Lowering the speed limit to 30km/h might result in more deaths and injuries on Auckland's roads, according to a prominent road safety campaigner.
Road safety campaigner Clive Matthew-Wilson, editor of car-rating Dog and Lemon Guide, says if motorists think the speed limit is too low, they'll just ignore it.
He cites a study carried out by the UK Department for Transport, which found 86 percent of drivers in 20m/h zones - roughly equivalent to 30km/h - broke the speed limit. About a fifth were caught going more than 30m/h (about 50km/h).
"Motorists tend to drive at a speed that feels right. If the speed limit is set unreasonably low, drivers tend to ignore it," said Mr Matthew-Wilson.
"If the low-speed areas are confined to areas of known high-risk, motorists tend to obey the law. If the lower speed limits are blindly applied everywhere, drivers tend to ignore the law."
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The UK research found a 44 percent year-on-year increase in injuries in low-speed zones after they were expanded to cover all residential streets in cities like Manchester, Edinburgh, Leeds and Birmingham, and a 53 percent increase in deaths.
"Imposing blanket 20m/h zones simply means that drivers ignore them all, including the roads where you really need lower limits like on narrow residential streets or outside schools," UK Automobile Association president Edmund King told The Times.
Auckland Transport's proposal covers the CBD from the Northern Motorway in the west, the Northwestern Motorway in the east, and as far south as Spaghetti Junction. Town centres across the super city like Mission Bay would also be reduced to 30km/h.
Mr Matthew-Wilson said it would be better for city planners to focus on "traffic-calming" solutions such as speed bumps, chicanes and narrowing streets - this way drivers have to slow down, regardless of what any sign says.
Patrick Morgan of Cycling Action Network says evidence from Christchurch's first two years of having a 30km/h CBD speed limit proves it works - but agrees that traffic-calming measures are just as, if not more important.
"There does appear to be a benefit when you lower speed limits by themselves, but accompanied by changes to how the streets look, education and enforcement, you get much more benefit," he told Newshub.
Christchurch lowered speed limits in the city to 30km/h in March 2016. A comparison of the two years before and the two years since found a 25 percent reduction in crashes that resulted in injury, and a 36.5 percent reduction in reported injuries themselves.
In contrast, in streets around the CBD which remained at 50km/h there was a 13.5 percent increase in crashes and only a 5.4 percent reduction in injuries.
"No one's saying speed reduction alone is what we need to do to reduce the risk on our roads, but it's the closest thing we've got to a silver bullet," said Mr Morgan.
"The evidence is overwhelming that speed limits of around 30km/h, when there lots of people on foot and bikes, are definitely effective in saving lives. No question about that."
Wellington experienced similar results. A 2014 report found injury crashes dropped by 82 percent in suburbs with a 30km/h limit, while they rose slightly elsewhere, Stuff reported in 2015.
Mr Matthew-Wilson and Mr Morgan agree that separating cyclists and drivers would be the best option. As the former put it, "Cars have airbags. Pedestrians and bikes don't."
Auckland Transport has been dedicating more roading space to cyclists lately, but not always with successful results.
Plastic poles installed along lanes in Sandringham and Mt Albert last year to provide a physical barrier between motorists and cyclists have been abandoned after most of them were destroyed by wayward drivers.