Mini-roundabouts - Getting them Right!

DESIGN DETAILS

At a T-junction there is one side-road; at a mini-roundabout, every approach is a side-road...

The central island

A fundamental point here so often forgotten is that virtually all drivers (UK and western Europe) know instinctively what to do once they identify the central island of a roundabout. They will:

  • slow down
  • give way/yield to vehicles from the right
  • pass to the left of the island once the way is clear

This applies to ALL roundabouts however large or small!!

NB I was sent details of a proposed traversable roundabout of about 7m for a crossroads. Excellent, but UK designers should be aware that a mini-roundabout is a form of overrun area so the traffic calming regulations concerning overrun areas may apply. The side-slope may not exceed 15º and any vertical face must not exceed 6mm.
I recommend this for all countries.

For more details, check the UK Regulations.

So the central island:

  • Must NOT be sunk in a hollow

  • Island diameter - normally 2- 4m for 3-arm junctions, up to 6m diameter for 4-arm junctions, but in any event large enough to ensure adequate deflection for crossing and right (UK) turning traffic
    (Do not experiment with     a flush island when raised island is needed)

  • Normally dome island about 20-25mm per 1m diameter; do not exceed 100mm. For larger islands consider a shape with 10-15º side slopes and a nearly flat top

  • Locate by examining inside edge of deflected light vehicle swept paths for the three right (UK)/left (Europe & N America) turns (3-arms) or for the four ahead movements (4-arms)

  • Construct in flexible material (UK HRA/DBM) - preferably no kerbs/curbs or setts - they tend to break out
    (I am looking at designs which use shallow kerbs at about 15 degrees like the French use so extensively, with a relatively flat topped island. This design might be appropriate for those sites with larger central islands which I am recommending for crossroads situations as above.)

  • Must be white reflectorised (UK); white, or bright colour strongly recommended for rest of world

  • In the UK, must not have a vertical edge more than 6mm and must not have a side-slope greater than 15º
    (recommended in all cases, even where not legally required)

  • Additional (white) concentric ring(s) have been used in some instances - these are now not recommended.

  • Will need regular maintenance

  • In the UK the three arrows are required. I do not recommend these for the rest of the world. While they show well in plan, they are hardly visible to drivers.
Small roundabouts
  • Keep simple
  • Well raised but not to obstruct entering drivers view of right/left turn indicators on circulating traffic
  • Safe overrun area on the edge up to 1m - do not use high kerbs on the inside
    The Americans have quite large overrun areas (truck aprons) using a relatively substantial kerb to prevent their misuse. In effect they design TWO roundabouts, one for trucks and one for light vehicles. It is this thinking which has led me to understand that the central island of a mini-roundabout is a stand-alone truck apron which must be as large as necessary,  i.e. a significant roundabout, to deflect light vehicles.

    Our UK size criterion was based on the wrong element - the solid centre disappearing instead of the truck apron.
American small roundabout at Baltimore near University

American small roundabout
Baltimore near University
Note wide truck apron/overrun area.

Large roundabouts

  • Ensure that they are sufficiently domed or that the level surface is above roadway height. Some flat low-profile ones where I used to live in Kent have over-running problems particularly as the circulating carriageway slopes towards the central island and therefore the flat island is below the level of the approaching traffic lanes.  

Remember, the dome or raised central island is a most important clue to approaching drivers of the presence of the roundabout.

The approaches

No-one should be overshooting your mini-roundabouts; if they are, something has gone badly wrong...

Failure to design the approaches correctly is responsible for much over-shooting often resulting in accidents. 

The approach layout must get drivers out of "auto-pilot". This is probably the single most important aspect of mini-roundabout design that seems to have gone wrong in the UK.

Safety Auditors are urged to do a technical review of all of their sites with the following comments very much in mind.

  • NEVER use bifurcation arrows (these are used on priority junctions and give drivers positively misleading information)

  • Avoid any arrows, except to encourage double lane use (where more than one entry lane may be used towards the same exit)

  • Work centre-line progressively offset to off-side on approach to cause:

  • Visual break with opposite C/line (T-junction - usually former major road axis)

  • The approach to be split into two lanes where possible (start at 2m lane widths)
    (The visual effect of this is very important; such narrow lanes are safer for cyclists too; UK designers should ignore DMRB minimum lane width requirement of 3m at the yield line - it is inappropriate for two lane entries.)

  • Give-way line just behind outer swept path not usually on inscribed circle circumference (ICC)

  • In UK the give-way triangle may be added to the mini-roundabout give-way line (single pad). But if you decide to use the give way sign you must then use the triangle and double give way line. I do not normally recommend this.

  • Avoid just a single wide lane with central hatching . Use buff surfacing (usually anti-skid) especially on single lane approaches. Make sure that any hatching or central marking does not resemble a layout that could suggest priority e.g. a (former) T-junction layout.

  • Avoid kerbline bulges. Profiling the nearside kerbline carefully will usually be satisfactory, but the short abrupt kerb "blisters" often used are ineffective and sometimes dangerous.  Evidence is mounting that they cause accidents rather than prevent them.  On its own a kerb blister is not a speed reducing feature. It can seriously reduce entry capacity too.

How approach configuration makes a huge difference to vehicle paths
The red paths are for right turning (2nd exit) and the blue paths show where a separate lane is provided on approach which is normally used to turn left (1st exit). This approach detail, important enough in itself for warning drivers in advance of the presence of the junction, has a good effect in the junction too. Splitter islands may be used in either arrangement but they have little effect where there is just one wide lane marked.

Wrong!

Right!
By having only single lane approaches it is virtually impossible to stop drivers from cutting across the junction and creating dangerous conflict angles. By off-setting the centre-lines early, splitting the approaches into two lanes and deflecting vehicle paths parallel to the nearside kerb, drivers tend more to take the paths shown. Crossing angles are much less severe.

A superb example of a misleading approach to a mini-roundabout is illustrated below at Shrewsbury.

Featherbed Lane, Shrewsbury
This site was discussed at my seminar for Shropshire in July 1999 and illustrates well typical problems converting T-junctions. The issues are so important that I have moved these images here from the "problems" page.
Featherbed Lane junction showing bifurcation arrow - never use these!
Closer to the junction from east
Note these two views approaching the mini-roundabout at Featherbed Lane, Shrewsbury from the Ring Road. The bifurcation arrow and the general layout all give the impression of an approach to a T-junction, Lane 1 being the through lane and Lane 2 being the right turn stack lane. Even the warning sign suggests this. But what about the ADS? and would most drivers notice the detail anyway? And where is the mini-roundabout central island? It seems too small and inconspicuous.
Looking east from the yield line
Looking east across the junction from some distance
Look at the second picture which tells the truth about the junction and then look at the third which shows the skid marks. I wonder how many unwary drivers are deceived here every day. From the opposite direction note the lack of clarity on approach. Drivers are approaching something, but what? - it could easily be just a pedestrian refuge! And note too that the street lighting seems to indicate a through route - this needs to highlight the junction, one light on each corner and preferably a change in colour e.g. SON.

Lessons to be learnt

  • Make sure, from the road layout, that drivers PERCEIVE the presence of the mini-roundabout in good time

  • NEVER use bifurcation arrows on the approach to any roundabout unless the bifurcated direction is obviously dissociated from the roundabout; and even then use with great care.

Yield/Give-way lines

The location of these is dependent upon the outer swept paths of the largest circulating vehicles. These lines act as the guidance for circulating drivers as well as instructing entering drivers where to give way/yield. It follows that these lines will usually take the form of a spiral towards the kerb ending nearly parallel to it. At mini-roundabouts it is usually a mistake to place these on the circumference of the inscribed circle. This encourages drivers to cut across the central island. Rather drivers should be directed to enter the roundabout (nearly) parallel with the nearside kerb so that they are directed to the nearside of the central island.

Crossfalls & drainage
(most normal roundabouts and all mini-roundabouts)

We have created a serious problem of excessive speed on UK roundabouts due mainly to our convention for draining the circulating carriageway inwards (TD16/93 & still in TD16/07) so favouring drivers on the roundabout. This is creating both safety and capacity problems. And it is inhibiting for entering drivers facing a continual stream at high speed. After all, the give-way rule is meant to be advisory and drivers on the roundabout can be expected to slow down, or at least that was the original intention.

So let's get re-designing our roundabouts in accordance with TAL 9/97 (although that leaflet did not mention outward falling drainage; it seems UK DfT were not aware of continental crossfall practice). DfT/HA are now taking this issue seriously.

A crucial factor in design is to ensure adequate slowing on entry. A truck entering a roundabout too fast will be more likely to overturn somewhere around the circulation area; but once slowed sufficiently on entry a truck will not accelerate to roll-over speed on a circular roundabout whatever the crossfall provided it is consistent.

Drain outwards

  • Raises central island (up to 400mm)

  • Eliminates crowns - main cause of HGV overturning

  • Keeps circulating speeds low

  • Brings circulating carriageway into better view on approach

  • Reduces need to adjust or relocate underground equipment

  • Avoids drain gullies next to central island

Sloping sites

  • Keep central island high

  • Avoid too much re-shaping

  • Gullies sometimes needed by roundabout island
    or

  • French practice appears to be to maintain a consistent outward radial fall of up to 2.5%.

Signs

In the UK we mostly get this WRONG!

  • Locate mini-roundabout circulation sign
    (UK dia 611.1) where it will be seen!

    • Up to 15m (not 1.5m) from Give Way (Yield) line

    • On a splitter island perhaps

  • Use Roundabout warning sign (UK dia 510) before junctions or

  • Use Advance direction sign (ADS)

  • Do not incorporate mini-roundabout symbol on ADS

On small roundabouts in urban areas:

  • Use turn left/right not pass left/right (UK use 606 not 610) (Horizontal white arrow on blue circle is the internationally recognised sign)

  • Avoid chevrons unless carefully integrated into the vista

  • Side slopes and planting must not mask view of right/left turning indicators of circulating vehicles

  • Avoid too many bollards (pass left/right) on splitter islands, often just one will be sufficient

  • Maintain route continuity through all roundabouts - direction signs must show which way numbered routes go

  • Locate direction/destination signs at the junction carefully and ensure they are visible for the drivers to whom the sign relates

Lighting

On mini- roundabouts

  • Try to create a continuity change in the colour or style of lighting - this will help drivers to identify the change point (link into node)

  • Light the whole junction brightly, white or high pressure sodium (SON)

On small roundabouts

  • Light the island with post top unit (Mercury, SON or white)

  • Introduce environmental art, landscaping etc. (local laws may not allow this in some countries)

  • Minimise sign lighting units on central island

  • Keep light columns away from the edge of the central island. (I still see this!)

If not lighting your roundabout (which will be the exception) ensure high retro-reflectivity of signs and a very clear layout in light coloured materials.

Design features are examined in detail at the seminars.
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