Mini-roundabout sign

Mini-roundabouts - Getting them Right!
Capacity of (mini-) roundabouts

I use a rule of thumb in my book and continue to promote this. Because mini-roundabouts are so interactive, drivers may be forgiven when occasionally they all look at one another wondering who is going to make the first move. This three-way stand-off is relatively common and it has to be said that different drivers will accept very different conditions before entering a busy mini-roundabout. Frank Blackmore invented a capacity rule based on the area of the junction. He argued that the capacity was in direct proportion to the linear size of the junction expressed by a formula that summed the entry widths and added a value for the square root of the remaining area. This was based as much on his research on the TRL test track as anything else; it suggests the possible capacity if the area is used most efficiently. It relates to a fundamental formula that I illustrate in my seminars:

Q = N/t

where Q is the flow,
N is the average number of vehicles in the system at any one moment and
t is the average time taken by any vehicle to pass through the system.

In my capacity work I assume a reasonable degree of one-to-one; if one driver cannot proceed then another usually can.
This works reasonably well for small junctions and seems to relate closely to the formulas that the Americans have used.

For a stream entering a mini-roundabout and turning right:

QR = 1200 - QC

where QC is the circulating traffic in vehicles per hour equivalent.

For a stream entering a mini-roundabout and turning left:

QL = 1500 - QC

For a stream entering a mini-roundabout and turning left and right:

QL&R = 1200 - QC

This formula results in a straight line graph at 45, which is close to the graphs produced by others. There are slight end effects that we can ignore here.
The French have identified that when trying to enter a roundabout the traffic turning off at the first exit (left in the UK) does have an inhibiting effect on the next entering stream around. 
This arises particularly with a single lane approach where drivers are turning left and right from the same lane. Entering drivers cannot be sure exactly what the drivers from the previous approach are doing.

In my book are various worked examples of how I have applied my formula. It has proved most useful to gauge the likely scenario.

 

Penntraff - Dec 2014
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