Junk Maths from NZTA

The Herald reports:

A Waka Kotahi NZ Transport Agency claim that a speed drop of 20 kilometres an hour on an 83km stretch of highway would delay commuters by an average of one minute has been labelled as “junk maths” and “disingenuous” by a traffic engineering specialist.

But as anger in Hawke’s Bay grows about the claim, the agency is standing by it.

State Highway 5 from Esk Valley in Hawke’s Bay to Rangitaiki, near Taupō, is one of New Zealand’s most dangerous highways.

After 250 injury crashes, and 16 deaths, Waka Kotahi NZTA decided to propose a blanket speed limit of 80km/h on the 100km/h stretch of road. Submissions on the proposed changes can be made up until 11pm on May 9.

In a Hawke’s Bay Today Talking Point on Saturday Waka Kotahi’s Emma Speight wrote:

“Our technical assessment of the road found that although the posted speed limit on SH5 between Rangitaiki and Esk Valley is 100km/h, the mean speed that people travel at is 81km/h.

“This means lowering the speed to 80km/h will increase the average travel time by less than a minute.”

This is appalling use of maths, and NZTA should be ashamed.

The NZTA claims would only be valid if motorists didn’t just drive at an average of 81 km/hr on the road but drove at that exact speed for the entire journey. That at no time did they ever ever drive faster than 81 km/hr.

That is of course nonsense.

Let’s take a simple example. If the average speed is 81 km/hr the 83 km trip would take 61.5 minutes on average.

If half that distance you drive at the maximum speed of 100 km/hr that takes 25 minutes. The remaining 41.5 kms would take 36.5 minutes so average speed for the rest would be 53 km/hr.

If you lower the speed limit to 80 km/hr then the 41.5 kms that took 25 minutes would now take 31 minutes, so the increase is six minutes not one minute.

That’s just a simple example. The actual way you shiould measure it is very simple.

Drive a car from one end to another where you drive at the speed a standard car handles safely for each stretch of road and never exceeding 100 km/hr. Then repeat the exercise but never exceed 80 km/hr. The difference between the two is what the time impact will be.

If NZTA can’t do such a simple calculation but instead relies on junk maths, that is a huge worry.

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