Improvements over the current system at City Care

As City Care’s current mowing rounds are calculated solely on distance, comparisons with the rounds created by the GRASS program are limited. We were, however, able to compare the relative performance of our 8-hour daily rounds with one of the single 40-hour weeklong rounds created by City Care. It showed that having daily rounds could reduce total travel by up to 12%, a significant saving in terms of fuel costs.

As you can see below, this particular weekly round created by City Care was found using Travelling Salesman Problem (TSP) software for all the locations in the south-east of Christchurch to which a Gang mower has been assigned:


Single Weeklong Route

Figure 1. Single week-long round constructed by City Care


When the mower operators are given a week-long round such as this one, they travel as far as they can along it each day of the week (up to eight hours) then return to the depot in the evening. The following morning, they return to the point in the round at which they had finished the previous evening. This translates to five daily rounds, which we constructed given the estimated mowing and travelling times:


Daily Routes from Weeklong Route

Figure 2. Division of week-long round into daily rounds


Given the same locations (each assigned equal urgency) from which to construct five daily rounds, each with a maximum length of eight hours, our GRASS program found a solution which contained 12% less travelling time than the solution given in Figure 2.


GRASS Daily Routes

Figure 3. Daily rounds found by the GRASS program


While this test was only undertaken for just one of the rounds compiled by City Care, it must be noted again that comparisons are largely irrelevant due to the use of other criteria, over and above distance, used by the GRASS program in creating the daily rounds. However, savings are likely to be of similar scales for the other rounds and other types of mowers. This is because the depot is taken into account for every round every day, instead of just once at the start and end of each week.



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Paul Stewart & James Tipping, November 2002