The Stonehenge Effect - Why Not All Traffic Jams Are Equal - A Soliton For the Masses

This weekend was set to be the first sunny weekend of 2014 - and only a week away from Easter.  We were expecting problems with the volume of traffic and jams westbound on Friday and Saturday morning and we anticipate more heading eastbound this evening.  However, the jams have been a little odd, to say the least.  On Friday afternoon,  we had the anticipated westbound tailbacks at the Countess roundabout in Amesbury, but traffic through Winterbourne Stoke was steady and not excessively heavy.  Strangely, the Highways Agency camera that is located near to the Solstice Park in Amesbury was out of action.  Strange that; it seems to happen an awful lot when the A303 is busy.  "Conspiracy " you cry!  Well, you might be right, the Highways Agency can't always be allowed to get away with the excuse of coincidental equipment failure or incompetence.  Perhaps they really are switching off cameras that might cause them embarrassment?

Expecting heavy westbound traffic, SWMBO left Winterbourne Stoke at 4:00pm on Friday - often a peak jam time - and drove to Barnstaple in North Devon.  She later reported that the drive was easy and traffic was very light.  Odd, given the massive tailbacks east of here

Eastbound traffic wasn't playing ball either.  From late morning to late afternoon, the eastbound traffic was virtually at a standstill; tailing back as far as the A36 interchange at Deptford.

Eastbound traffic at Winterbourne Stoke

Spire FM, our local radio station was expressing a degree of surprise as no-one appeared to know what was causing the jam.  Curiouser and curiouser.

On Saturday morning, we held a STAG meeting to discuss activities in the run-up to Easter; a period we expect to be hellish as it is going to be the first potentially warm and sunny Bank Holiday since the new Stonehenge Visitor's Centre opened and the half-baked changes to the Longbarrow Roundabout completed.  We agreed to take a look at the A303 at Stonehenge Bottom and in Winterbourne Stoke and leaflet the standing traffic if the opportunity arose.

When we got back to Winterbourne Stoke from Shrewton, we were staggered to see that there was so little traffic to be seen on the A303.  We really though there had been an accident and we checked all our usual sources of information.  No, nothing.  Puzzled, we fired a text message to Stagman and Watch Bustard to tell them that traffic was light.

Saturday Afternoon in Winterbourne Stoke - Where's the traffic?

Seconds later, we got a somewhat incredulous reply from Stagman who was by the side of the westbound A303 opposite Stonhenge Cottages - just to the east of Stonehenge Bottom.   Stagman pointed out that he was, as we exchanged text messages, wandering between two lanes of standing traffic distributing STAG leaflets.   Good to see that he was purposeful in this activity as otherwise he might have been accused of ganderflanking - we can't be having any of that can we?

Compare what was going on at Winterbourne Stoke above to the situation at Stonehenge Bottom at exactly the same time:

Westbound A303 - Stonehenge Hoves Into View

 The view back towards Amesbury looked like this:

Westbound Traffic and the Stagmobile

Don't worry, Stagman's sign is double-sided, so the westbound travellers could see it and passengers could snap the QR code on their mobile phones and get linked to the STAG petition.

We were so STAGgered by the contrast that we recorded 30 minutes of westbound A303 traffic as a video.  We've either speeded it up a wee bit or most of the traffic was travelling at over 400mph.  Either way, the clip only last about 18 seconds!

Clearly, something odd has been happening.  Despite it being a fine and sunny weekend, it seems the level of traffic on the A303 has been surprisingly light; certainly much less than we had anticipated.  We expect that we are going to see that sort of traffic density from mid-Maundy Thursday into next weekend.   So this odd phenomenon over the weekend seems to be the first confirmed case of the "Stonehenge Effect".

So what is the Stonehenge Effect?  Well, it seems that the closure of the A344 and the "visit Stonehenge by timed appointment (preferred) policy" has driven the majority of those who used to stop on the A344 for a few minutes, or those who might pop in for a 30 minute look-see, to do something else.

Those of you who regularly use the A303 know exactly what they do.  They get parallel with the stones, just west of Stonehenge Bottom, they slow, or even come to a complete stop, then a passenger or even the driver will whip out a smart-phone or camera and take a snap - or even a selfie!   "It's only a second or ten," they would plead if stopped by Wiltshire Police's finest.  "What harm can it possibly do?

Well, dear traveller.  It seems that you have been the victim of something called a soliton - a self-reinforcing solitary wave.  It's a well recognised phenomenon on roads and motorways that is also called a traffic wave, a stop wave, or a traffic shock.  It's usually created when some muppet brakes hard in traffic for no apparent reason, or changes lane without indicating, forcing traffic behind to brake as well.  Now cars can join a queue of standing traffic faster than they can leave the queue, so normally the soliton travels back down the road in the opposite direction to the normal traffic flow as shown in the diagram below.  The blue bulge represents the volume of traffic on a "normal" road.  Here the traffic is trying to go from right to left.  The big red arrow shows that when the soliton forms, it moves against the traffic - once the event that caused. it in the first place has ended.

However, at Stonehenge Bottom, we have a nightmare situation.  Lots of similar soliton-generating events are happening at exactly the same point on the A303 - by Stonehenge.  In this case, once the soliton has formed, the start point stays exactly where it was and the end keeps on travelling back against the traffic flow.   It should be fairly obvious that a major jam is going to form even with relatively light traffic and that is why we have been having problems all winter.

Soliton's also form when sheer volume of traffic blocks the A303 at choke points or turnings - such as the ends of each dual-carriageway section of the road where traffic is compressed back into a single carriageway.  When the westbound traffic is really busy on the A303, a second soliton is often formed west of Winterbourne Stoke near Deptford and the rear of the soliton moves back east towards Winterbourne Stoke and collides with the leading edge of the Stonehenge soliton.  The result is a massive and persistent jam - such as the ones that will begin next weekend.

What can be done about it and how can we stop the soliton's forming before the A303 is dualled?  Fairly simply, maybe.  Traffic management, pure and simple.  The best way to achieve this is probably counter-intuitive, but it would simply to be to turn the A303 back into a single carriageway road, on all the dualled sections, from Thursday lunchtime to Monday lunchtime.  That would get rid of all the 2-into-1 soliton's.  The Stonehenge soliton can be prevented if the traffic is kept moving continuously through the World Heritage site.  The best way to do this would be to prevent cars from seeing the stones.  If this can't be done, because it conflicts with World Heritage Site aspirations, etc., then the section of road must be policed effectively with the full economic costs of doing this falling to the Department of Culture, Media and Sport.

So, we now have another meaning for our SH2 symbol - in addition to parasitising HS2 with its meaning of "Stonehenge Dual-carriageway" it now can also mean "Soliton Here As Well!"