In the news

You know if you've had a drink, you don't know exactly what speed you're going.

Sure, but 1 pint is still under the limit for most people and many people do drive on that basis. There's also the time factor. If I have a pint at 7pm can I drive at 9pm? If I have 6 pints starting at 7pm can I drive at 7am? Yet somehow we can have a concept of "units" and such for drink driving to give people an idea. Much as you may not like it many people teeter around the edge of the drink drive limits purely because the limits are so high (they're half or lower in other bits of Europe) and the risk of getting caught is so low.

And people know when someone is too pissed to drive, but somehow people aren't capable of knowing whether they're going faster than 20mph on a bicycle? Really?

I'd suggest that a good number of people who regularly go over 20mph already have a speedometer in the form of a GPS, and if not, a significant percentage of people capable of going over 20mph on the flat will know quite well that they're going over 20mph.

It's all pissing in the wind as it doesn't have a hope in hell of coming into force, but the argument that you need to have a speedo is just a shit one - as the drink driving limits show.

In Richmond Park (where they were often enforcing the 20mph speed limit) the approach by the police seemed quite simple:

• someone doing ~23mph on the flat = meh
  
• someone freewheeling down Sawyer's Hill at 28mph = meh
  
• someone actively pedalling down Sawyer's Hill at 35mph = pulled over