How did Waka Kotahi calculate the travel time increase for the Hawke’s Bay Speed Review?
Since the launch of the Hawke's Bay Speed Review consultation people have raised questions about the likely increase in travel times on SH5, if the proposed lower speeds are in place.
When calculating travel times we have several research-based findings we can apply, which together provide a range of likely impacts on travel time. There’s not a single answer because every person uses the road differently.
That’s why we work with mean speeds which are averages, and these are based on the assumption that people are driving at or below the posted (and proposed) speed limit.
We measure travel time increase in two ways to find out what the range of the estimated increase will be. In our initial consultation we predominantly applied commentary around method one (see below), with some conversation on method two.
The re-calculation applies method one to data from a different source, that is significantly more detailed. We believe that the more detailed data will provide a more accurate representation of what the average person will experience driving the length of the corridor.
Method 1: the time difference between the current mean speed people are travelling and the proposed speed limit.
This method assumes that where people are currently driving above 80km/h, they will now drive at 80km/h and where they are driving below 80km/h their speed will remain unchanged.
The data we used when we first launched the consultation collected data over a two week period and broke it down into four similar sections. Applying method one to this data showed a very small increase of under a minute.
In our re-calculation of the likely impact on travel times we have used a different data source, a longer period of three months and we broke the corridor length into smaller segments of 2km. The smaller segments better reflect the different typography of the road. Because of this we expected some variation in the outcome.
Our additional analysis does have a different result. The more detailed data suggests the increase in travel time northbound will likely be about 3.5 minutes, and 4 minutes southbound.
Method 2: The time difference between the posted speed limit and proposed speed limit.
Based on the second methodology the increase between the posted speed limit of 100km/h to proposed speed limit of 80km/h the increase would be just over 11 minutes for the entire proposed length of 76km. This method works on the assumption that people are driving at the posted (and proposed) speed limit over the entire length, including the very windy parts, of SH5.
Taking into account these two methods and our re-calculation we believe that people will experience between 4 to 11 minutes increase in journey time on State Highway 5.
Find out more on how we calculate speeds.