The MIDI-roundabout
Idealised drawings of four-arm midi-roundabouts
Some drawings of idealised layouts to illustrate deflection problems...
Here is a series of four drawings of symmetrical idealised crossroads. Each illustrates central island sizes required to provide deflection at 60m radius for approx. 2m wide vehicle paths. The island size is determined by the deflection required which in turn is determined by ICD and other factors such as road widths on entry and exit, flare, etc. In this series of drawings with reducing ICDs the first is the only one with a solid centre. In the following three layouts it is only the truck apron which remains, and it is always designed to be large enough to deflect light vehicles, and that will often mean a diameter larger than 4m.
1. Small Roundabout with solid central island

What we are seeing in effect here are two roundabouts! One designed for trucks, coaches etc. and one designed for light vehicles.

In the UK this would be legal.

The size of the truck apron here is determined mainly by the road widths. Reducing these locally would help pedestrians, but remember refuge (Splitter) islands are very effective.
(See Millennium Vision page)

2. Larger mini-roundabout

There are many cross-roads like this in the UK and thousands in America.

To deflect traffic by the central island alone requires quite a large island. Deflection would be assisted here by the use of build-outs on the approaches, but avoid significant reduction of the ICD. In the absence of vertical deflections, the central island is the only deterrent against "straight-lining" by drivers.

In my seminars we look at a possible way of dealing with one of the sites at Hove which is remarkably similar.

In the UK this might be illegal!
or require special authorisation.
In the U.S. this could usefully replace the
thousands of 4-way STOP intersections
that perform so badly.

3. Large crossroads

This is a relatively extreme case where a midi-roundabout might be tried on a crossroads with tight corner radii and wide approach roads. As with the previous layout narrowing the approaches will make a better layout altogether but which may still require over 4m for the central island.

Again, this might be illegal in the UK!

4. Medium-size flared crossroads

7.3m is the typical width of many UK roads so this illustrates well the need for a central island larger than 4m. Many highway authorities will not now install mini-roundabouts in these circumstances which is a pity because they can work well provided there is sufficient deflection, achievable using a larger central island. In addition I would recommend raised splitter islands at a site like this, but these alone might be illegal although not if headed by a solid part with a bollard included.

Still technically illegal in the UK!

Note for UK designers: TD16/93 (now superceded) requires deflection to be provided on the approaches where possible. On mini-roundabouts this is nearly impossible, but the deflection must be provided somewhere on all crossing paths unless physical vertical deflection is provided to a sufficient standard. The above three drawings of the MIDI-roundabout illustrate well the method of design. TD54/07 replaces TD16 but remains a poor document - See my comments on this HERE.

* * * * *

Central island design

This rather large drawing illustrates to approximate scale my suggestions regarding the central island. The island is a conic sectional volume or frustum which is essentially flat on the top. The side angle must not (UK) exceed 15 or 1:4 (Traffic calming Regulations). I have shown the side slope grey in plan but it should remain white "on the ground".

I believe that a serious problem has developed with mini-roundabouts generally and that is drivers' lack of respect for the central island. Many drivers choose to ignore the island although it is unlawful to pass to the wrong side of it. Designers bear a lot of the responsibility for this by designing poor layouts. So for these larger junction layouts I strongly recommend this form of island to deter drivers from running straight across the intersection.

Reality has it that designers will lay this island on the existing carriageway profile which may be flat, level or quite variable. I favour a general resurfacing of the intersection and the opportunity will then be taken to have a relatively even profile with a steady outward drainage fall of around 2%. The island will fall into place on top of that and the central island height will be dictated by the type of vehicle likely to pass over it; grounding must be avoided. The centre of the island should perhaps be marginally raised to drain over the edge. Overall height must not exceed 125mm and I generally recommend no more than 100mm. If the carriageway slopes away at more than 2% take care that a low-loader or low-floor bus for example does not ground.

I recommend too that the central island must be wide enough to deflect (as per drawings above) but that there should be room for most buses and coaches to circumnavigate such islands even if slowly. It can be very uncomfortable for passengers when buses and coaches lurch if they have to climb vertical deflections such as this.

Links to other pages:

Home | Crossroads | Drawings | Millennium Vision

Penntraff - January 2012
Pages designed by: