As someone who has commuted from Howden to Bristol for work over the last 20 years, I have instinctively recognised this interesting phenomenon. When I'm driving to work I tend to drive at about 66-69 mph actual speed on the motorway, so around less than 70 mph indicated speed and it usually takes me 4 hours at 214 miles. I usually stop to powder my nose on the M1, just before hitting the horrible A42/M42. After 10 minutes I rejoin the motorway and then start to recognise lorries that I've already overtaken and sometimes it takes a while to catch them up.
So, the reason why is the paceometer, as your speed increases there is a diminishing return between how much you have to go faster to save a small amount of time. At 30 mph, you cover the same distance in half the time if you are doing 60 mph, simple and everyone know this. At 70 mph if I increase by 10 mph to 80 mph I would save a total of 23 minutes if I didn't have to slowdown or stop, so in the real world driving it is actual considerably less than 23 minutes, from my experience it would be less than 15 minutes saved.
Key Speedometer (MPH) to Minutes per 10 Miles Conversions:
and why it's just not worthwhile speeding on our motorways
So, the reason why is the paceometer, as your speed increases there is a diminishing return between how much you have to go faster to save a small amount of time. At 30 mph, you cover the same distance in half the time if you are doing 60 mph, simple and everyone know this. At 70 mph if I increase by 10 mph to 80 mph I would save a total of 23 minutes if I didn't have to slowdown or stop, so in the real world driving it is actual considerably less than 23 minutes, from my experience it would be less than 15 minutes saved.
Key Speedometer (MPH) to Minutes per 10 Miles Conversions:
- 10 mph = 60 minutes
- 15 mph = 40 minutes
- 20 mph = 30 minutes
- 30 mph = 20 minutes
- 40 mph = 15 minutes
- 50 mph = 12 minutes
- 60 mph = 10 minutes
- 75 mph = 8 minutes
- 100 mph = 6 minutes
- 120 mph = 5 minutes

