Arrival Turning Flows
Average Daily Traffic
Number of Entry Lanes ne
Flare Length L'
Entry Radius R
Entry Angle Phi (Ф)
Entry Width E
The HCM model uses the number of entry lanes only and does not use the Entry Width.
The entry width is the narrowest width at the yield line perpendicular to the traffic path.
It excludes gutter pans.
E has a range of 12 ft (3.65 m) to 54 ft (16.5 m).
Capacity is very sensitive to E and capacity can be increased by widening E without increasing the number of lanes. Narrow lanes have significantly less capacity than wide lanes.
However over wide lanes should be avoided as they become inefficient and the capacity increase from widening rapidly fades. Single lane entries can be an exception to this as discussed below.
The following graph shows capacity related to Entry Width with flow and all other parameters fixed.
E is typically 14 ft (4.2 m) to 18 ft (5.5 m) on single lane roundabouts. However larger values can be used to accommodate a design vehicle.
The "effective" entry width (that actually used by the entering traffic) should be input to avoid possible overestimation of capacity.
If the receiving circulating width is less than E, E will be reduced to the circulating width. Even then the effective width may be less.
Below is an example of a roundabout with an effective entry width about half the physical entry width, as the used and non-used sections are very distinct. The poor effectiveness of the entry has reduced capacity by about 50%.
The geometric capacity model has been extended to model US style single lane roundabouts with very wide entries that are wider than the receiving section of the circulating road which acts as a constraint. The circulating width and number of circulating lanes is now input for this and other purposes.
However, situations where the width of the receiving section of the circulating road is equal or wider than the entry width but where the relation between the entry and the central island are such that entry path overlap is such that the Entry Width is not fully used will not be known by Rodel, unless this is recognized by the user and the "effective" entry width is input rather than the physical entry width. If effective entry width is not input, capacity can be significantly overestimated.
Increasing E increases accidents so E should be kept to a minimum and capacity increased by increasing either V and or L'. When increasing the entry width, the fast path radius must be kept sufficiently small to control entry speeds.
Single lane entries are a special case.
As a single lane is progressively widened, capacity can increase due to three capacity mechanisms until a full two lane entry is achieved:
However, this is totally dependent on their being two circulating lanes to receive the traffic as the entry morphs from one to two lanes.
The geometric capacity model has been applied to situations where the number of opposing or conflicting circulating streams differs to the number of entering streams.
This was not included in the original model as UK Roundabout Guides required that the numbers of circulating lanes were the same as the number of entry lanes. As this is no longer case the enhanced model applies the geometric equations to any combination of entry and circulating lanes.
The effect on entry capacity is modest at low circulating flows but increases as circulating flow rise. The graph below shows capacity for circulating flows on 1 – 4 lanes.
© Rodel Software Ltd