This project was designed to determine: * whether the current design guidance on ramp acceleration lengths provided by the Austroads guides are still valid for today’s Australian vehicles * appropriate acceleration lengths for entrance ramps that take into account suitable grade correction factors that represent a defendable balance between affordability, operational performance and safety. A literature review identified the source of the Australian design parameters and compared them to those used in the United States. This was followed by data collection for various vehicle classes at a number of locations in Queensland. A crash history review was also undertaken simultaneously. The key findings include: * The relationship between grade and acceleration rates adopted by drivers is complex; grade appears to have limited influence on acceleration rates adopted by drivers, especially on geometrically constrained ramps. * Based on the limited data collected, the grade correction factors provided in the current Austroads Guide to Road Design Part 4C: Interchanges do not appear to reflect the capabilities of passenger vehicles on the road network. * There are other factors affecting vehicle acceleration at entrance ramps, e.g. horizontal alignment, forward sight distance, visibility to freeway through lanes (design form elements), which may play a more significant role in driver behaviour than vertical alignment. * The relationship between ramp length and merge speed appeared to be weak, but there may be an optimal length that would enable safe merging without significantly increasing construction costs. However, the findings to date are inconclusive and the following areas of further investigation were recommended: * Additional speed and acceleration data needs to be collected from other jurisdictions. There is a need to collect acceleration data from ramps constructed prior to the adoption of current acceleration lengths, such data will enable a better understanding of driver behaviour and vehicle performance at constrained locations. * The concept of specifying an acceptable level of acceleration based on anticipated traffic composition may be a possible alternative to current design practices. * Human factors and design forms have impacts on drivers’ selection of acceleration rate that are not clearly understood, and require further research and modelling. The recommended investigations may lead to the optimisation of geometric design elements which will result in significant financial savings ultimately, through: * reduction in pavement construction, which may also reduce the amount of earthworks * reduction in impervious areas, which would reduce the costs of drainage and stormwater quality improvement equipment * reduction in the provisioning of road furniture, including but not limited to lighting and road safety barriers. (Author/publisher)
Samenvatting