#32 Road Diets

Road diets are described generally as a road space reallocation strategy or, in the case of Austroads (2021), a “road-narrowing measure”. While accurate, neither description adequately captures the intent or potential of road diets and the multiple objectives that underlie their design and implementation. Recent research[1] notes the diverse types of road diets implemented internationally and examines their effectiveness in different contexts. The United States (US) first introduced road diets in the 1970s and the approach has more recently been adopted in other countries, including Australia. The classic scenario for road diets involves converting 2-lane, 2-way undivided roads into 1-lane, 2-way roads with a centre (or median) 2-way turning lane. A pre/post treatment diagram adapted from Austroads (2021) illustrates the basic concept, variations of which can be applied in other scenarios. In this diagram, cycle lanes and the median turn lane are added, while the single through lanes in each direction retain their original 3.3m width.

Aims of a road diet may include crash reduction, speed reduction, traffic calming, reduced risk for vulnerable road users, and encouragement for alternative and active transport modes. Depending on the surrounding environment, road diets can be relatively low-cost.

Effectiveness

Austroads (2021) has indicated a 35% reduction in casualty crashes and small to moderate mean speed and 85^th percentile speed reductions from road diet implementation. However, site-specific evaluations of different examples across the world show highly variable effects. US research indicates casualty crash reductions of 6% to 56%, with most studies arriving at figures somewhere between these extremes.

Some studies have shown increased crash and injury risk for specific road user groups, and small increases in vehicle speeds, delay and congestion at some specific sites. Overall results, however, are consistently positive and the benefits are clearest for sites with low to moderate traffic volumes. Further research in Australia is recommended to generate more robust findings around local implementation.

Considerations

From a capacity perspective, the concept of road diets can be seen as counter-intuitive and is not without its critics in relation to mobility and other issues (including safety). However, evaluations show that in most cases, traffic volumes and flow for motorists are not greatly affected.

Traffic volumes are a key factor when considering road diet design and implementation. Austroads suggests that volumes of 20,000 VPD or less are good road diet candidates. Site-specific planning, assessment and design are essential elements of a successful road diet project.

[1] Boyle et al., (2023). A Comprehensive Study of a Road Diet Implementation in the US and Abroad. Current Urban Studies 11, 447-473.