Car Care: Best policy is to always drive to the conditions
Recommended speed for taking highway bends has been determined by thorough NZTA testing
Have you ever wondered how those advisory speed recommendation signs when approaching and taking certain bends on our main roads are calculated?
Peter has asked the question after a trip from Auckland to Wellington. He found he could consistently and comfortably take all the bends at slightly higher than recommended speed.
This started him thinking about how those speeds were calculated. Is there any in-depth science or geometry used to calculate each bend’s individual curvature, are the speeds set to strike a balance between wet and dry weather conditions, or is it done by somebody driving the various routes and simply making a recommendation on the appropriate speed?
We asked NZTA media spokesman Andrew Knackstedt. His explanation:
“The advisory speeds on curves are actually based on ‘comfort criteria’ which roughly relates to limiting values of side friction of 0.21g (g is the gravitational constant) for speeds less than 35 km/h, 0.18g for speeds between 35 km/h and 55 km/h and 0.15g for speeds greater than 55km/h.
“The appropriate speed is determined using a device called a ball bank gauge (effectively a ball bearing in a circular tube).
The particular curve is driven at a range of speeds and the ball bank reading is plotted on a graph which gives the appropriate advisory speed which is reported to end on 5km/h e.g. 15, 25, 35, 45, 55….85, 95 km/h.”
So no guesswork involved. Each bend is tested individually and driven multiple times by the sounds of it to come up with the appropriate speed for all motorists in all weather conditions.
Although many motorists like Peter can, and do, consistently drive quite safely in excess of the advisory speed limits, we shouldn’t forget there are plenty of situations where those recommendations should act as a driver alert to potential danger.
Light vans travelling with little to no extra weight in the rear, motorhomes, towing regardless of the actual extra load, driving with under-inflated tyres and travelling in poor weather conditions such as ice and snow, or after heavy rain, are when those advisory speeds should be taken a lot more seriously.
For drivers who use cruise control regularly on the open highways, some caution should be exercised also. On a fine day it may be easy to drive most bends above the advisory speed limits with cruise control on. But there will be occasions when it is better left off — especially if there are a number of bends and advisory speed warnings in place.
Sudden braking to reduce speed once you have entered into a corner is when a vehicle can very easily and quickly start to lose control.
Vehicle design is also another factor. In theory, a low-slung sedan will always corner better than an SUV of similar age and size.
Advisory speed limit signs should act as subtle warnings and drivers are wise to negotiate each bend with the conditions and traffic flow in mind.
I’m sure there are many drivers who get annoyed at drivers who slow for every bend unnecessarily, then speed up once on a straight. It makes overtaking difficult and risky at times.
If you are one of those drivers who don’t travel the open roads very often and get a little nervous when approaching bends, keep an eye on that rear view mirror and let other traffic pass whenever possible.