Why don’t we have more green right arrows?

Green arrows have been a bugbear in Christchurch for many years and we've way fewer than other centres.

A while back I asked for information as part of this year's Annual Plan to see what the budget was for installing them and what could be required. I got the response back yesterday - it's got great info and I've got some more questions. I think it probably needs a public briefing, so councillors can learn more and discuss it and the public can see too.

Apart from cost, it looks like it comes down to whether drivers would be okay waiting a bit longer at lights heading in one direction, to allow others to have a green turning arrow coming from another direction...

"Funding: Currently this type of work is funded through the Minor Road Safety programme. Within this funding stream there are two component programmes: - High Risk Intersections - School Safety Each component is programmed independently, with a multi-criteria assessment used for prioritisation.

Currently this programme is over-committed with safety projects identified out until FY2033, with additional projects on a waitlist. To accommodate more works under this programme would require an increase in the budget. To ensure that the budget is ring-fenced for specific projects that provide right turn phases at intersections, a separate programme component within Minor Road Safety could be established or a separate line item created with the LTP.

Infrastructure Challenges: Whilst it may seem like there is minimal work required to install more arrow signal displays, it can be very costly to provide right turn phases at intersections due to issues with underground infrastructure. In order to add the additional arrow signal displays for right turn phases there needs to be spare capacity in the electrical cables that link the displays to the signal controller cabinet.

Due to the age and condition of this cable in several locations, there is insufficient capacity in the existing cable to accommodate additional signal displays. A lot of underground traffic signal infrastructure was damaged during the earthquakes and we are still dealing with the aftermath. Cable has been damaged, as have the ducts which are run underground to get cable across road carriageway. A lot of these ducts were blocked up with silt as a result of the earthquakes, which in some cases means we are unable to run new cable through existing ducts.

We are currently having to re-cable a lot of intersections where damage is critical. The extent of works required varies from site to site, dependent on the condition of the infrastructure. A full intersection upgrade is generally around $600k but can be up to $850k. A partial upgrade could be in the vicinity of $150-200k.

Recent Projects: In addition to the Minor Road Safety programme, this type of work can overlap with other programme budgets such as capital projects (e.g. Major Cycle Routes) and Traffic Signal Cabling Renewals. In the last five years (2020-present) we have provided additional right turn phases at 23 intersections throughout the city (including seven sites on the Four Avenues).

Wider Impact: When looking at installing green right turn arrows, while there are safety benefits, the full impacts need to be considered. These impacts depend on the specific intersection layout, traffic volume, pedestrian activity, and surrounding network dynamics.

Installing green right turn arrows can lead to increased intersection delays. For the arrows to operate the opposing straight through movements would need to be stopped. This reduces the amount of green time given to what would typically be the lanes carrying the most traffic.

However, sometimes when there is a heavy right turn, the green arrows can lead to an improvement in overall intersection delay. As most of our signalised intersections operate as part of a coordinated network, any changes that lead to increased delay to the main through movements will often lead to much wider adverse impacts and may impact other intersections.

Installing green arrows can also change travel behaviour and one consequence is that the right turn can become a busier movement. They can attract more people to turn right at a particular intersection, and the impacts of this should be considered as it may or may not be the desired outcome.

In summary, the installation of green turning arrows is complex and is currently being delivered where possible through the minor safety programme or the intersection re-cabling programme. Staff do not recommend adding a single line to the LTP for the installation of arrows as they must be considered holistically."

And here's an article about it from 2023.