TRCLC 17-10

Summary:

This study assessed the impact of traffic calming measures to the speed, travel times and capacity of residential roadways. The study focused on two types of speed tables, speed humps and a raised crosswalk. A moving test vehicle equipped with GPS receivers that allowed calculation of speeds and determination of speed profiles at 1s intervals were used. Multi-regime model was used to provide the best fit using steady state equations; hence the corresponding speed-flow relationships were established for different calming scenarios. It was found that capacities of residential roadway segments due to presence of calming features ranged from 640 to 730 vph. However, the capacity varied with the spacing of the calming features in which spacing speed tables at 1050 ft apart caused a 23% reduction in capacity while 350-ft spacing reduced capacity by 32%. Analysis showed a linear decrease of capacity of approximately 20 vphpl, 37 vphpl and 34 vphpl when 17 ft wide speed tables were spaced at 350 ft, 700 ft, and 1050 ft apart respectively. For speed hump calming features, spacing humps at 350 ft reduced capacity by about 33% while a 700 ft spacing reduced capacity by 30%. The study concludes that speed tables are slightly better than speed humps in terms of preserving the roadway capacity. Also, traffic calming measures significantly reduce the speeds of vehicles, and it is best to keep spacing of 630 ft or less to achieve desirable crossing speeds of less or equal to 15 mph especially in a street with schools nearby. A microscopic simulation model was developed to replicate the driving behavior of traffic on urban road diets roads to analyze the influence of bus stops on traffic flow and safety. The impacts of safety were assessed using surrogate measures of safety (SSAM). The study found that presence of a bus stops for 10, 20 and 30 s dwell times have almost 9.5%, 12%, and 20% effect on traffic speed reductions when 300 veh/hr flow is considered. A comparison of reduction in speed of traffic on an 11ft wide road lane of a road diet due to curbside stops and bus bays for a mean of 30s with a standard deviation of 5s  dwell time case was conducted. Results showed that a bus stop bay with the stated bus dwell time causes an approximate 8% speed reduction to traffic at a flow level of about 1400 vph. Analysis of the trajectories from bust stop locations showed that at 0, 25, 50, 75, 100, 125, 150, and 175 feet from the intersection the number of conflicts is affected by the presence and location of a curbside stop on a segment with a road diet.

Problem:

Traffic calming has its origins in the Dutch “Woonerf” schemes of the 1970’s, and since then has been further expanded and spread throughout the northern Europe, but especially in Netherlands. However, it is still relatively new to many areas of Tennessee and the United States and it is not implemented to the extent of Europe. Traffic calming measures (TCM) are used for the purpose of reducing speed and regulating traffic volume to acceptable levels. These measures can have positive or negative traffic flow and safety impact to the community which is worth evaluating. If not properly implemented, traffic calming can lead to delays, traffic accidents, lower travel speed and overall dissatisfaction to the community. Traffic calming techniques can be grouped into two categories (1) non-physical measures such as educational programs and (2) physical measures which are of three types: horizontal measures which use forces of lateral acceleration to discourage speeding; vertical measures which use forces of vertical acceleration to discourage speed such as speed humps, speed tables etc. and narrowing which use a psycho-perceptive sense enclosure to discourage speeding. This study focused on simulating calming features that physically impact the drivers’ speeds including (1) speed humps (2) speed tables and (3) raised crosswalks

Traffic calming is not only a transportation element to most communities but can be an enhancer of the safety of drivers and pedestrians, enhancing livability, improving air quality, change social behaviors and promote walkability, it is a great way of creating a sustainable city. The goal of this study is therefore to assess traffic calming programs through microsimulation in the context of urban transport system functioning with respect to the impact on modal split, routing decisions and in result, on observable traffic flows. As a result, this will help to properly represent the impact of engineering measures and the examination of speeds and travel times alterations for each type of measure are essential for livable communities.

To achieve primary aim of the study several primary tasks were involved: comprehensive literature search was undertaken to uncover published and unpublished reports and papers on Traffic Calming Schemes, programs and simulation. Selection of Study Segments, several roadway segments with known traffic calming in Metro Nashville area have been selected for the study. Traffic Calming Microsimulation which involves defining, simulating and evaluating the Impacts of Traffic Calming measures including raised Median Island, traffic circle, speed hump, raised crosswalk, crosswalk refuge etc. VISSIM software was used for simulation. Traffic Calming Sensitivity Analysis in which the impact of varying Traffic Calming were analyzed through simulation. The operational impacts from sensitivity analysis were documented and graphically plotted for comparison purposes.

 Furthermore, the study evaluated the use of Road diets for traffic operations and safety improvements. Over the course of the 20th century, four (4) lanes roads became prominent (normal throughout the country). No engineering guidance during that period encouraged consideration of a three-lane alternative. A few years later due to increase in motor vehicles, many roads became congested and hence more lanes were added to increase capacity without careful examination of the pedestrian’s safety. Nowadays, these roads are unsafe and therefore, a solution which tends to improve safety by reducing highway fatalities and serious injuries is needed. One such a solution is the use of road diets. Road diets involve reallocation of road space through reduction of the number of motorized traffic lanes. The freed-up space can be used for the following features: (1) A bike or parking lane which can be used to increase the buffer between vehicles and pedestrians walking along the sidewalks or fixed objects, (2) space for pedestrian refuges, (3) a reversible center lane, and (4) addition or widening of footpaths. These types of public safety treatments can result in a crash reduction, improved livability, and community support.  They also encourage walking, bicycling, and transit use [54]. However, these road diets can also result in negative aspects which are worth evaluating. The tradeoffs include: delay, longer queues and travel times, increase of rear-end crashes, increased emissions, and diversion from the corridor.  In order, to address the congestion concern due to road diet installment, the traffic signal timing along the corridor may be optimized to improve the progression of traffic and allow easy access of motorists at stop-controlled intersections. This research evaluated the road diet impacts using Microsimulation and Surrogate Safety Measures (SSAM) by assessing the capacity impacts and safety effects of road diet conversions.

Research Results:

This study used VISSIM microsimulation to simulate the impact of speed calming features and spacing to capacities of residential roadways. GPS collected speed data were collected along the study then simulated through VISSIM to characterize driving behaviors for speed tables (17-ft and 21-ft wide tables), speed humps and a raised crosswalk. About 40 trips were generated using test vehicles equipped with GPS were obtained from drivers who had no idea of the purpose of the study to avoid skewing the driving speeds. The valid numbers of runs were used to develop speed profiles and for further computations of accelerations and decelerations at the approach and exit of the respective calming features. The new approach for estimation of capacity in a road segment with presence of calming measures was developed in this study. The study concluded that there was a minimal difference in capacity resulting from a 17-ft wide speed table compared to 21-ft wide speed table. However, the 17-ft table produced slightly higher reduction in capacity when spaced at 350ft, 700ft and 1050 ft, the speed tables of 17 ft when compared to 21 ft speed tables. The capacity reductions were 30% for 1050 ft spacing, 34% for 700ft and 37% for 350 ft spacing. For all type of calming features evaluated, the addition of number of calming measures in a link resulted into linear decrease in capacity. Furthermore, from this study, it was found that the traffic calming devices significantly reduce the speeds of vehicles, and it is best to keep spacing of 630 ft or less to achieve desirable crossing speeds of less or equal to 15 mph especially in a street with schools nearby. The study therefore concludes that speed tables are slightly better than speed humps and raised cross-walks in terms of preserving the roadway capacity. The report also concludes that spacing the speed tables or speed humps or raised cross-walks at 1050-ft instead of 350-ft improve capacity by approximately 13%.

The study found that road diets can decrease lane crossing distance and reduce vehicle speeds through converting an existing four-lane, undivided highway to two through lanes and a center, two-way left turn lane (TWLTL). This design allows left-turning vehicles to exit the traffic stream while waiting for a gap to complete their turn and frees up space that can be relocated to other uses. One of the uses is for transit lanes. Although it is commonly noted in the literature that transit vehicles on a road diet have a negative impact on traffic safety and operation, the magnitude has not been systematically quantified. These buses or freight vehicles dwelling at loading areas near the intersection of a road diet temporarily block car traffic to provide bus priority. For the purpose of estimating effective traffic management strategies at critical locations on a road diet, the effects of these obstructions were recognized or accounted for in this study. In this study, a microscopic simulation model developed to replicate the driving behavior of traffic on urban road diets roads has been used to analyze the influence of bus stops on traffic flow and safety. The impacts of safety were assessed using surrogate measures of safety (SSAM).

The results indicate that presence of a bus stops for 10, 20 and 30 s dwell times have almost 9.5%, 12%, and 20% effect on traffic speed reductions when 300 veh/hr flow is considered. Also, up to the traffic flow level of about 600 vehicles per hour, the effects of dwell times of 10, 20, and 30 s duration, are nearly the same. Additionally, a comparison of reduction in speed of traffic on an 11ft wide road lane of a road diet due to curbside stops and bus bays for a mean of 30 s with a standard deviation of 5 s  dwell time case was conducted. Results show that a bus bay stop with the stated bus dwell time causes an approximate 8% speed reduction to traffic at a flow level of about 1400 vph. For safety analysis, an analysis of the trajectories from bust stop locations at 0, 25, 50, 75, 100, 125, 150, and 175 feet from the intersection shows how the number of conflicts is affected by the presence and location of a curbside stop on the link of a road diet. the number of rear-end conflicts is significantly greater when the bus stop is at the intersection. The conflicts appear to as the blockage moves away from the intersection until there was no statistically significant difference at approximately 100 feet from the intersection. For crossing conflicts, again, there were more conflicts the closer the curb stop is to the intersection on a road diet. In this case, the conflicts decrease linearly with distance until about 100 feet.

Presentation

Final Report