Intersection Capacity Analysis according to the Highway Capacity Manual (ICA)

VD functions are usually used to model volume-dependent travel times on links (Impedance and VD functions). They can also be used to model volume-dependent wait times for turns or complete nodes (Node impedances of turns VD function and Node impedance of nodes VD function).

By contrast the Highway Capacity Manual (HCM) published by the US Transportation Research Board contains internationally recognized guidelines on calculating the level of service and other performance indicators for intersections, based on the detailed junction geometry and various control strategies. Visum computes performance indicators such as capacity, delays or LOS either according to the guidelines defined in the operation model HCM 2000, 2010, HCM 6, or HCM 7.

Note: In the following, the implementation of the HCM 2000 in Visum is mainly described. In the more recent editions, HCM 2010, HCM 6th Edition and HCM 7th Edition, the calculations differ from HCM 2000 in some areas. We therefore strongly recommend that you refer to the original editions when looking for applicable formulas.

We have highlighted the sections in the HCM 2010 that differ from the HCM 2000. The HCM 6th Edition and HCM 7th Edition are continuations or extensions of the HCM 2010, major changes compared to HCM 2010 are described in the corresponding sections. The HCM is only available in English.

For intersection points of the same level, the calculation differentiates between the following control types (attribute Effective control type at node):

Visum offers two different models for the analysis of roundabouts:

  • The method developed by R. M. Kimber, (Kimber 1980), (Kimber, Hollis 1979), (Kimber, Daly 1986), which is also described in the British guideline TD 16/93 "The Geometric Design of Roundabouts", is based on the empirical study of numerous roundabouts and the statistical adjustment of a model which estimates capacities in dependency of the geometry (Roundabouts according to the TRL/Kimber method).
  • Methods described in the HCM editions (Roundabouts according to the HCM method ).

The method according to TRL/Kimber has the advantage of taking comprehensive empirical results on the influence of geometry on the permeability of a roundabout into consideration and has been successfully implemented for nearly three decades.

The method according to HCM is recommended if in theory you prefer consistency for all control types (roundabouts also according to HCM like signalized and two-way stop nodes) within a project. Furthermore, the method is not dependent on observations which were only obtained through driving behavior studies in Great Britain.

Not the control type, but the effective control type is decisive for the calculation. These two values differ, as signalized nodes are considered yield-controlled node if no signal controller is allocated to them or if the signal controller has been switched off (signal controller attribute Turned off).

Notes: Throughout the model description, special provision for right or left turns relates to right-hand traffic. For Visum models with left-hand traffic, the roles of right and left turns are reversed (User Manual: Selecting the direction of traffic).

U-turns are never considered in HCM 2000. In Visum it is possible to treat U-turns as far left turns through the corresponding setting in the procedure parameters for intersection impedance analysis (in left-hand traffic accordingly as far right turns). This calculation is then no longer HCM conform. From HCM 2010, u-turns at two-way stop nodes are included. Here, the processing is performed according to HCM 2010 in Visum.