Jonathan Sigel Section A December 19 th, 2016 Literature Review: Final Function and Purpose of a Roundabout: Roundabouts are a location in which multiple roads are joined together in a circle, with an impassable central median located in the middle. Depending on the location of the structure, roundabouts can also be referred to as rotaries or traffic circles. In some areas, traffic circles are considered a distinct entity, marked by their comparatively low speeds and lack of a yield-atentry law (New York Department of Transportation). A major benefit of a roundabout is that they contain no signs or traffic signals. A driver can pass through a roundabout without slowing down at a stop sign or traffic light. Knowledge of how to pass through a roundabout in the state of Massachusetts is essential, because they are far more common than in other parts of the country (Massachusetts Registry of Motor Vehicles, 2011). Numerous laws govern the movement of vehicles in a roundabout. First, traffic is meant to flow through a roundabout in a counter-clockwise manner. Drivers must yield to any vehicles already in the roundabout and make proper usage of their turn signals for the benefit of other drivers and pedestrians. A driver must also know how to use the inner and outer lanes to efficiently pass through a roundabout. A quarter or half turn requires that a driver must use the outer lane, while a three quarter or U-turn must be performed in the inner lane (Massachusetts Registry of Motor Vehicles, 2011). By obeying the traffic laws, drivers should be able to pass through a roundabout quickly and without incident.
Safety Advantages and Safety Issues in Roundabouts When implemented properly, roundabouts are a safe and efficient road formation. Per studies conducted by the Washington State Department of Transportation, roundabouts built in places where signalized intersections once were decreased fatal accidents by ninety percent. Nonfatal collisions between vehicles, and collisions between cars and pedestrians, also decreased significantly. Figure 1. The above chart shows the percent reduction of each type of collision, after implementing a roundabout in the place of a signalized intersection. Fatal accidents, as well as accidents that afflict injury, are greatly reduced (Washington State Department of Transportation).
The reasons for the decrease in accidents include low travel speeds and one way traffic (Washington State Department of Transportation). The law requiring cars to yield to vehicles already in the roundabout is also a major advantage of the edifice. Roundabouts in locations where this law was not enforced were found to be significantly less efficient (Morrow). Additionally, the absence of traffic lights prevents drivers from speeding up to get past a light before it turns red (Washington State Department of Transportation). These advantages often make roundabouts a superior alternative to traditional signalized intersections. However, flaws can still exist in the design of a roundabout. Improper marking and signage is a large issue in roundabout design (Traffic Transfund of New Zealand). When a roundabout is built over a previously extant intersection, the signs and road markings are not always properly adjusted. These discrepancies have the potential to confuse drivers, which may lead to accidents (Traffic Transfund of New Zealand). Therefore, it is recommended that all old markings and redundant signs are removed when constructing a roundabout. In order to ensure that all road users are safe, provisions must be granted to cyclists and pedestrians Design flaws that endanger pedestrians include the position and quality of cross walks (Traffic Transfund of New Zealand). Cross walks that are placed improperly or not fully visible - either due to lack of maintenance or lack of light are very dangerous for pedestrians. The Traffic Transfund of New Zealand recommends that cross walk paths link together and are placed far away from the roundabout itself. Narrow roads and large roundabouts may endanger cyclists, because they force bikers to share lanes with traffic (Traffic Transfund of New Zealand). Another possible solution is for cyclists to not utilize their bikes while in a roundabout. The Wisconsin Department of Transportation recommends that cyclists dismount their bikes and walk through roundabouts in order to stay safe. Widening a roundabout not only increases its capacity, but also makes cyclists
more safe. Other issues that may impede the flow of traffic are excessive vegetation and poorly placed light posts (Traffic Transfund of New Zealand). These issues can be addressed by removing surplus plant life and adding more light posts. Improving Roundabout Efficiency Numerous factors determine the design and efficiency of a roundabout. These factors include the placement of pedestrian crosswalks, the width of the roads feeding into the roundabout, the size and position of the central median, and the presence or absence of lanes (Corrigan, 2014). The impact of crosswalks varies based on their position and the number of pedestrians that need to use them. If one pedestrian must traverse multiple crosswalks in order to reach his or her destination, the walker stops numerous lanes of traffic for a brief period. The distance of the crosswalks to the roundabout may also influence the impact that they have on traffic. The size of the streets is crucial to the efficient flow of traffic (Corrigan, 2014). The capacity of the roundabout is determined by the size of the circular lanes and connecting roads. Equations for calculating the capacity of a portion of a roundabout commonly incorporate the road length, road width, and the number of weaves in a road. Road weaves are defined as the turns that a driver can take within the roundabout (Çalişkanelli, 2009). This calculation can be applied to both single and double lane roundabouts. (United States Department of Transportation). When investigating ways to alter the efficiency of a rotary, capacity is one of the most important factors, as it dictates how many cars the roundabout can hold at a given time (Corrigan 2014). A roundabout with low capacity is incapable of handling a large amount of traffic output from the connecting streets, resulting in congestion. However, larger rotaries may take up too much space, and negatively affect nearby businesses.
The arrangement of lanes in a roundabout greatly influences the behavior of drivers. The presence of lanes makes it clear to drivers where to drive, allowing them to pass through more easily. The absence of clear lane lines can lead to disorderly traffic patterns, which may cause accidents and congestion. However, the addition of lanes carry their own disadvantage: as mentioned previously, in roundabouts with inner and outer circles, the recommended procedure changes (Corrigan, 2014). Many drivers may be unaware of the guidelines governing a multiplelane roundabout, which could produce numerous issues. The central barrier of a roundabout plays a large role in determining its capacity and the layout of its lanes. In order to create the circular shape needed for a roundabout to function, a central barrier is necessary (Corrigan, 2014). This central barrier is commonly called a median. The size and position of the median can be altered. A smaller median would allow a greater capacity within the roundabout, thereby creating a smoother traffic flow. Additionally, the median does not need to be placed in the exact center of the roundabout; some medians are closer to the outer edge of the roundabout, producing an asymmetrical flow of traffic. Due to the factors of size and location, the position of a median can decide the presence and arrangement of lane lines (Corrigan, 2014). Current Methods for Modeling Traffic Many current traffic models focus on two important factors: the capacity of streets, and the number of cars spilling back. Cars spill back when they are unable to move into the section of traffic that they wish to enter. Traffic builds up behind slowed or stopped cars causing vehicles to accumulate. A traffic model that focuses on capacity and spillback is called a queue model (Grether, 2012). In a queue model, cars are arranged into queues: lines of cars waiting for their turn to pass through the end of the street (Grether, 2012). Streets are split into multiple
nodes, each with its own queue. Each succession of cars has a certain number of vehicles it can hold, which is determined by the street s physical characteristics (Grether, 2012). The goal of these models is to minimize spillback. Excessive spillback may eventually cause all nodes to reach maximum capacity, which in turn results in severe traffic delays. Splitting the nodes in a queue model into separate lanes provides more accurate results (Grether, 2012). Other iterations of this concept split nodes based on the intention of the driver: drivers who intend to go left, right, or straight would all be sorted into separate lines (Grether, 2012). An alternative to queue models utilizes raw traffic statistics to perform regressive analysis: this model provides estimates for how long it will take for a given car to pass through a given number of streets. This method is considered less accurate than the queue model, largely due to its failure to account for the capacity and spillback within separate nodes (Grether, 2012). Current Methods for Modeling Traffic in A Roundabout Although methods for modeling traffic in a roundabout are similar to the methods used to model standard traffic, the roundabout s complexity necessitates the input of additional variables. Vehicles passing into the traffic circle from connecting streets are still modeled using a queue based approach. However, when entering the roundabout itself, critical gap acceptance theory should be considered (Çalişkanelli, 2009). Critical gap acceptance theory states that cars should account for the distance between cars while transitioning from the minor to major stream of traffic (Çalişkanelli, 2009). In the context of a roundabout, the major and minor streams refer to traffic in the roundabout and the connecting streets, respectively (Massachusetts Registry of Motor Vehicles). For every car entering the roundabout, there is a critical gap time, in which the distance between vehicles is great enough for a car to enter the roundabout. In roundabouts with many lanes, if the time needed to move through the connecting street is greater than the critical
gap time, traffic may stall (Çalişkanelli, 2009). Additionally, some roundabout models may use statistical regression. Regression is often used to compute the maximum capacity for a given traffic queue, but some simulations utilize nothing but statistical regression in order to calculate the flow of traffic in a roundabout. The application of statistical regression to traffic in a roundabout is referred to as the Ashworth and Field method, named after its creators. (Çalişkanelli, 2009). For initial or rough approaches to a given roundabout, the Ashworth and Field method could be useful for obtaining quick results (Çalişkanelli, 2009). Additionally, the method could be further refined to consider more factors (Çalişkanelli, 2009). However, Çalişkanelli concludes that models incorporating critical-gap time are superior to methods, like the Ashworth and Field method, that only make use of statistical regression. Regarding single lane roundabouts, another option exists. A model proposed by Rulili Wang utilizes a queue based approach and randomized driver behavior. Driver behavior not only includes their start and end points, but also reckless choices that they might make. Driver recklessness and the throughput (total cars outputted) by the model were major points of observation. (Wang, 2002) Combining this randomized approach with the laws and models for a double lane roundabout could produce a versatile system for simulating roundabouts. (Wang, 2002) Engineering Plan A. Problem Statement Numerous roundabouts in Worcester have been modernized, but the roundabout in Newton Square has not been improved. As a result of the roundabout s poor design, the slow and congested traffic passing through the Newton Square often causes delays and accidents.
B. Engineering Goal: By programming a computer simulation of the Newton Square roundabout, numerous iterations of the model will be created that will show different designs for the roundabout. These different models will be measured against a set of criteria to determine an alternate design for the Newton Square roundabout. Drastic changes, such as adding or removing streets, will not be considered. This is due to the high cost and political issues associated with changing the Newton Square roundabout. Such a large-scale project would be incredibly costly when compared to the more cost effective methods addressed earlier, such as repositioning crosswalks. Additionally, numerous businesses located near the Newton Square roundabout would find themselves inconvenienced by any long-term construction occurring nearby (Corrigan, 2014).
Figure 2. A satellite view of the Newton Square roundabout. Notable features include the five roads of varying width, and the asymmetrical design of the roundabout itself. (Picture from Google images.) C. Procedure In order to develop a new design for the Newton Square roundabout, a traffic simulation will be programmed. The simulation makes use of the concepts found within the queue traffic model and critical gap model to increase accuracy. The initial models will first be of basic street and intersection layouts, before advancing to simulating a generic four street roundabout. Numerous layers of complexity, such as pedestrians and vehicles of varying sizes, will be added to increase the accuracy of the models. All of this will be coded within Game Maker Studio 1.4. A transition to Game Maker Studio 2 may be possible when the new program is released. After successfully coding the logic needed to simulate a generic roundabout, the program will then be adapted to simulate the unique layout of the Newton Square roundabout. Data from traffic studies conducted by the state of Massachusetts will be used in the simulation, in order to further improve its accuracy. Also included in the data will be the data obtained from Corrigan s team during their attempt to redesign the Newton Square roundabout. The combination of these two sources will provide ample information when programming a simulation. A late addition to the model will be a visual mode for the program, that generates an animated picture of the roundabout. When the simulation of Newton Square is complete, many different iterations of it will be programmed. These new iterations will address the risk factors delineated previously, such as road width, lane lines, and the position of the central median. A large focus during the testing of all models will be the performance of the simulation before and after school hours. Many of the issues recorded in the Newton Square roundabout are a result of school buses jamming traffic
(Corrigan, 2014). The simulation will collect various data points each time it is executed. These data points will include information such as the time elapsed, the number of accidents that occurred, and the number of cars that successfully pass through the rotary. These outputs will be compared to the outputs of the original model and a set of criteria in order to verify their effectiveness. An effective design for the Newton Square roundabout would allow many cars to pass through it in a given timeframe, minimize the number of accidents, be intuitive to navigate, and have a low cost. A cost analysis will be performed outside of the simulation to determine the price of implementing the simulated solution. Efficiency and safety will be given the highest priority when choosing a new design for the Newton Square roundabout. The cost will be considered the third most important attribute, followed by how intuitive it is to navigate.
Citations 4329 Modeling Roundabout Traffic Flow as a Dynamic Fluid System. (n.d.). Retrieved December 4, 2016, from http://www.bing.com/cr?ig=ccafd84ad1a2402a9c702da2b1e0ef3f&cid=00b6 85155609663428D38CF4573867C7&rd=1&h=QGJdjfJzifhIwqa9wPWoVQemc- PhRIcTuepo5zlgYjc&v=1&r=http://www.math.washington.edu/~morrow/mcm/4329.pdf &p=devex,5066.1 Appleton, I. (n.d.). Transfund: Ins and Outs of Roundabouts. Corrigan, M. (n.d.). Redesign of Newton Square. Retrieved November 15, 2016, from https://web.wpi.edu/pubs/e-project/available/e-project-030714-112944/unrestricted/newtonsquare_complete_submission.pdf How Roundabouts Work. (n.d.). Retrieved December 1, 2016. Massachusetts Registry Of Motor Vehicles - Driver's Manual. (n.d.). Roundabouts. (n.d.). United States Department of Transportation. Retrieved December 3, 2016. Simulation of Urban Traffic Control: A Queue Model Approach. (n.d.). Retrieved November 15, 2016, from http://www.sciencedirect.com/science/article/pii/s1877050912004619 Wang, R. (n.d.). Modeling traffic flow at a single-lane urban roundabout. Retrieved December 04, 2016, from http://www.sciencedirect.com/science/article/pii/s0010465502003624 What is a Roundabout? (n.d.). Retrieved December 1, 2016, from https://www.dot.ny.gov/main/roundabouts/background WSDOT - Roundabout Benefits. (n.d.). Retrieved December 04, 2016, from http://www.wsdot.wa.gov/safety/roundabouts/benefits.htm