Speed Management in Rural Two-way Roads – Speed Limit Definition Through Expert-based System



Speed Management in Rural Two-way Roads – Speed Limit Definition Through Expert-based System

Authors

Nuno Gregorio, Universidade De Coimbra, Ana Bastos Silva, Universidade De Coimbra, Alvaro Seco, Universidade De Coimbra

Description

Development of a discrete choice mathematical model for the definition of speed limits in interurban two-way roads, using an expert-based system.

Abstract

Until recently, the design of road infrastructure involved mainly concerns related to the base speed value and to requirements imposed by criteria ensuring high levels of service. In fact, until the 1980’s technical recommendations usually based the definition of geometrical and functional solutions on concerns without major consideration of road integration in the surrounding environment. Thus, road networks were mostly designed to provide uninterrupted flow conditions and a high level of mobility.
However, the recent and growing urbanization of the adjacent spaces along roads led to the emergence of dispersed built-up areas, with non-easily identifiable boundaries with consolidated urban areas and the rural regions. In this increasingly complex road environment, traditional road design and management principles, namely including speed limit setting approaches, tended not to be appropriate, which very often has had implications in terms of speed related severe accidents.
Therefore, it is today consensually accepted that only an integrated approach is able to take into account, in a coherent and balanced way, the interests and needs of all the involved users and stakeholders. These requirements include not only the need of a high level of service of motorized vehicles and a suitable standard of urban living, but also safety concerns associated to other users and local residents. This vision has been leading to different approaches on speed management along the road and particularly to the definition of the maximum speed appropriate to each section.
During the last decade, new speed limits setting methodological approaches have emerged, based on new design models and tools, which take into account road geometric, safety and operational characteristics. Among others, the Australian family of applications (XLimits), later adapted in other countries such as USA and New Zealand, is a relevant example of this type of approach. This model enables the selection of adequate speed limits by taking into consideration a wide set of explanatory variables, aiming at describing the infrastructure, land use, local safety and operational characteristics. However adapted to the standard road environments of the countries for which they were developed, these models tend to require the availability of a wide range of information, not always easily collected. This is the case, for instance, of local accident and speed distribution data, which is often in many other contexts impossible to obtain.
Having this into account, this research work aimed to develop a methodology of speed management with a widespread use applied to single carriageway roads in interurban areas, crossing different road environments with a mixed use. More specifically, this work focused on the development of a decision support methodology for the definition of the appropriate maximum speed in each road section, based on criteria related with speed consistency and road layout homogeneity, as well as on the willingness of drivers to naturally accept a change in the legal limit.
For this purpose, an analytical model able to accurately estimate that speed limit was developed based on a set of objective and easily measurable and obtainable explanatory variables characterizing the section under analysis and its surrounding areas. A real database covering the observation of national or regional road sections was collected and used. This model is intended to be based on internationally accepted design principles, as those taken into account in the previously referred approaches, but calibrated for other road environments (in this specific case, the Portuguese road network).
The resulting methodology is a Multinomial Logit model, a discrete choice technique, and it was carried out using a case-study involving four different tracks of interurban roads crossing different environments. The model was estimated resorting to the use of values chosen by four traffic safety experts recorded for each road segment in both directions. Limitations imposed on speed resulting from other properties of the road, namely restrictions based on the road track, were not taken into account.
The paper firstly includes a literature review of the subject under analysis. This is followed by the description of the methodological approach used in this work, including the estimation of the Multinomial Logit discrete choice model. Afterwards, the validation of the estimated model is carried out, by applying it to an independent data set relative to another road track. The obtained results are then analyzed and discussed, having into account the current posted speed limits in the sample roads in order to evaluate how much they differ from the model outputs. Finally, conclusions and discussion of future work are presented.

Publisher

Association for European Transport