On 2004 Jul 19 , at 5:16 PM, Chris KEENAN wrote (in response to my question about the 112 km/h speed limit):
It's the conversion from 70 mi/h (which is also what it is here in the UK) -
the speed limits haven't yet gone metric; when they do, they will be changed
to round metric values.
I accept Chris's explanation but want to make a couple comments.
I had thought of that possibility but wasn't sure the other speed limits listed were all round metric values that had been converted from the original round English unit values. Those values were:
1. Speed limits
• Freeways/Motorways 112 km/h (80km/h towing)
• Regular non-urban roads 100 km/h (80km/h towing)
• Urban areas 48/50 km/h
When I checked more carefully, I found that they probably were, as follows.
Back-converting to mph reveals:
112 km/h = 69.61 mph
80 km/h = 49.72 mph
100 km/h = 62.15 mph
48 km/h = 29.8 mph
50 km/h =31.07 mph
That could suggest that the original mile per hour speeds were:
70,
50,
60,
30 and 30,
respectively.
If so, the "forward" conversions would have been:
70 mph = 112.6 km/h (rounded to 112)
50 mph = 80.45 km/h (rounded to 80)
60 mph = 96.54 km/h (rounded to 100)
30 mph = 48.27 km/h (rounded to 48 for some cases and to 50 for others?)
I'm mystified by a couple of aspects of this.
One aspect is the inconsistency of the rounding. The value of 112.6 was rounded to 112 instead of the "rounder" 110, while 96.54 km/h was rounded all the way up to 100 instead of the nearer approximation of 96 or 95. And the value of 48.27 was seemingly rounded to 48 for some uses but to 50 for others.
The other aspect is the apparent possibility that the signs can be greater than the legal limit. If the official legal limit (still in old English units) is 60 mph which converts to only 96.54 km/h, then the newly posted limit of 100 km/h is too high, higher than the speed set by law. Similarly, the one conversion of 30 mph to a rounded 50 km/h is too high since 30 mph is only equal to 48.27 km/h.
In the US, at least, officials are not allowed to round up to anything greater than the official legal speed. When our 55 mph national maximum was in effect and some states posted metric equivalents, they were not allowed to round to 90 km/h because that would have been greater than the official 55 mph (which equals only 88.50 km/h). So they were required to post the metric speed limit as "88 km/h". (The could round down but not up.) The converted value began to be known as "crazy eights".
Perhaps British officials have greater leeway in making such decisions and adjustments than do their American counterparts.
Regards,
Bill Hooper
Fernandina Beach, Florida, USA
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Make it simple; Make it Metric.
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