Analysis of the weaknesses in the present freeway capacity models for Sweden

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Abstract

The aim of the work is to scrutinize the present Swedish capacity manual and new models from a project in the early 2000's. It only covers freeway facilities. By using new and extended empirical data the models that have to be improved can be further developed. The result shall be the freeway models in the new Swedish Capacity Manual. The manual for freeway facilities include a time-space model for bottleneck analysis of urban and rural freeways. The work has been based on the documentation for the present models. The models have also been compared to other international models to check the reasonableness. The project includes three parts, freeway mainline section, freeway weaving and freeway merging. The review of the models revealed several deficiencies. - The Swedish capacity model for main line section doesn't take into account the gradient, which is an important factor and is one of the most significant parameters according to German experiences. - Another large knowledge gap is the fact that the capacity model doesn't take into account lane width and distance to obstacle. - The model for weaving has one large weakness, it is based on empirical data where the on-ramp flow is almost equal with the off-ramp flow. The model has then been design with a total weaving flow, which means that the model is insensitive to flow cases where e.g. the on-ramp flow is much larger than the off-ramp flow. - The model for on-ramp merging does only take into account the on-ramp flow, other international models also have ramp length. The work with the new capacity models during 1998-2001 had a lack of empirical data, which gave the result that relation between different parameters where difficult to find. E.g. the weaving model has only 6 hours empirical data. The models should preferable add the following parameters: - Gradient for main line - Lane width - Distance to obstacle - Separate on- and off-ramp flow in the weaving model - On-ramp length With the data collection system STRESS new data have been analyzed and a new model for oversaturated conditions has been developed. Also a new model for capacity in weaving segments has been developed. The new model takes into account both on-ramp and off-ramp flow. The TPMA model for link capacity has been further developed with a parameter that takes into account the gradient. A time-space model for calculation and identification of bottlenecks should also be developed. The time-space model should be answering the following questions: How to calculate the emergence of bottlenecks in a system of adjacent interchanges? Which bottlenecks arise because of geometric design? How to conduct an analysis of the geometric configuration's impact on the occurrence of the bottleneck? How to model the changes in the geometric design to eliminate or reduce the bottleneck in time and/or place? The state of art shows that only one model is adequate and gives a relatively fully cover of different on-and off ramp configurations, the model is Freeval and is a part of HCM 2010. The Swedish model will use the method for time and space calculation in HCM 2010. © 2011 Published by Elsevier Ltd.

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Strömgren, P. (2011). Analysis of the weaknesses in the present freeway capacity models for Sweden. In Procedia - Social and Behavioral Sciences (Vol. 16, pp. 76–88). Elsevier Ltd. https://doi.org/10.1016/j.sbspro.2011.04.431

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