This weekend saw the first ever day time closure of the M25, with the section north of the A3 closed to allow work on the M25-A3 junction improvements to take place. The predicted chaos did not happen, probably because people made many journeys on other days.
There are, broadly speaking, two approaches to these sorts of junctions. The first is to have a roundabout at the end of the slip roads. People leave the motorway, and choose which way to go when they reach the roundabout.
The alternative is to have eight specific connecting roads. The four “left turns” are simple curves, built on the land. In contrast the four “right turns” have to cross over and therefore have to be flyovers. If the motorway runs north to south, and the other road east to west, these would each cover N-W, S-E, E-N and W-S. This approach is called “free flow” in the trade - because traffic never has to stop at a roundabout.
Almost all motorway-non-motorway junctions follow the first approach, while almost all motorway-motorway junctions follow the second.
The second approach clearly offers a much better experience for the driver, because there is no need to stop. This also makes it less likely that the traffic will back up as there is less congestion. That in turn reduces the risk of queuing traffic on the motorway itself - reducing the risk of collisions, as well as reducing the number of lorries forced to move into the middle lane, slowing other traffic.
Although better for motorists, the second approach is much more expensive to build. Flyovers involve a lot of concrete, after all. They also use up a lot of land - the M25-M4 junction is about four times as land intensive as the M25-A3 junction.
Our country is skint, with the government only meeting its own fiscal rules with implausible assumptions about future spending. We need to think about saving money. But roundabouts alone are not the answer.