DiExSys LLC

Centennial, CO, United States

DiExSys LLC

Centennial, CO, United States
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Durso C.,University of Denver | Allery B.,DiExSys LLC
Transportation Research Record | Year: 2012

The decision to add lanes to a freeway is motivated by the need to relieve congestion. Practicing engineers and planners generally believe that the decreased congestion that results from adding lanes is associated with some degree of improved safety, yet the majority opinion of researchers is that accident rates increase as the number of lanes increases. In more than 70 years of modern road building, these conflicting views have not been reconciled. This paper first examines the relationship of traffic flow parameters, such as volume, density, and speed, with safety by calibrating corridor-specific safety performance functions. On the basis of an understanding of this relationship, a possible explanation of the effect that adding lanes has on safety is formulated. An empirical examination of the relationship of flow, density, and speed to the crash rate on selected freeways in Colorado suggests that, as the flow increases, the crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded, the crash rate rapidly rises. The rise in the crash rate may be because an increase in density without a notable reduction in speed produces headways so small that it becomes difficult or impossible to compensate for driver error. This model suggests that, after the construction of additional lanes, crash rates initially decline because of the lower traffic volume and density per lane. However, as development and rerouting occur, freeways with more lanes are expected to have higher crash rates that are attributable to the increased opportunities for lane change-related conflicts.


Kononov J.,DiExSys LLC | Durso C.,University of Denver | Allery B.,DiExSys LLC
Transportation Research Record | Year: 2012

Speed-flow relationships for a typical basic freeway segment are understood well at present and are documented by the successive editions of the Highway Capacity Manual. Recent freeway studies showed that speed on freeways was notaffected by low to midrange traffic flow. Increases in flow and density without a reduction in speed have a significant influence on safety. However, current literature lacks constructive discussion of this influence. Empirical examination of the relationship between flow-density, speed, and crash rate on selected freeways in Colorado suggests that as flow-density increases, the crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded, the crash rate rises rapidly. The rise in crash rate may be caused by flow compression without a notable reduction in speed: resultant headways are so small that drivers find it difficult or impossible to compensate for error and avoid a crash. This paper calibrates performance functions for corridor-specific safety that relate crash rate to hourly volume-density and speed and proposes an algorithm for a variable speed limit intended to slow traffic in real time in advance of a high speed-high density operational regime. Deployment of such an algorithm has the potential to improve safety and reduce travel time variability.


Kononov J.,DiExSys LLC | Hersey S.,000 South Holly Street | Allery B.,DiExSys LLC
Transportation Research Record | Year: 2012

Decisions to run traffic on freeway shoulders during peak periods are motivated by the need to relieve congestion. Practicing traffic engineers generally believe that the decreased congestion resulting from running traffic on hard shoulders (i.e., hard shoulder running) is associated with some unspecified degree of improved safety, yet the majority of researchers agree that accident rates increase as the number of lanes increase, even if full shoulders are provided. Despite many years of modern road building, these conflicting views have not been reconciled. This paper first examines the relationship of traffic flow parameters, such as volume, density, and speed, to safety with calibrated performance functions of corridor-specific safety. On the basis of an understanding of this relationship, a possible explanation of the effect on safety of hard shoulder running is formulated. Empirical examination of the relationship of flow, density, and speed to the crash rate on selected freeways in Colorado suggests that as flow increases, the crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded, the crash rate rises rapidly. This rapid rise in crash rate may be caused by an increase in density without a notable reduction in speed and the resultant small headways that make it difficult or impossible for drivers to compensate for error. This model suggests that during hard shoulder running, crash rates decline because of the lower traffic volume or density per lane and that the safety benefits of a reduced volume or density per lane outweigh the adverse effects of the lack of provision of a full shoulder.

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