On an already busy road with N Vehicles passing distance D between points P1 and P2 per minute, if their time to travel distance D increases 33% (because their speed reduces 33%) then their will be 33% more vehicles between point P1 and P2 at any given time frame, meaning cars will be closer together meaning people trying to pull out onto the thoroughfare from a side road will get 33% more difficult! Until Gridlock where people cannot pull of the thoroughfare as exit route is blocked by people waiting to pull into the thoroughfare.
Actually I think the problem is worse than the off the top of my head calculation above, as if you look at it in reverse, an increase in speed from 20mph to 30mph is a 50% increase in speed so you can get 50% more vehicles passing between point p1 and p2 in optimal conditions in any given time frame. Therefore a reduction from 30mph to 20mph must be the same but opposite effect.
Typical government swine… but a quart does does not fit into a pint pot.
But the establishment tend to have ten-gallon pots for themselves eh! And they think in terms of their own private helicopter/jet/first-class-ticket/service lives eh! And also I reckon they use that statistic trick of the reverse statistic whenever that is beneficial to the propaganda they spew.