When the rains came

Tina and I got back from the Isle of Wight two days ago.  That may seem an odd way to start the blog, because it will not mean anything to many readers, so I must explain.  The Isle of Wight is a smallish island just off the south coast of England, famous for hosting sailing at the town of Cowes, and for the unspoilt countryside.  Over the last few days, it has been in the news headlines because of the Isle of Wight Music Festival.  This is an annual open-air festival, attracting 55,000 people for three days (Friday, Saturday and Sunday) to hear excellent music and join in the event in parks and fields just outside the main town of Newport.   Only this year, things went wrong.  The ground was wet, it rained very hard on Wednesday and Thursday and some of the campsite was flooded.

And the car parks were so wet that normal cars had difficulty manoeuvring (so many cars needed assistance from tow vehicles).  Because traffic was entering the festival site more slowly than expected, it started to back up onto the main highway, and it backed up more and more.  Before long, there was gridlock around the festival site.  Local people and businesses were affected as well.
As holidaymakers on the island, not there for the festival, we managed to keep away from the gridlock, but there were what the press call "horror stories" of people sleeping in their cars, and the ferries to the island could not dock because there was no way to clear the cargo of vehicles once they reached the island.  On Friday, various plans were implemented to clear the gridlock and sort out the worst of the problems, with considerable success.

Over the weekend, an emergency plan has been put into place to clear the site (here) and it seems to have worked.
As an observer, almost on the spot, I wondered what lessons could be drawn from the experience of the arrivals to the festival, who met the gridlock.  I imagine that one could devise an interesting case-study along the following lines, drawing on three strands of O.R. methodology.
  1. There was a queueing aspect.  How do you get vehicles off the highway and into the festival site?  Each one has to leave the traffic on the highway and enter one of the access gates.  The more gates, the more servers (in queue parlance).  However, the rate of service changed because of the state of the ground in the festival campsite and car parks.  Each service became several times as long as it would have been under "normal" conditions.  In consequence, the queues for service grew, because one of the first lessons that you learn in a course on queues is that the average service rate multiplied by the number of servers must exceed the average arrival rate, otherwise the queue for service grows and grows.
  2. There was a network aspect.  The highways of the Isle of Wight form a network.  Each arc of the network has a number of properties, including a capacity for vehicle flow.  In a course on networks, one could discuss the capacity of the network as a whole - assuming that there was free flow in those arcs and through the nodes.  Unfortunately, the principal highways close to the festival converged on one roundabout (at Wootton Common).  Traffic from one ferry came to the roundabout from the east, traffic from another ferry came from the north-west, and the road to the festival was the third arm of the roundabout, to the south-west.  Local traffic also needed to use these roads (including, sadly, traffic for services at the crematorium close to the roundabout).  The lesson one could draw for this is that network flow models are useful - up to a point, but do not work when the assumptions about them fall down.
  3. There could be a simulation aspect.  It would not be difficult to build a simulation model of the traffic on those three highways, and investigate what happens when the service rate at the festival site changes.  And that model would demonstrate how the congestion would spread around the vicinity of the event, and how gridlock would follow.  And such a model could be extended to test alternative strategies in case such weather-related problems happened again.


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