Variable speed limits? Totally random would be a more accurate description

So far, I have managed to resist the temptation to go into full-on, ‘grumpy old man’ mode on this blog, but today I’m going to smash my self-restraint

I started work in Bristol at the end of July, meaning that I now commute from Cardiff to Bristol early in the morning (and complete the return journey after the conclusion of the evening rush hour) on Monday to Thursday of each week. It’s a round trip of 74 miles and usually takes about 45 minutes each way. It’s a reasonably pleasant journey, accompanied by the Today programme in the morning and Radio 2 in the evening – but I digress

The thing that I have found increasingly irritating in the past six weeks is the seemingly totally random operation of the variable speed limit system that operates on the M4 around Newport. I understand the principle – sensors in the road detect traffic flow and average speeds and algorithms in the software controlling the system then adjust the speed limit to maximise traffic flow and reduce delays. I applaud the aim

The problem is that the system appears to operate on a whim, with speed limits often changing apparently arbitrarily from gantry to gantry, and all apparently with no regard whatsoever to actual traffic flow or road conditions. An example from yesterday might help illustrate the point : travelling east towards the Brynglas Tunnels at 6.35am, the first gantry in the system displayed a 60mph limit – traffic was light and traffic flow was good; as we moved up the hill past High Cross, traffic built up and began to slow to an average speed of probably 50mph, but the next gantry was displaying the national speed limit sign, and this continued all the way through the remaining section, including the run up to the Coldra interchange where congestion was heavy and the average speed fell to 40 or 45mph. Returning home last evening, the whole section of the motorway from Coldra to Brynglass was set at 50mph, but there was literally not another car ahead of me on the road for at least half a mile, and there was no congestion at all even in the approach to the tunnels (something that has been almost unprecedented in my experience of travelling at this time so far)

The upshot of this is that people who now regularly travel the route have clearly lost all confidence in the variable speed limit system and largely ignore it, choosing to travel at a speed which is appropriate to the actual traffic density and weather conditions at the time. Bizarrely, the system appears to actually CAUSE congestion because some drivers respond to the random limits imposed by breaking sharply from (for example) 70mph to 50mph, to comply with the overhead gantry signs causing vehicles behind to slow down and/or move out to overtake them, and leading to an overall ‘backing-up’ of traffic where there would otherwise have been none

I don’t know what the statistics are on comparative accident and delay rates on this stretch of road since the introduction of the £20m scheme (and it would be interesting to see them), but it would be fascinating to know whether schemes of this type have been evaluated scientifically. My personal experience would suggest that where there is an apparent disconnect between the system and the road conditions that it’s supposed to be regulating, then its impact on actual driving behaviour is negligible


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