If in your view of a Roundabout reveives cars from four directions, each car with its own destination choice, and they each drive around the Roundabout until they reach their exit, well, there will be path conflicts.
Instead invert the question. At each entry pre-sort into three lines of vehicles, each headed to the same exit. How should each line of vehicles proceed to reduce, minimise, or eliminate path conflicts?
The right turn Rs can all be handled with dedicated slip lanes. So the problem reduces to the Left Turn Ls and the Centre paths C.
It turns out the “best path conflict reduction entry point” for the C and L lines of vehicles is directly across the roundabout, and from the inside, not the outside.
Using Right-handed entry paths makes the left turn angle a more tolerable obtuse angle, rather than the acute angle that results from left side entry. In LHD roundabouts flipping the entry to from-the-left would be sensible.
Almost all roundabouts, LHD and RHD alike…
Many guides list Merging and Weaving as trade-offs from using Roundabouts, and their good qualities must offset these downsides. They all say it, so it must be true.
All those guides and statements assume that vehicles headed on all three(3) directions of travel should all, immediately, be injected to the roundabout, whether necessary or not. Assumed, so it becomes true.
Most guides recommend separating our the R traffic, the right-hand-turns, in many cases keeping them off the roundabout itself through a dedicated slip lane. The missing analysis is also deciding what to do with the C and L traffic.
Test of Concept… The Small R=5U Version
After being unable to get anything working with an R=4U circle, something bigger was required, and called in.
Note some of the junctions between tyhe descendind two-lane fondue stick and the tint roundabout get rendered poorly. As drawn, some of the sticks block part of the intersection. But these do not block the flow. If confused watch the other intersections that render less poorly.
[No criticism of the AI and screen rendering here at all. My choice was to cram 4 roads together into a very tight space. It is pleasing they connect and work. Thank you for handling what I threw at it so well.]
Proof of Concept… The R=8-10U Octagon Version
Since it is an octagon, people differ on its true radius or diameter.
Building on the small-scale success, the next goal was to maintain transit speed. Since most roads follow the ground this meant inverting the Fondue Pot, putting it above the mainlanes, and the Fondue Sticks now reach up into the pot. It was impossible to get a circle and four fondue sticks together simultaneously. So the Octagon was born. The octagon also assisted with shifting the entry target one vertex upstream, to maintain transit speeds. Yay.
Fair warning, if you must change the spacing between the entry and exit on one side, leave the entry alone and move the exit. If you adjust the entry of the Fondue Sticks, a risk is never getting Humpty Dumpty back together, deleting the whole lot, and re-plopping.
Most of the intersection is built on mini stilts to assist with plopping. The four upward launch points each sit on the ground. Unevenness here might make placing a challenge. Hopefully the mini-stilts help.
Demonstration of Concept
See the save game linked in the collection. Watch two no-weave roundabouts in action.
On top left is an R=5U two-land roundabout. Left and Right turns each move along 1/4 of the circle, from different entry opoints so thjey leave together.
At Lower right is a No-Weave Roundabout that performs more like a Windmill, specifically a …
“Left turn Left departing, Late&Late Windmill”
Eachg left-turn ramp makes up 1/4 of the roundabout path. Making the incremental cost of adding these to the mainlanes themselves almost trivial.
And, for the adventurous, so long as you have not enabled Uturns which introduces merging and weaving…