Wikipedia:Reference desk/Archives/Miscellaneous/2016 February 4

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February 4

Speed limiter

What I don't understand, and the article doesn't say, is why the speed of HGVs on motorways should be limited so much below the speed limit. So, erm, why are they? --Dweller (talk) 14:39, 4 February 2016 (UTC)

Not sure if you want references or just an explanation, but if it is the latter, I think the government justifies it by using statistics, which shows that accidents at high speed involving HGVs are more likely to cause death than those involving only cars. Hopefully someone will have the time to research and give you references if that is what you expected. --Lgriot (talk) 14:53, 4 February 2016 (UTC)

This page gives stopping distances for LGVs (fig2) and cars (fig3); stopping distances, and braking times, are much greater for the large vehicles. -- Finlay McWalterᚠTalk 15:01, 4 February 2016 (UTC)

Thanks. I wonder how much analysis they've done of the relative risks posed by people taking stupid risks around slow-moving lorries on motorways. --Dweller (talk) 15:08, 4 February 2016 (UTC)

Yes, they need to study the totality of the system, not just accidents directly involving the vehicles themselves. Having all vehicles moving at a similar speed definitely has safety advantages, as less lane changing and passing occurs, and when it does, it can be done more leisurely, taking time to find the safest place to pass. This is the reason why many freeways in the US also have a minimum speed limit, in addition to a maximum. Having separate lanes for slow moving traffic also helps, but this either requires adding new lanes or causes traffic jams by making those lanes unavailable to the rest of the traffic. StuRat (talk) 16:52, 5 February 2016 (UTC)

The RAC Foundation says "Although motorways carry around 21 per cent of traffic, they only account for 5.4 per cent of fatalities and 4.7 per cent of injured". The same FAQ (A9) says "Failed to look properly was the most frequently reported contributory factor and was reported in 44 per cent of all accidents." Taken together, that doesn't suggest HGVs on motorways is a major safety issue. List of countries by traffic-related death rate show that the UK has the safest roads of pretty much every developed country and Reported Road Casualties Great Britain shows that number has been declining since the 1960s. That latter article indicates that only among cyclists and pedestrians is the decline in jeopardy, which to me suggests safety engineering effort should be directed to low-ish speed urban collisions between vehicles and pedestrians and cyclists, and not to motorways. -- Finlay McWalterᚠTalk 15:36, 4 February 2016 (UTC)

The fact that motorways are safer than other roads has no relation to the question of whether HGVs on motorways are more dangerous than other vehicles on motorways. Nor does failure to look being the primary cause, since that doens't distinguish between an HGV driver failing to look and a moped driver failing to look. I was also under the impression that the HGV speed limit was as much about emissions as safety (since they are generally more efficient around 60mph than around 70mph). MChesterMC (talk) 16:29, 4 February 2016 (UTC)

(EC, my first and last refs support Finlay's suggestion - speed limiters do seem to help, but cyclists and pedestrians are over-represented in road fatalities.) Looks like more and more comparative analysis is coming available. Here is a nice study that says they can't assess the speed limiters yet, but it's still interesting reading with nice relevant figures on safety [1]. This book chapter [2] has some data but still complains about paucity. This [3] is a comprehensive review, with more good refs and some survey data on limiters, and this [4], assesses the fatality savings of many safety measures, including speed limiters. SemanticMantis (talk) 15:47, 4 February 2016 (UTC)

I don't believe the limiters are "much below the speed limit". Limiters are set to 90 kph, or just over 56 mph. The speed limit on motorways is 60 mph. So they are slightly below the speed limit.--Phil Holmes (talk) 09:45, 5 February 2016 (UTC)

Precisely. The national speed limit for HGVs is 60mph on multiple carriage ways (dual carriageways and motorways) and 50mph on single carriageways. So not 'so much below'. Also, the article does say why, "harmonisation within the EU".--Ykraps (talk) 10:40, 5 February 2016 (UTC)