Manufacturing Traffic Jams

As regular readers know I am a frequent car driver, topping 320 Km per week. Whilst this is nowhere close to the distances covered by truck drivers or even the travelling salesmen left in business, it does mean I spend quite a lot of time on the road. In the past it used to be the M25 and these days it is often the M4. Both are very busy motorways in South-East England (and if you think that no motorway is ever not busy, I suggest going along the M40 or the M69). The traffic along them is often congested. The western arc of the M25 seems particularly worse than it used to be, a factor I put down to so many people like myself commuting in on a daily or weekly basis from South-West and West England and the West Midlands to work in London. What exacerbates the congestion on the M25 and now increasingly on the eastern Berkshire (I know that part of the county has been fragmented into a number of consolidated authority ‘city-states’) stretch of the M4 is the variable speeds approach. This is explicitly warned about on the M25 but also now occurs on the M4.



Now, I am a person who adheres strictly to road speed limits and I have no issue about speed cameras. However, what I do not like is how the speed limit is jerked back and forth on the M4 and M25. The speed limit for a motorway is 70mph (112 kph) but the matrix signs daily alter this down to anything from 40-60mph (64-96 kph). Often you have ‘Queue caution’ appearing as well. This is ironic, because as I have discussed before: http://rooksmoor.blogspot.com/2009/06/rubbernecking-far-less-common-than.html  putting up any sign on the motorway causes people to slow down automatically so creating the queue that they were seeking to warn people about. The other day I actually saw a sign that said ‘Strong winds forecast’, not that they were happening then, they might as well have said ‘Long-term recession forecast’ in terms of how much it may have modified the average driver’s behaviour. The signs appearing saying ‘Fog’ always strike me as a waste of electricity. Anyway, what you find is that you are slowed down, sometimes jumping from 70 mph to 40 mph and then back up to 50 mph or 60 mph, or any combination of these speeds. What this tends to do is not actually moderate the flow in the way that I assume the Highways Agency intends, it simply creates a jam wherever the signs start.


What is the expectation of the ‘Queue caution’ sign? One might expect people to slow up so that they do not crash into the unmoving cars. They would slow up even if you put a smiley face on the matrix boards. In addition, motorways are designed so that you can see far into the distance. Particularly at this time of the year you can see the cluster of red lights long before you come close to them. Is there an assumption that we see a queue warning sign and decide to leave the motorway and try an alternate route? There are a few places where not going down the motorway will get you to your destination faster than using the motorway. I know that most days it is easier to get into Dorset via the single carriageway roads through Newbury and Salisbury than going along the motorways and dual carriageway route through Winchester and Southampton: M3-M27-A31. However, most of the time, taking the motorway is the most direct route, especially going into a city like London. The whole reason why motorways were invented was to move vehicles between cities more effectively so it seems ironic that we are now being encouraged to leave them.


Of course, not all of the blame can be put on whichever human or computer decides to put these random speeds into the mix, some of it depends on the drivers too who simply exacerbate a difficult situation. Many drivers on the M4 simply ignore the signs and keep going as fast as they can until they actually come to the start of the jam. There will always be those, like myself, who obey the speed limit immediately and the bulk of drivers who will not seek to match the limit but will simply lift their foot off the accelerator as they process the information. There are also those who speed along the hard shoulder with their emergency lights flashing away pretending that they are having a break down right up to the next junction. The other major challenge is that so many drivers see the queues as a good opportunity to switch lanes believing that in doing so they will suddenly get clear of the jam. This is particularly troublesome when an articulated lorry decides it is time to switch lanes and so a space the length of five cars is forced to open up. All of this behaviour simply adds to the jam.


I am not saying that without variable speed limits jams on motorways would not come to an end. However, it is clear that without the signalling of queues and the constant slowing down and speeding up on the M25 and M4 and probably other motorways I do not frequent too, the queues would be shorter both in length and duration. This is apparent, when suddenly for no reason the queue breaks up and you find yourself travelling again at a decent speed. If you can see an accident by the roadside you know the course, but these days more often it is simply the exclamations about queues which is creating these things and you then find the road is perfectly clear and that there is, in fact, no reason why you could not have been driving at 60-70 mph for the past two miles. My father, among others, feels that variable speeds on motorways is a game being played by bored staff at some traffic monitoring centre. I agree with him on very few things, but for now, these seems as rational explanation for this irrational behaviour as any other.