Not strictly a total signs post this, but recent discussions over on SABRE (and on Twitter) suggest that there is a serious inability to get these features right. This in turn has major safety implications as misuse of mini-roundabouts can cause confusion and thus collisions.
Naturally, the Design Manual for Roads & Bridges manages to take the design of them and turn it into something horribly complicated which is why I suspect a lot of the bad design creeps through as people try to translate TD54/07 into something workable on a narrow urban road. In fact, the DMRB actually puts so many design constraints on minis that it’s almost not worth bothering. Small wonder that we instead get rural A-road design in town centre as The Ranty Highwayman has discussed here.
Disclaimer: you are obliged to follow DMRB if working on trunk road schemes. On local roads, unless the highway authority specifies DMRB compliance, there is some degree of flexibility applicable. This is NOT a design guide and you use advice here at your own risk.
The first thing that needs to be questioned when designing a mini-roundabout is an obvious, but overlooked, notion. Is a mini-roundabout actually needed to begin with? They are not traffic calming devices as many erroneously suggest they are; their primary function is to reallocate priority at difficult junctions. Using them where there are obvious dominant flows can result in drivers ignoring the mini entirely until that fateful moment a rare vehicles does indeed emerge from the side road.
The second thing is, does the site have sufficient space? To be safely traversed the mini needs to have an adequate Inscribed Circle Diameter (ICD). This provides the envelope of the entire junction. To force lower speeds through the junction a smaller ICD is preferable, but this will reduce capacity. If bigger ICDs are needed then ideally traffic calming on all approaches should be provided. This is often a detail that designers get horribly wrong resulting in disproportionate approach speeds and increased risk of collisions.
I personally would never advise using a single mini-roundabout on a skewed crossroads – if space permits then a double mini should be provided. If space does not permit it then a different method of junction control needs to be applied because traffic will never circulate around the white blob no matter how much it is asked to.
Consider Non-Motorised Users (NMUs). A cyclist may struggle with mini-roundabouts particularly as the vast amount of road markings can become very slippery. Not only that, turning across the central disc is often a case of having to force into the primary position, which in turn causes less talented drivers to close pass or cut-up the cyclist. Where practical, cyclists should be taken off the carriageway and around the junction. Where this is not practical then the mini-roundabout needs to be as low speed as possible.
There is a lot of debate about the use of entry lanes. More than two is a no-no, but narrow entry lanes can force a better line through the junction. Again, consider NMUs!
And lastly, get the signs right…