I don't think I have seen a speed camera on a bit of open motorway. They only tend to be where there are speed limits in urban area's and around roadwork’s.
You won't see many permanent cameras on 70mph stretches of motorway, but that doesn't stop the Think! van from sitting on the bridge when they realise it'll net them more money than actually stopping dangerous speeding outside schools etc.
In the 40 roadworks (why so often 40 now when it used to be 50?) SPECS cameras tend to be the norm. The truckers usually start their bully-boy tactics once you've passed the last SPECS camera but not yet reached the NSL sign.
My point was where there is a defined speed limit - 20, 30, 40, 50 then it’s simpler to just stick to the speed limit and I think you will find that my rule works every time because this is were the congestion is, also your multiplier rule does not work in these instance (e.g. 10 miles at 30mph is 20 mins, 10 miles in a 30mph zone at 45 saves you about 5 to 6 mins at best on a clear run). These are the environments the speed camera’s are in.
On a single-carriageway road, I'd say you're right. Though given a clear shot, your time savings of doing 40 in a 30 are even greater than doing 80 in a 70. Still, I agree that 5 minutes is neither here nor there on a 20 minute journey.
I tend to stick to the 30 limit in genuine residential and urban areas these days. I must admit that I still tend to do about 40 in commercial areas with 30mph limits, as there are no kids or little old ladies to run over there, and I consider the 30 limit to be inappropriate. Once I hit a non-residential dual carriageway, I'll generally do about 10 over the limit and keep an eye out for cameras.
Also, I think you are deluding yourself if you think your average speed over a whole 280 mile journey from door to door is 70 mph. To get an average speed of 80 MPH on an average 280 mile journey from door to door with 5 or 10 miles of urban travel at each end would mean you would have to be doing well over 100 on some sections of the route
. I find it hard to get averages over 60 MPH on such journeys even when I have been “tonking on a bit” in between - fuel computers are wonderful things they really do make you take a fresh look at you driving habits particularly when you check your averages.
Sorry, but I'm not being delusional at all
. It depends on the run and how you time your journey. I can drive from Chester to Glasgow and do 90+% of the journey on cruise control. If you want to split hairs, then of course you can't do the whole journey at 80mph, but 90+% will do for me. The point is that my whole journey involves several hours of motorway travel. If I've spent 3.5 hours cruising at 80 on the motorway, then I've still saved my half hour over sticking to the speed limit regardless of how slow I end up going at either end of the journey.
I have been driving for 25 year and in the days before speed camera have done virtually every speed you can imagine (in a standard road car) and this is just experience talking. By driving fast you just think you are getting there hours before everyone else when in fact its only minutes at best, hit a bit of congestion and everyone arrives together – the only difference then is the number of points on your licence.
I've been on the roads for about 20 years, and in that time, I've clocked up over half a million miles. Trivial by a trucker's standards, but more than a lifetime of driving for someone who does the average 8-10K per year. As you can imagine, I spend a lot of my driving time on the motorways.
I don't kid myself that I'm getting there hours before. I've done the maths and can see a half hour saving on the type of journey I often do, just by sticking the needle on 80 instead of 70. As I previously said, if that half hour makes the difference between me getting home before the start of the rush hour or getting stuck in the middle of it, then the saving is magnified considerably.