Why is speeding so commonplace?

The root problem seems more that people are expected to drive at least at the speed limit, if not over. When a driver travels at 3-5kph under the speed limit, following cars tend to overtake as soon as possible or get angry and start behaving dangerously. There are very large number of people out there who will consider you to be a lunatic for doing 48km/h with a 50km/h limit. In Dublin at least, a 50km/h limit is tacitly interpreted as a 60km/h limit, a 60 as an 80, etc.

And it's not purely a social problem; it starts in the Rules of the Road, which says things like:

You must progress at a speed and in a way that avoids interference with other motorway traffic.

Avoid driving too slowly

In normal road and traffic conditions, keep up with the pace of the traffic flow while obeying the speed limit. While you must keep a safe distance away from the vehicle in front, you should not drive so slowly that your vehicle unnecessarily blocks other road users. If you drive too slowly, you risk frustrating other drivers, which could lead to dangerous overtaking.

While it does remind you to obey the speed limit, this comes into conflict with the greater message here: "keep up with the pace". Since it seems like the majority of other road users are almost constantly speeding, this reinforces the notion that you should never travel below the speed limit, and if anything, increase speed to match traffic in front.

It's frustrating - if you're driving within a city, given the time spent in traffic or stopped at lights and the relatively short distance, it doesn't really matter if you travel at 45 or 55. But it's considered a greater crime to err on the low side. Why?