Titbits: Cost of bike paths

Titbits: A series on the recommended design of cycle infrastructure in the ACT from the opaque world of the ACT and Australian Engineering Design standards.

PART 2: Cost of bike paths

Several different materials are used to build paths in Canberra, and I have always wondered about the cost of path construction. Preliminary estimates from ACT budget data showed that it was possible to build 30 km of bike paths for every single kilometre of dual-lane carriageway. This is an argument to spend more on paths but does not answer the question of the cost of path types. 

The Australian road safety and standards body Austroads has a standard for cycling path design: the Guide to Road Design Part 6A: Paths for Walking and Cycling (2017). Appendix C.4 has some information about paths. As expected, the exact costs can vary depending on the jurisdiction but the relative costs for different paths types are clear from the Austroads data. 

Below is a chart that compares the cost of the path over a 20-year period for three different materials: decomposed granite, asphalt, concrete and boardwalk. The capital costs for construction for an unpaved decomposed granite is cheapest of these types but unpaved surfaces erode easily and therefore the maintenance for unpaved paths is high. Paved surfaces are more expensive to build but are hardier so that the maintenance costs are better. The capital costs of asphalt are only a little more than decomposed granite but more expensive than concrete to maintain. Nevertheless, despite concrete being almost twice the price per km of asphalt, the “lifetime cost” over twenty years concrete and asphalt paths are similar.

Figure 1: development of total costs over 20 years depending on the path material (year 0 is the capital cost)

Asphalt or concrete, which is better? 

It depends. In wet areas and areas prone to flooding concrete appears to be a better option. It also does not get as hot in the summer, but glare can be a problem. Moisture can cause longitudinal cracks on asphalt and roots can cause cracking across the path. Both can be reduced by root and moisture guards (600 mm deep) as is required for all new paths (estates) under ACT planning standards (MIS05 and ACTSD). The ACT specification for paths is ACTSD-0501 (see enlarged portion below).

Figure 2: ACT Standard Drawing (enlargement) ACTSD-0501

What about maintenance? 

A well-constructed asphalt will last 20 years. Canberra’s paths are of poor quality due to lack of maintenance and maybe in some cases older than 20 years. Primary and Main Community Routes are likely to be asphalt. Why would the ACT government leave a path unpaved if asphalt is cheaper in the long term?Unpaved paths are very expensive as they erode with every storm. It seems strange that the ACT government would build them. The only reason appears to be that in a “natural” environment an unpaved may have better “amenity”. In other words, the reason is aesthetic, certainly not because it is cheaper. Unpaved paths may also be chosen to discourage bike use as their poor traction qualities make riding more difficult.

Table C.1 Example of life cycle costs source: Guide to Road Design Part 6A: Paths for Walking and Cycling 2017 (Austroads)

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