Empirical study of heavy vehicle operating speeds on mountainous downgrades in the Western Cape, South Africa
Oncke, Jandre (2016-03)
ENGLISH ABSTRACT: Heavy vehicles play a major role in the economic well-being of a country through its significant role in moving freight. Therefore any adverse effect to the road freight, from being stuck in traffic to being involved in a fatal accident, will have negative impacts on the economy and on the safety of the persons involved. Therefore it is paramount to understand how and to what extent factors such as road geometrics, vehicle characteristics and weather conditions affect the safety of heavy vehicles. There are several countermeasures addressing the issue, with the most common approach being to supply drivers with better information. This is done through the use of a grade severity rating system (GSRS) or weight specific signs (WSS) that direct heavy vehicle drivers to adopt a slower and more suited speed for the relevant downgrade. However, there are no such countermeasure or system deployed in South Africa, and simply adapting the current GSRS and WSS might not suffice, since most of these models are outdated and does not account for newer technology or regulations. Thus, this research paper aimed to gain a better understanding of how the select geometric and vehicle characteristics influence the operating speed of heavy vehicles on downgrades, and ultimately to formulate a mathematical equation that can be used to estimate the operating speed of heavy vehicles on downgrades. The equations can be used to design WSS for South African mountain passes.
The variables that have been identified as important in this study include, operating speed, gross vehicle mass, gradient, downgrade length, curve radius, superelevation and stopping distance. The operating speed is taken as the dependent variable and measure of safety, since it is the most common relatable variable among the diverse and complex nature of heavy vehicle operating systems. In order to collect data of the select variables, the critical section for each mountain pass was identified. This was done by following descending heavy vehicles and determining the relevant speed profiles. This was done multiple times for each pass, so that a combination graph could be obtained, which was analysed to identify the critical section. Data for the variables were collected using various devices and means. The operating speed and number of axles were collected by video recordings, and gradient, downgrade length and curved radius were collected with instruments such as a GPS and theodolite. The mass of the heavy vehicles were obtained from the weighbridges situated along routes of the relevant mountain passes.
Analysing the data were primarily done through regressing the data, with the calculated operating speed through the critical sections as the dependent variable. Three regressions were done, namely a general regression, regression with a change in horizontal alignment and a regression with a compulsory stop at the base of the downgrade. Equations were found for heavy vehicle speeds on steep downgrades in general and on downgrades that subject operating speed to small curve radii. A logical exercise was also done, in which it was attempted to determine driver familiarity with the select downgrades. It was found that only half of all the observed heavy vehicle drivers exhibited familiarity with the given pass. Based on the finding that more relevant information is needed by the drivers and the lack of a sufficient means of conveying information to heavy vehicle drivers, it is fitting that an improved alternate to the GS 505 signs be found. The two equations found in this study can be used to this end.
AFRIKAANSE OPSOMMING: Swaarvoertuie speel 'n belangrike rol in die ekonomiese welstand van die land deur die beweging van goedere. Enige nadelige inwerking op swaarvoertuie, van in verkeer vassit tot ongelukke, sal ŉ negatiewe impak op die ekonomie en die veiligheid van die betrokke persone hê. Daarom is dit belangrik om te verstaan hoe en tot watter mate faktore soos pad-geometrie, voertuigeienskappe en weerstoestande die veiligheid van swaarvoertuie beïnvloed. Daar is verskeie teenmaatreëls om die kwessie teen te werk, met die mees algemene benadering om beter inligting aan bestuurders te voorsien. Dit word gedoen deur middel van ŉ grade severity rating system (GSRS) of ‘n weight specfic sign (WSS), wat beter en meer direkte inligting aan swaarvoertuig bestuurders gee, sodat ŉ meer geskikte spoed gehandhaaf kan word. Daar is egter geen soortgelyke stelsel in Suid-Afrika nie, en net die aanpassing van die huidige “GSRS” en” WSS” sal nie voldoende wees nie, aangesien die meeste van hierdie modelle verouderd is en nie rekening hou met nuwe tegnologie of regulasies nie. Hierdie studie is daarop gemik om 'n beter begrip van hoe en tot watter mate die geometriese- en voertuigeienskappe die spoed van swaarvoertuie op afdraandes beïnvloed. Die studie sal dan verder gevat word om 'n wiskundige vergelyking te formuleer, wat gebruik kan word om die spoed van swaarvoertuie op afdraandes te bepaal. Die vergelykings kan gebruik word om “WSS” te ontwerp vir Suid-Afrikaanse bergpasse.
Die veranderlikes wat geïdentifiseer is as belangrik in hierdie studie sluit in: spoed, voertuigmassa, gradiënt, lengte van die afdraande, draairadius, verkanting en stop afstand. Die spoed word geneem as die afhanklike veranderlike en as die maatstaaf van veiligheid, want dit is die algemene veranderlike tussen swaarvoertuie ongeag van die voertuig se dryfstelsel. Om die data te bekom wat benodig word vir die studie, moet die kritiese punte van elke bergpas eers bepaal word. Dit is gedoen deur swaarvoertuie te agtervolg terwyl hulle teen die bergpas daal, waarna die relevante spoedprofiel bepaal is. Dit is meer as een keer gedoen vir elke bergpas, sodat 'n gekombineerde grafiek van die spoedprofiele saamgestel kon word, wat dan ontleed is om die kritiese punt te identifiseer. Data vir die veranderlikes is ingesamel met behulp van verskeie toestelle. Die spoed en aantal asse is verkry deur video-opnames; die helling, afdraande lengte en draai radiusse is ingesamel deur middel van ŉ GPS-toestel en ŉ teodoliet. Die massa van die swaarvoertuie is verkry uit die weegbrûe langs roetes van die betrokke bergpasse.
Ontleding van die data is hoofsaaklik gedoen deur regressie, met die swaarvoertuig spoed deur die kritiese punte, as die afhanklike veranderlike. Drie regressies is gedoen, naamlik 'n algemene regressie, ŉ regressie met 'n verandering in horisontale belyning en 'n regressie met 'n verpligte stop aan die voet die bergpas. Twee vergelykings is geformuleer om swaarvoertuig spoed op afdraandes te bepaal. Die eerste vergelyking is vir bergpasse in die algemeen en die tweede vergelyking maak voorsiening vir die kritieke draai radius. ŉ Logiese oefening is ook gedoen, waarin bestuurder bekendheid rakend die afdraandes bespreek word. Daar is gevind dat slegs die helfte van al die waargeneemde swaarvoertuigbestuurders vertroudheid met die verskeie bergpasse getoon het. Gebaseer op dié bevinding, word dit voorgestel dat beter en direkte inligting aan swaarvoertuig bestuurder oorgedra word, veral op bergpasse. Die twee vergelykings wat geformuleer is in hierdie studie, kan gebruik word vir die doel.
Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98520