Hartmann / Alexandrina / Bald Hills Road Intersection Upgrade
The intersection of Hartmann, Alexandrina, and Bald Hills roads plays a significant role in the Mount Barker local road network and in an effort to improve traffic management and safety, Council has endorsed the installation of traffic lights.
Traffic data collected by the council reveals that the intersection is currently operating at near full capacity at peak times, necessitating an upgrade to accommodate present and future traffic volumes effectively.
Three potential solutions were considered before the recommendation was put to the council to install traffic lights to ease congestion at the busy intersection. Other options were a dual lane roundabout, and a single lane roundabout.
Why was a single lane roundabout discarded?
A single lane roundabout was investigated by both consultants engaged in 2021 and 2023, and they both came to the same conclusion that a single lane roundabout will not perform adequately for any of the future scenarios.
A single lane roundabout does not have the same throughput capacity as a dual lane roundabout or multilane traffic signals, which greatly affects the future queue lengths and delay to unacceptable levels. Hence, single lane roundabout option was not pursued further.
Why was a dual lane roundabout investigated?
It was determined that a single lane roundabout will NOT be adequate to accommodate future volumes on the intersection, and the only possible roundabout solution was a dual lane roundabout. There were 2 potential dual lane roundabout sizes (20m and 12.5m) that were investigated, however the 12.5m dual lane roundabout had clashes in vehicle movement due to Alexandrina Road/ Bald Hills Road being a designated B double (26m) route. The only way to solve the clash was by increasing the diameter which would make it the same size as the 20m dual lane roundabout.
How were the options accessed?
The options underwent a Multi Criteria Analysis (MCA) based on the role of the intersection in our district with the following weighting structure used:
- 30% weighting to intersection level of service, pedestrians and cyclists movement, and safety
- 20% weighting to vehicle turn movements, which relates back to level of service
- 15% weighting to the impact on trees
- 5% weighting on land required
| Criteria | Comments | Score |
|---|---|---|
| Level of Service | While the dual lane roundabout will have free flowing traffic during interpeak hours, the traffic flow on the Hartmann Road leg during peak hours is highly dependent on the gap obtained from Alexandrina Road turning right into Hartmann Road. Whereas the traffic signals, which will cause slight delays in Alexandrina Road and Bald Hills Road when they stop at a red light will still ensure all movements get an opportunity to flow regardless of other movements. | 5 - roundabout 4 - traffic lights |
| Vehicle turn movements |
A roundabout gives priority to through movement and movement out of Hartmann Road is highly dependent on the right turn movement from Alexandrina Road. This problem can be resolved by signalising the Bald Hills Road leg, however this option still has all the impacts of a 20m roundabout with additional
signals and maintenance costs. A signalised intersection still gives priority to through movement, however right turning vehicles will get a chance to turn. |
3 - roundabout 4 - traffic lights |
| Pedestrians and cyclists safety |
Hartmann Road, Alexandrina Road and Bald Hills Road are all significantly high volume roads. Even with a wide storage refuge, pedestrians and cyclists will have difficulties finding a safe gap to cross two lanes in each direction which may encourage unsafe crossing behaviors thus putting the vulnerable user group at risk. Although cars may seem to slow down on approach to a roundabout, they can still go through it at a max speed of 30 km/h, which causes fatal impact to pedestrians and cyclists should a crash take place. However, signalised intersection can be phased to give safe and frequent crossing opportunities to pedestrians and cyclists. |
1 - roundabout 4 - traffic lights |
| Impact on trees |
There are significant remnant native vegetation at the intersection which get impacted from the intersection upgrade. At this stage only a concept design has been prepared so actual numbers of trees impacted is subjected to detailed design. From the concept design we can see that a roundabout impacts a high number of trees, whereas the signalised intersection only impacts a low number of trees. |
1 - roundabout 3 - traffic lights |
| Land required | The dual lane roundabout has a large footprint and requires acquiring of private land from a couple of properties. Whereas, the traffic signals can be accommodated in the existing road reserve. | 1 - roundabout 5 - traffic lights |
Did the proposed solution take into consideration community needs?
Although a consultation specific to the intersection had not been undertaken, community needs have been considered. We have listened to a lot of feedback and strategic direction (including but not limited to the Paech Road consultation and Markyt survey) to form this opinion and put this solution forward.
As shown in the MCA, multiple user groups and impacts were considered when proposing the solution and it was not just a technical approach. Significant importance was given to improving level of service, whilst still making the intersection safe and accessible for pedestrians and cyclists, whilst minimising impact on the native vegetation and residents.
As we move to a creating a more active community, it is crucial to consider and accommodate different user groups and hence that is why traffic signals was put forward as a preferred solution.