Grant McLachlan: Lower rural speed limits often worsen congestion

COMMENT: Be careful what you wish for when suggesting the lowering of speed limits on rural roads. Often it results in unintended consequences.

In 2007, the then-named Transit New Zealand (now NZ Transport Agency) published a planning policy manual setting out how it wished to integrate land use around its national roading network. The manual tried to avoid adjacent activities affecting the efficient flow of traffic.

The quantity, quality, and proximity of accessways onto the highway system influences the design, signage, and speed limits of highways – and vice versa.

The NZTA has every reason to protect its patch. For decades, townships and developments free-rode on highways as their main point of access without building a roading network for local traffic. What results is called "ribbon development". Think of a town near you whose 50km/h zone has been extended again and again.

When towns grow along highways instead of away from them, speed limits are affected in a variety of ways. In tourist trap towns, many business owners want pedestrian crossings installed and speed limits lowered to 30km/h. Although the law clearly states that traffic must slow to 20km/h around school buses, many school parents want speed limits permanently lowered for safety reasons.

But it's the lowering of speed limits which affects traffic the most.

Traffic travelling slower affects response times, which influences the quantity and proximity of accessways. In a 100km/h zone, the NZTA suggested sight distances from accessways be 282m whereas for 80km/h zones it be 203m.

The minimum distance between accessways for 100km/h zones is 200m whereas for 80km/h zones it is 100m. Lowering the speed limit by 20km/h can potentially double the number of accessways along a rural road.

But it's not just state highways that are affected. Many councils used the manual as a guide. The Dairy Flat highway between Albany and Silverdale used to be State Highway 1. Accessways were few and far between along the limited access road. Following the completion of the extension to the Northern Motorway, the local council lowered the speed limit, allowed ribbon development, and then lowered the speed limit again "for safety reasons".

A rural arterial clustered with letterboxes and crosses speaks for itself. Matakana Rd is another where, as soon as the speed limit was lowered, lifestyle blocks and roadside stalls cluttered the once open road. Waitoki, Riverhead, Whitford, Karaka… I bet you can name a few more examples.

With urban boundary expansion, rural arterials will become important perimeter arterials and connectors. But the Auckland Council is allowing development along these open roads and proposing lowering speed limits.

I challenge anyone to drive around Auckland's fringe. Driving from one side of Rodney to the other is a confusing mess.

Once the Puhoi to Warkworth extension to the Northern Motorway is completed, the existing State Highway 1 will become another Dairy Flat highway. Already, the two flattest and straightest passing lanes have been removed and speed limits lowered to allow for construction traffic access. Further large developments were proposed for the area before the motorway construction started.

Other "future proofing" projects in Warkworth will achieve the opposite of what was intended. The much-heralded 1600m long Matakana link road – which is meant to spirit motorists from the motorway to "Matakana Country" – will be a gauntlet of at least six intersections through a 50km/h "light industrial" estate (more likely to be fast food and retail strip malls) and a new residential zone.

A Warkworth-based transport planner specialising in cycling and pedestrian projects proposed lowering speed limits throughout Auckland to 80km/h in rural areas and 30km/h in urban areas. Developers who read such suggestions salivate.

They'd be thinking of how many high-rises they could build in Warkworth or which farms to landbank along the remaining stretches of straight open road that should be passing lanes for growing satellite suburbs.

It's a vicious cycle. Lower a speed limit, allow more development, lower the speed limit again.

From an economic perspective, once a council allows a momentum of lifestyle blocks amongst productive farmland, the value and cost (such as rates) of maintaining productive farmland becomes prohibitive and they too are carved up into more lifestyle blocks.

Ironically, councils look for large rural land holdings on urban boundaries to rezone. Similarly, NZTA looks for large holdings to route dual carriageways. Lifestyle blocks are effectively a hindrance of their making.

Central to the solution is using productive farmland to limit rural road congestion. Lifestyle blocks should be limited to where they won't cause adverse long-term effects. That way, lowering speed limits won't be necessary nor will there be limitations down the line.

• Grant McLachlan is an infrastructure specialist.