Monday, 1 May 2017
Austroads has published a report that investigates how the geometric design components of a roundabout may contribute to bicycle crashes.
National, state and local government strategies are being developed to get more people cycling for their improved health and other economic and social benefits.
Providing more separated, safe and attractive facilities help encourage people to ride bicycles, however there are some treatments, in particular some roundabout layouts and alignments, that pose both a real and perceived crash risk and deter people from taking up cycling.
Attempts to accommodate or attract cyclists with marked bicycle lanes in the circulatory lanes at roundabouts have been found to be counterproductive in terms of increased crash risk, and therefore methods must be found to better cater for cycling where roundabouts have been or are to be constructed.
Each year, on average about 37 cyclists were killed and about 5200 hospitalised on roads across Australia.
This study included an Australian and New Zealand crash analysis which found that most of the crashes at roundabouts occurred at urban local road roundabouts, in 50 km/h speed limit zones. The crashes predominantly occurred on the circulating lane near the entry for an approach road and were right-adjacent type crashes.
Following the analysis, 17 roundabouts across Queensland, New South Wales and Victoria were chosen for in-depth investigation to identify the factors that contributed to the crashes.
A geometric analysis identified that the entry geometry of the roundabouts investigated would permit relatively high entry speeds, in excess of the target speed of less than 30 km/h. This target speed was adopted for analysis purposes, however, further investigation to determine an appropriate speed to prevent or minimise fatal and serious injury outcomes for crashes involving motor vehicle and cyclists is needed.
The motor vehicle speeds on the entry and circulating lanes were estimated using the ARNDT crash prediction model. For the purposes of this investigation, the ARNDT model was used to assess geometric alignments to achieve lower approach speeds and it was found that a roundabout with a radial-type of alignment, used in countries in Europe, achieved approach and circulating speeds of less than 30 km/h.
Sight distances were examined and it was found that the available sight distance to vehicles approaching from the right did not meet the design requirements. There is some research which indicates that restricting the sight distance on the approach to a roundabout reduces the approach speeds of vehicles, however, this requires further investigation to develop design criteria.
Possible treatments were identified, with the entry path curvature being the main component that needed to be increased to slow the entering vehicles. On local roads, with smaller design vehicles, the entry curve radius could be reduced to achieve a speed of ≤ 30 km/h, but on the arterial road roundabouts, reducing the entry speeds was not achievable due to the larger design vehicles on these roads. On the local roads, the alignment of the roundabout to achieve the desired entry speed was similar to the alignment of a radial-type roundabout.
The report suggests amendments to Austroads Guides and recommends further investigation into motor vehicle/cyclist crash outcomes, the effect of restricting sight distance on the approaches to a roundabout, and the development of design guidance for urban local road roundabouts.
Report link: Bicycle Safety at Roundabouts