Browsing by Author "Dr. Joseph Hummer, Committee Chair"

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The Effects of Bus Stops on the Saturation Flow Rate of Signalized Intersections
(2004-07-19) Holt, Daniel Lester; Dr. Billy Williams, Committee Member; Dr. John Stone, Committee Member; Dr. Joseph Hummer, Committee Chair
The total number of vehicle-miles traveled on our roadways has rapidly increased resulting in an increase in traffic congestion and a decrease in the operational integrity of the transportation system. To reduce these numbers of vehicle-miles, agencies attempt to influence and educate users on choosing the alternative mode of travel known as transit. For the potential users to conveniently use transit, agencies must conveniently locate bus stops so as to provide a high level of service. Though transit use has increased to provide approximately 8 billion passenger trips nationally, the number of vehicles and vehicle-miles traveled on our roadways has not decreased. With these bus stops usually located directly in the traffic stream, detrimental impacts to the traffic flow cannot be avoided as a bus stops and consequently blocks the traffic flow. Therefore, a realistic measure of the effect these transit vehicles have on the transportation system is needed. The 2000 Highway Capacity Manual's (HCM) bus blockage adjustment factor equation found in Chapter 16 uses an average bus blockage time of 14.4 seconds. However, according to Chapter 27 of the 2000 HCM, when an average deceleration and acceleration time of 10 seconds for a bus to enter and exit a bus stop is applied to this 14.4 value, a total bus dwell time of only 4.4 seconds remains to actually serve its passengers. An additional review of Chapter 27 revealed that 15 seconds is a recommended average bus dwell time and along with the additional 10 seconds that accounts for the deceleration and acceleration time of a bus, Chapter 27 recommends a total bus blockage value of 25 seconds. With this disparity in bus blockage times, this research set out to formulate new analytical equations that more accurately estimate the effect of bus stops on the saturation flow rate of a signalized intersection. These analytical equations were based on the assumption that the maximum impact of a bus stop occurs during the effective green time period. The near-side bus stop was shown to prohibit the progression of vehicles through the intersection, but that the effective green time and the cycle length were established as non-factors in evaluating this effect. However, the far-side bus stop does allow vehicle progression, but this progression is limited by the available vehicle storage space between the bus and the stop bar and the point in the signal cycle when the stop is performed. These equations were validated using CORISM simulation runs of a simple, saturated roadway network with a single bus route and bus stop that resulted in a simulated and an ideal saturation flow rate. A predicted saturation flow rate was then calculated to compare against the simulated saturation flow rate. Statistical testing and a sensitivity analysis conclusively showed that the magnitude of the proportion of right-turning vehicles at the intersection, the distance of the bus stop from the stop bar, and the actual number of bus stops have a direct effect on the value of the saturation flow rate. After a comparison of the adjustment factors from the derived equations against the HCM bus blockage adjustment factor, it is found that the 14.4 value in the HCM bus blockage equation does not accurately estimate the severity of the effects of a near-side bus stop and should be changed to the value of 25 seconds from Chapter 27 of the HCM. For the far-side bus stop, the new analytical equation developed here that is more sensitive to the parameters that can aid in estimating the effects of a far-side bus stop should be utilized to distinct it from the equation for a near-side bus stop.
Operational and Safety Impacts of U-Turns at Signalized Intersections
(2004-04-15) Carter, Daniel L.; Dr. Joseph Hummer, Committee Chair; Dr. Billy Williams, Committee Member; Dr. Nagui Rouphail, Committee Member
With rapidly growing urban areas and construction of new developments, efficient access to the roadway network becomes a relevant issue. In the effort to balance safety, mobility, and access, many transportation officials are in favor of designs that employ raised medians on the main road. However, this decision draws much controversy from those opposed to the lack of direct access that comes with raised median designs. One of the issues in this controversy is the effect of increased U-turns at adjacent intersections. The purpose of this research is to determine the operational and safety effects of U-turns at signalized intersections. The operational analysis involved measurements of vehicle headways in exclusive left turn lanes at 14 intersections. By regression analysis, I obtained an equation to estimate saturation flow reduction based on intersection characteristics. This equation indicates a 1.8% saturation flow rate loss in the left turn lane for every 10% increase in U-turn percentage and an additional 1.5% loss for every 10% U-turns if the U-turning movement is opposed by protected right turn overlap from the cross street. The safety study involved a set of 78 intersections. Fifty-four sites were chosen randomly, and twenty-four sites were selected based on their reputation as U-turn 'problem sites'. Although the group of study sites was purposely biased toward sites with high U-turn percentages, the study found that 65 of the 78 sites did not have any collisions involving U-turns in the three-year study period, and the U-turn collisions at the remaining 13 sites ranged from 0.33 to 3.0 collisions per year. Sites with double left turn lanes, protected right turn overlap, or high left turn and conflicting right turn traffic volumes were found to have a significantly greater number of U-turn collisions.