Design treatments for increasing traffic capacity in road diet projects

• Online Access: http://hdl.handle.net/2047/D20323949

Road diets are found to be powerful paradigm in adding more functionality to roads while also improving safety and efficiency. With road diets, of which the most common type is to reduce the number of car lanes from two lanes per direction to one, right-of-way can be allocated more efficiently among road users. The feasibility of a road diet depends on the ability to provide a high traffic capacity with a limited number of lanes. This thesis focuses on design features that can contribute significantly to a successful road diet by helping preserve traffic capacity using limited space. While some of the design features this thesis addresses are well-known, it also elaborates on some less-studied features as well. The thesis begins with a road diet case study on Cummins Highway, an urban arterial in a residential part of Boston, reducing it from two lanes per direction with a raised median to a single lane per direction, retaining the raised median. Several of the design features covered in this thesis were motivated by needs and opportunities presented by this case. One of the features addressed in this study is auxiliary through lanes, which are lanes added at signalized intersections to increase capacity. The design considerations for them are explored in the literature, as well as in a model that was developed to be used as a simple guideline. The importance of harmony between signal timing - particularly, the length of the green interval - and auxiliary lane length is highlighted. Informal flares, making the road wide enough for one car to pass another who is waiting to turn, but not formally marking it as a turn lane or auxiliary lane, is an important but neglected design feature for road diets. They are space efficient, providing critical functionality for traffic flow while consuming less space than a formal auxiliary lane, leaving space for other valuable functions such as crossing islands. Analysis of field observations helps highlight their critical dimensions and factors. Shadow lines in median breaks - that is, broken lines that continue the edges of a median through an intersection - are another understudied feature that can be valuable in road diets, in particular in combination with informal flares. When left-turning drivers wait in an intersection for a gap, these lines are hypothesized to draw them deeper into the median break by reassuring them that they are not occupying the opposite lane, making it easier for cars coming from behind to pass and reducing risk of rear-end collision. While no study shows how effective they might be, field observations show that the median break space is used very inefficiently without them. Roundabouts and mini-roundabouts have been given close attention because of their ability to carry more traffic per lane than is usually possible with signalized intersections, making them interesting for road diets. Mini-roundabouts, which are smaller and have a traversable center island and splitter islands for large vehicles, can be ideal for constrained rights-of-way. Parameters affecting their capacity are reviewed, and differences between U.S. guidelines and Dutch design are discussed. Finally, bus stop location and type are analyzed. Previous studies emphasize the value of in-line bus stops located downstream of signalized intersections. We develop and test (using simulation) a theory for how the distance of an in-line bus stop from the stop line and the length of the green interval length affect traffic blockage.