Anyone who goes anywhere near South Road knows that it’s a traffic bottleneck. It gets congested even at times when other roads usually aren’t. Yet there is a simple short-term solution available that the government is largely missing: fix Marion Road.

The Marion Road corridor (which includes Holbrooks Road and East Avenue) is the most obvious alternative route to South Road, yet it could be so much better than it is now. The route in its current state suffers from severe bottlenecks at multiple points, which need to be fixed to make it a quality alternative route. Anyone who uses this route will be familiar with these bottlenecks:

• The Grange Road, East Avenue and Holbrooks Road intersection is probably the worst, with effectively only a single lane each way for north-south traffic and very limited capacity.
• The second worst is probably the pair of intersections that connect Marion Road to Holbrooks Road via Henley Beach road, which also limits traffic to a single lane for north-south movement, and is at capacity.
• Along Marion Road proper, the worst section would have to be the intersections with Cross Road and Anzac Highway, and the short stretch of road in between which also includes a tram crossing. It is clear that a third lane in each direction is needed here.
• The intersection of Marion Road and Sir Donald Bradman Drive also needs an upgrade, as the department acknowledges, but their current plans for this are deficient.

One might look at this list and think a lot of work is needed, but the main take-away should actually be the opposite. It’s remarkable how little work would be needed to transform Marion Road into a first-class high-capacity alternative to South Road. It already has good capacity at the intersections with Daws/Oaklands Road and Sturt Road, making the southern section already pretty good (although the Sturt Road intersection currently carries a bit too much turning traffic, which I’ll suggest a solution to below). It only requires a few major bottlenecks to be fixed to increase its capacity substantially. This would create a high-capacity alternative route connecting the North-South Motorway to the Southern Expressway via Grange Road, Holbrooks Road and Marion Road (or via East Avenue and Holbrooks Road, if coming from Port Road), bypassing all the congested parts of South Road, buying time for completion of the North-South motorway and providing a better alternative route during construction.

The necessary upgrades are as follows.

At the intersection of Holbrooks Road and East Avenue with Grange Road, it is reasonably obvious what to do – realign both roads to meet at a single intersection. The DIT already has plans for this, albeit deficient ones in that there is no westbound left-turning lane on Grange Rd for turning onto Holbrooks Road, which would be a problem in the evening peak, especially when a pedestrian holds up left-turning vehicles, blocking an entire lane of through traffic on Grange Rd. Their plans also weirdly involve widening Holbrooks Road to the west, when widening it to the east would make much more sense. Nevertheless, it’s good to see this intersection getting a long-overdue upgrade. (EDIT: Updated plans for this upgrade project now include a left-turn lane from Grange Rd to Holbrooks Rd, although it is still too short.)

Holbrooks Road will need to be widened to two lanes each direction with a median strip (in most places) for right-turning traffic. Fortunately, a significant portion already has two lanes each way, and for the rest there appears to be a wide enough verge along much of its length to permit this to be done relatively easily, while the partial purchase of some front yards will be needed in the southern section.

To fix the intersections of Marion Road and Holbrooks Road with Henley Beach Road, my first thought was to join the two roads together at a single intersection as shown on this map, by purchasing about 35 homes in Underdale (similar to what was done in the 90s to link Tapleys Hill Road to Brighton Road at the intersection with Anzac Highway). However, I think a better option may be to keep it as two separate intersections and widen this short stretch of Henley Beach Road to accommodate the same amount of north-south traffic as a single intersection would, with much less disruption to the neighbourhood and only a slight (about 150m) increase in travel distance for north-south traffic, compared to the realignment option (which probably equates to about a 10 second time penalty).

This would be done by widening Henley Beach Road to four lanes each way in between the two intersections, with two straight lanes and two right-turning lanes in each direction. The two left-turning lanes from Marion Rd to Henley Beach Rd would direct traffic into the two right-turning lanes, and similarly for the left-turning lanes from Holbrooks Rd, thus creating two uninterrupted lanes for north-south traffic without any lane changes required. This requires 30m of road width (kerb to kerb, not including footpaths) in this middle section, which can be done with only about 15 full property acquisitions plus a similar number of partial acquisitions involving front yards and car parks. The approximate footprint of this road is shown on this map. Because T-junctions have fewer traffic light phases than intersections, two lanes for north-south traffic should be sufficient.

Moving south, the next bottleneck intersection is at Sir Donald Bradman Drive and Marion Road. The government also has plans to upgrade this intersection, but those plans fall far short of what is needed, which is to have three lanes each way on both roads, in both directions, through this intersection. This could be done by purchasing houses along the eastern side of Marion Road, and part of VIP’s car park (building a new car park for them in the adjoining land where the houses were). Then they could avoid demolishing the large commercial building on the southwestern side of the intersection. Alternatively, the government could purchase front yards on both sides of Marion Road in order to avoid demolishing houses. Either way, we need three lanes northbound and southbound to cope with traffic volumes that Marion Road will attract once the other bottlenecks along the corridor are fixed.

The largest upgrade needed along Marion Road is the dual intersection upgrade of the Anzac Highway and Cross Road intersections with Marion Road. The government also has plans for this as part of their level crossing removal project, but again, they are insufficient in their current form. This entire stretch of Marion Road needs three lanes in both directions, uninterrupted, with bike lanes alongside (which are curiously absent in the government’s current plans). There also need to be two right turning lanes for turning onto Cross Road (eastbound) and Anzac Highway (in both directions). The current plans only show dual right-turning lanes at Anzac Highway, and only for turning towards the city, and even they are far to short to be of much help.

The final part of this upgrade is not an upgrade of Marion Road as such, but a new interchange to access the Southern Expressway directly from Diagonal Road and Seacombe Road, as shown in this map. This involves an onramp and an offramp meeting at an intersection on Seacombe Road just west of the bridge (the onramp would pass over the expressway), and realigning Diagonal Road to meet this intersection. This new interchange would divert a large amount of traffic that currently uses Sturt Road and Marion Road to access the expressway from Diagonal Road, which will free up capacity on Marion Road, making it well able to handle the additional traffic volumes from all the upgraded intersections above. It will also help divert some traffic from Marion Road to Diagonal Road & Tapleys Hill Road, freeing up Marion Road to take more traffic away from South Road.

These upgrades are sensible and should be done as soon as possible, as they will go a long way towards relieving congestion on South Road in the short term, and will continue to be important and worthwhile even after the north-south motorway is completed.