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Hits and misses of Kenya’s speed limits
Max limits are set in part to prevent excess and to give everyone the same benchmark so they tend to travel at or near the highest speed they are allowed, so all travel at similar speeds.
Based on some strong themes in DN2 Motoring, does Kenya have the right speed limits for optimum safety and traffic flow? AG
The limits we already have are generally okay (though not always optimal), but one is missing! Its absence is the prime cause of delays and potentially dangerous disruption in the traffic system. What we lack is a minimum speed limit on highways.
The scientific certainty is that traffic is most efficient and safest if it flows. The exact speed at which it does so is secondary. And somewhat arbitrary.
Max limits are set in part to prevent excess, but more importantly, to give everyone the same benchmark so they tend to travel at or near the highest speed they are allowed, so all travel at similar speeds.
On the main roads of much of the world, the near-max speed is also the ambient speed! By and large, traffic - all of it! - flows like a conveyor belt.
Indeed, roads and cars are designed on that basis. And that pattern works very efficiently…and relatively safely. No one is held up, there is very little overtaking, and because conflicting interaction is minimised, collisions are rare. Journey times, even over long distances, are predictable to within a couple of minutes. And rapid.
This would still work if some of the cars went faster than the speed limit when conditions allowed. However, it does not work if a significant proportion of the vehicles travel at a much lower speed than the ambient flow.
So in many places, there are also minimum speed limits and, even where these are not legislated, mobile patrols are quick to side-line any “crawlers” obstructing the flow and thereby causing tailbacks and forcing incessant overtaking.
This is so important – to both flow and safety – that any vehicle that is not capable of maintaining the ambient speed is forbidden to use the big highways.
You will recognise where Kenya fits in that context, and legislators have tried to cope with our huge traffic speed disparities (and relatively limited road infrastructure) by setting a modest maximum speed – for any vehicle on any road – of 100 km/h and downgraded that to 80 km/h for large or less agile classes of vehicles, and PSVs.
That’s a narrow spread that theoretically leads to an adequate ambient speed. But how does that work – how can it possibly work - if a large proportion of the traffic rumbles and wheezes along major highways at 40 km/h, and as slow as 20 km/h where the road climbs a hill?
It doesn’t work. Huge tailbacks are generated, and any intercity journey becomes a dangerous succession of overtaking manoeuvres; journey times get longer and longer and are unpredictable. There are no winners by this formula. Everybody suffers.
Why do so many of our trucks, and some other classes too, travel so slowly? As no one designed a truck unable to cruise at 80 km/h and even climb moderate hills at 60 km/h (an arguable “minimum speed” limit) those that cannot maintain even that modest ambient speed are being put to work they were not designed to do, or they are grossly overloaded, or they are mechanically un-roadworthy.
Whether or not a minimum highway speed of 60kph (when external conditions allow) is formally legislated, it still offers a rational guideline on other policy decisions - what is and what is not allowed to use major roads; type approval on the balance between engine size and rated load capacity; the teaching and enforcement of multi-lane disciplines; intervention by mobile traffic patrols; and so on.
