Tuesday, 4 January 2011

Using Google Earth for something useful for a change.

Today I had an almost new pupil in West Kirby. Since we were having a double lesson, I decided to take him to the Croft industrial estate in Bromborough.

So onto the M53, and off at the Clatterbridge junction, then down through Spital to Bromborough.

Did lesson. Took him back.

Went back via Hooton.

But which is shorter? Which is quicker? Which uses the least fuel?

The difference in route starts at Clatterbridge and ends at the industrial estate.

Time to get my ruler out!

The light blue line shows the route when you come off the motorway at clatterbridge, and travel along urban roads to the Croft. The green line shows the route if you stay on the motorway to Hooton then travel along a 40 mile per hour dual carriageway to the same point.

I think it's fair to say that there's a clear winner in the "Shortest" category. 2.57 miles urban, against 7.15 miles on the quicker roads. I already knew the Hooton route would be longer, but I'm surprised to find that it's almost 3 times as long.

But which is quicker?

Well apart from 0.17 miles of motorway sliproad, the blue route is almost entirely 30 mile an hour urban stuff, punctuated by traffic lights, parked cars, zebra crossings, etc. A reasonable figure for the average speed on this route is perhaps 20 miles per hour. That's a mile every 3 minutes. So the blue route would take about 8 minutes.

The longer route consists of 0.75  miles at 30 miles an hour limit, 2.15 miles with a 40 mile an  hour limit, and about 4.25 miles of 70 mph motorway. The 30 mph bit is straight and wide, with just a single roundabout to slow things. So I'm going to assume an average speed of 25 miles per hour for this bit, meaning this would be covered in a little under 2 minutes. The 40 mph bit contains about 8 sets of traffic lights, so I'm going to say the 2.15 miles is done at an average speed of 35 miles an hour. And that works out at roughtly 4 minutes. Finally, 4.25 miles of motorway, done at slightly over a mile per minute (60 mph =1 mile per minute) would take about 4 minutes. Add these together and you get a nice round 10 minutes, so the long route takes a bit longer, even though the roads are faster. Not a huge difference in time though. If the speeds and times are average and approximate, there are times when it would actually be quicker to go this way.

Finally, although it's further to drive, and actually takes a little longer, is it possible that the green route uses less fuel?

Driving at a constant 30 miles an hour uses far less fuel than travelling at a constant 70. I did some experiments a couple of years back, travelling at different constant speeds along the same stretch of road, and recording the average fuel consumption. Unfortunately I can't find the figures now, but although the recieved wisdom is that the optimal speed for fuel economy is 56 miles per hour, this did not fit my experience at all. Travelling in a high gear, at a constant speed of 30 miles an hour used a lot less fuel than travelling in a high gear, at a constant speed of 50 miles per hour, which in turn used a lot less fuel than travelling in a high gear at a constant speed of 60 miles an hour.

The problem is that on urban roads, you're less able to drive at a constant speed. You're forever altering your speed. If you read the road well, you can keep stopping and starting to a minimum, but you can't eliminate the changes in speed completely. So the equation here is that the car is travelling at a speed that should be more economical, but this advantage is offset by less consistency in speed.

In order for the car to use less fuel on the longer run, it would have to use 3 times as much fuel travelling at speeds between 10 and 30 miles an hour as it does travelling at a fairly steady 70 miles an hour. At a constant speed of about 70, my car does about 38 miles per gallon. So I'd have to be doing about 13 miles to the gallon on urban roads for this to be the case.

And it isn't. I get somewhere around 40-45 mpg. Therefore the car uses far less fuel going the blue way.

So to summarise, in future, I will only ever travel between Clatrterbridge and the Croft industrial estate via Bromborough and Spital. It's superior in every way!

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