following relationship:

Edge traversal time TF (s) = edge length (m)/edge traversal speed TF(m/s)

Edge traversal time FT (s) = edge length (m)/edge traversal speed FT(m/s)