31 research outputs found
Revised Raff's method for estimating critical gaps
No full textThe estimation of the critical gap was introduced in the 1970s to evaluate the capacity of vehicle and pedestrian movements at unsignalized intersections. The critical gap is the smallest gap that a driver is assumed to accept. The population of drivers, each with his or her own critical gap, will have a distribution of critical gaps; and it is this distribution, or its characteristics, that is the subject of this paper. These statistics for the critical gap are difficult to estimate; several methods have been developed and used. Raff’s method has been frequently used because of its simplicity. This paper examines the original method, an adaptation of the method that is frequently used, and a proposed adaptation of the method. Simplified traffic and driver behavior characterizations are used to demonstrate the ability of each process to predict the mean critical gap. When given exponential headways in the major stream and a uniform distribution of headways, unbiased estimates of the critical gap would be obtained by using a modified version of Raff’s method involving the accepted gap and the maximum rejected gap. The mean critical gaps estimated with a modified Raff’s method using the maximum rejected gap are slightly inferior to those from the maximum likelihood method. It is recommended that the maximum likelihood method be the preferred technique; it was used in measuring values for the Highway Capacity Manual. However, the modified Raff technique is an acceptable alternative
When common sense just won't do: Misconceptions about changing the behaviour of road users
No full textThis paper examines the paradox that a number of road safety measures popular with the general community have not proven cost-effective when subjected to rigorous evaluation. While examples of this can be found throughout road safety, it is perhaps most pronounced in the case of behavioural approaches. To demonstrate this point, the paper reviews a number of behavioural measures which have widespread community support, but limited road safety effectiveness, including driver training programs, harsher penalties, and the isolated use of mass media road safety campaigns. \ud
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Community support for these measures is often linked to their intuitive appeal. From the road users perspective it appears a matter of ‘common sense’ that they are effective. However, on closer inspection, this support is often based on misconceptions about crash causation, road user behaviour or ways of achieving behaviour change.\ud
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Two important implications emerge from this review. Firstly, in order to achieve their objectives, road user safety measures need to be based on sound behavioural principles, rather than on ‘common sense’ or intuition. Secondly, road safety agencies need to actively promote the effectiveness of successful road safety measures. This will not only improve support for these measures, but assist in shaping community perceptions about safe behaviour, which may in turn contribute to the acceptance of new approaches
The development of a package of integrated learning material to assist young learner drivers
No full textYoung drivers, including motorcycle riders, are over-represented in crash statistics (Macdonald, Bowland, & Hancock, 1994; Swain & Blake, 1995). This has led researchers to investigate issues relating to young driver crash involvement and driving skill. Early work attempting to link these two concepts has been fraught with methodological problems that rendered the results inconclusive (Mayhew & Simpson, 1995). However, more recent and methodologically sound studies indicate: (1) that driving skills improve with experience; and (2) that more experience is related to fewer crashes (Gregersen & Bjurulf, 1996; Triggs & Smith, 1996). Forsyth (1992) quantified this relationship by suggesting that accident involvement decreases with increasing experience. In addition, there is further indication that young driver crashes are also related to individual motivational factors such as risk-taking (Triffs & Smith, 1996)
A desktop model for computing Acceleration Severity Index (ASI) for rigid barrier impacts as a function of impact configuration
No full textAcceleration Severity Index (ASI) is a vehicle occupant severity indicator measured during homologation of road safety barriers. Published literature contains efforts to correlate occupant injury risk with ASI. Hence there is value in exploring how ASI might vary with impact configuration (impacting vehicle mass, speed and angle). This paper describes the development and testing of a desktop model for predicting ASI in impacts with rigid barriers as a function of impact configuration. The efficacy of the model is discussed and tested against published data
Using weigh-in-motion data to predict the likelihood of exceeding the capacity of a road safety barrier
No full textRun-off road and head-on crashes together constitute around 38% of all casualty crashes and a higher proportion (closer to 50%) of all fatality crashes in Queensland, Australia. These statistics are a fair reflection of the national condition. Vehicles leaving the travelled way are a significant contributor to Australian road trauma. The Australian National Road Safety Strategy proposes a number of infrastructure treatments for tackling these two crash types, including the use of an appropriate road safety barrier. Road authorities deploy longitudinal road safety barriers primarily to prevent errant vehicles from impacting with hazardous roadside objects that could cause an adverse outcome for either the occupants of the errant vehicle or third parties. However, road safety barriers are not equal and are differentiated in the first instance by their capacity to contain impacts of different speed, mass and angle of incidence. While roadway departure speeds and departure angles are well-addressed in contemporary academic literature and methodologies for road safety barrier selection, the mass-distribution of the in-service vehicle fleet is less well represented. This study proposes the use of data obtained from weigh-in-motion technology to represent the mass-frequency distribution of the in-service vehicle fleet. Combined with roadway departures conditions reported by others, a methodology is presented for calculating the likelihood of vehicle-barrier impact exceeding the road safety barrier capacities prescribed by the predominant global test protocols for road safety barriers. The methodology is used to consider how different roadway configurations and traffic compositions might influence the likelihood of barrier capacity exceedance. The results from modelling of various scenarios are reported. The results suggest that the relative likelihood of barrier capacity exceedance varies as a function of cross-sectional geometry as well as traffic composition, so suggesting that a “one-size fits all” approach to road safety barrier selection is not appropriate
Decompartmentalising road safety barrier stiffness in the context of vehicle occupant risk
No full textRoad safety barriers are selected for deployment on the basis of four basic criteria; costs, deflection performance, containment capacity, and severity outcomes. System specific severity risk to occupants of errant vehicles is not well established. Contemporary technical governance in the Australian context recognises three generic barrier types discerned by relative stiffness: rigid, semi-rigid, and flexible. This study explores how the occupant severity indicator Acceleration Severity Index (ASI) varies as a function of impact configuration and system stiffness. This study demonstrates that systems available to road safety practitioners may be better served by a continuum rather than a generic classification system
Feasibility of predicting light vehicle occupant injury disutility from impacts with road safety barriers
No full textA procedure for predicting disutility associated with injury severity outcomes for occupants of vehicles impacting longitudinal road safety barriers is not well established. Roadside hazard management procedures use results from empirical studies of generic barrier types. Published literature suggests that the injury severity outcomes of vehicle-barrier interactions are more complex, and are a function of (among other things) impacting vehicle mass, impact speed and impact angle, and a road safety barrier system’s stiffness (resistance to deflection). \ud
Presented is an exploration of the feasibility of a structural model that could be used to predict occupant injury outcome disutility arising from tracking light passenger vehicle impacts with road safety barriers as a function of the impact configuration and stiffness of the road safety barrier system. \ud
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The study concludes that such a structural model is feasible subject to development of a satisfactory link between barrier stiffness and likelihood of impact configuration and injury outcomes. The use of Head Impact Criterion (HIC) as a proxy to link vehicle accelerations to injury outcome is considered to show promise. \ud
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Calibration of such a model would require reporting of the impacted barrier in terms of the factors that are expected to influence system stiffness as well as the configuration of impact (vehicle mass, impact speed and angle) and the geometric circumstances (cross-section, number of lanes, lateral offset). Other variables expected to contribute to occupant injury outcomes such as vehicle age and safety rating, number of occupants and mode of impact (tracking or non-tracking) should also be collected
A study of the mass-frequency distribution of the registered light vehicle fleet in Queensland
No full textRoad safety barrier performance is a function of the mass of the impacting vehicle. However, knowledge of the mass-frequency distribution of the registered light vehicle fleet in Queensland is limited. A quantitative analysis of the mass of a proportion of the predominant body types comprising the light vehicle fleet is presented. While light vehicle mass appears to be increasing, the testing\ud
protocol for road safety barriers preferred by Australian/New Zealand Standard AS/NZS 3845.1:2015 (Standards Australia, 2015) is appropriate in terms of the mass of the test vehicle for both occupant severity and for barrier capacity
Effect of an upstream traffic signal on the capacity of a downstream two-way stop-controlled intersection
No full textThis paper investigates the platoon dispersion model that is part of the Highway Capacity Manual 2010 (HCM 2010) and is used to forecast downstream traffic flows for analyzing both signalized and two-way stop-controlled (TWSC) intersections. The paper focuses on the effect of platoon dispersion on proportion of time blocked, conflicting flow rate, and capacity flow rate for the major street left-turn movement at a TWSC intersection. Existing HCM 2010 methodology shows little effect on conflicting flow or capacity for various distances downstream from the signalized intersection. Two methods are suggested for computing the conflicting flow and capacity of minor stream movements at a TWSC intersection that have more desirable properties than the existing HCM method. Further, if the existing HCM 2010 method is retained, results suggest that the upstream signals model be dropped from the HCM method for TWSC intersections
