User Guide > Entry Capacity

Entry Capacity

The current version of Rodel incorporates two capacity options: Geometric and HCM (6 or 10).

HCM uses the Capacity Equations from the Highways Capacity Manual.

The geometric Rodel Capacity Algorithms are an extension of Kimber’s equations that model US conditions.

The enhanced geometric capacity model has been developed to model North American roundabouts; for example, single lane roundabouts with very wide single lane entries (to accommodate large trucks) that are wider than the receiving section of the circulating road. (Entries wider than the circulating road are not permitted in the UK.)

The geometric capacity model has also been applied to situations where the number of opposing circulating streams differs from the number of entry streams. The effect on entry capacity is modest with low circulating flows but increases as circulating flows increase. (This was not in the original model as UK Roundabout Guides required that the number of circulating lanes were not less than the number of entry lanes, which is now no longer the situation, so the model has been updated.)

Several Capacity Modifiers have been added to the enhanced model:

  1. The capacity Intercept is adjusted by the Light / Dark parameter as capacity is reduced by about 5% in dark conditions. Snow and icy conditions reduce capacity significantly but this effect can be reflected manually by using other Capacity Modifier as it depends on specific local conditions and cannot be generalized.
  2. Capacity Intercept modification alters the intercept for temporary reasons such as parked cars on an approach during the peak hour only. The adjustment to the intercept can be applied for the AM or PM peak only leaving the capacity intercept unchanged for the other peak hours.
  3. The effect of Crosswalks on capacity is also included. This uses the capacity factors from the tables in Exhibits 4-7 and 4-8 of NCHRP Report 672 "Roundabouts: An Informational Guide."
  4. The Confidence Level modifies the Capacity Intercepts to test the effects of pessimistic capacity on Queues, Delays and LOS. Roundabout intercepts are the mean of a normal distribution. All models implicitly use the mean value. Rodel uses the mean when the Confidence level is set at 50%. However, higher confidence levels can be used to test the consequences of sub mean capacities. If queues and delays on any leg become unacceptable with the sub-mean capacity then small modifications to geometry can increase capacity to remedy the situation. This produces more robust designs that will work acceptably even if the capacity turns out to be significantly sub-mean. The Confidence Level range is from 50% to 99%. Typically designing at 85% confidence level is considered a good tradeoff between risk and increases in geometry. However, each roundabout is case specific in this respect.

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