Karen K Dixon

Capacity for North Carolina Freeway Work Zones

Work zone capacity values for rural and urban freeways without continuous frontage roads were defined and determined. Data were collected using Nu-Metrics counters and classifiers at 24 work zones in North Carolina. The research included analysis of speed-flow behavior, evaluation of work zone sites based on lane configuration and site location, and determination of the location within the work zone where capacity is lowest. It was shown that the intensity of work activity and the type of study site (rural or urban) strongly affected work zone capacity. The data suggested that the location where capacity is reached is also variable based on the intensity of work. For heavy work in a two-lane to one-lane work zone configuration, the capacity values proposed at the active work area are approximately 1,200 vehicles per hour per lane for rural sites and 1,500 vehicles per hour per lane for urban sites. It is recommended that two distinct volumes be used when queue behavior in a freeway work zone is analyzed. The collapse from uninterrupted flow (designated work zone capacity) and the lower queue-discharge volume both should be considered.

Analysis of Fatal Crashes in Georgia Work Zones

Studies on work zone crashes typically examine a combination of injury, fatal, and property damage crashes to identify factors that contribute to unsafe conditions within work zones. Few studies focus on fatal crashes within work zones, although a clear understanding of the driver, roadway, and work zone conditions associated with fatal crashes will facilitate the development of strategies aimed at improving safety and reducing fatal as well as nonfatal crashes. The Georgia Department of Transportation recently performed a study to identify the manner of collision, location, and construction activity associated with fatal crashes in work zones. This study is expanded further to examine the difference between fatal crash activity within work zones compared with fatal crashes in non-work-zone locations. Using data from three work zone locations in Georgia, fatal crash activity within work zones also was compared with nonfatal crashes within work zones. Finally, fatal crash activity was examined to determine the influence of the work zone activity on the frequency of fatal crashes. The overall findings of the study indicate that the work zone influences the manner of collision, light conditions, truck involvement, and roadway functional classification under which fatal crashes occur. The study also indicates that fatal crashes in work zones are more likely to involve another vehicle than non-work-zone fatal crashes, and fatal crashes in work zones are less influenced by horizontal and vertical alignment than are non-work-zone crashes.

Effect of Wireless Communication and Entertainment Devices on Simulated Driving Performance

An analysis of the effect of wireless telephone communication using text and voice modalities as well as an Apple iPod on lane keeping, speed, speed variability, lateral speed, and lane position variability was conducted with a driving simulator. Participants (young adult licensed drivers) drove in an unusually curvy simulated driving environment while using wireless devices, controlling an iPod, and participating in conversations and word games. As expected on the basis of previous research, lane-keeping performance was robust for voice communication tasks; however, the text messaging and iPod tasks that required significant manual manipulation of the device resulted in significant decrements in lane-keeping performance. In addition, all wireless communication tasks and the iPod task resulted in significant increases in speed variability throughout the driving scenario. Lateral speed increases occurred for all wireless communication tasks other than the cellular phone conversation as well as the iPod task. Increases in lane position variability were observed for the text messaging conditions. In addition to establishing the dramatic performance decrement caused by text messaging tasks, this experiment suggests that driving performance may be affected by distraction in ways not captured by lane-keeping measures alone and explores potential alternative measures of driving performance that may be useful for identifying and quantifying the effects of distracted driving.

Normal Deceleration Behavior of Passenger Vehicles at Stop Sign–Controlled Intersections Evaluated with In-Vehicle Global Positioning System Data

Deceleration characteristics of passenger cars are often used in traffic simulation, vehicle fuel consumption and emissions models, and intersection and deceleration-lane design. Most previous studies collected spot speed data with detectors or radar guns. Because of the limitations of the data collection methods, these studies could not determine when and where drivers began to decelerate. Therefore, the studies may not provide an accurate estimation of deceleration time and distance. Furthermore, most previous studies are based on outdated and limited data, and their conclusions may not be applicable to the current vehicle fleet and drivers. The normal deceleration behavior of current passenger vehicles is evaluated at stop sign-controlled intersections on urban streets on the basis of in-vehicle Global Positioning System data. This study determined that drivers with higher approach speeds decelerated over a longer time and distance. Higher initial deceleration rates were also associated with higher app...

NORMAL ACCELERATION BEHAVIOR OF PASSENGER VEHICLES STARTING FROM REST AT ALL-WAY STOP-CONTROLLED INTERSECTIONS

Acceleration characteristics of passenger cars starting from rest are often used in traffic simulation, modeling of vehicle fuel consumption and emissions, and design of intersections, acceleration lanes for entrance terminals, turning bays, and passing lanes. Most of the previous studies developed four different acceleration models: the constant acceleration model, the two-phase model, the linear-decreasing acceleration model, and the polynomial acceleration model. However, most of the studies were based on outdated and limited data, so their conclusions may not be applicable for the current vehicle fleet and drivers. The normal acceleration behavior of current passenger vehicles starting from rest at all-way stop-controlled intersections is evaluated, and the previous acceleration models are verified with recent observations. A comparison is included between the acceleration behaviors of straight and turning maneuvers and the influence of speed limits on acceleration rates, and two new polynomial models are developed for driver acceleration behavior for turning maneuvers versus straight maneuver acceleration from a stopped condition.

Calibration of "Highway Safety Manual" Predictive Models for Oregon State Highways

The recently released Highway Safety Manual (HSM) published by AASHTO provides a comprehensive set of tools for evaluating and identifying opportunities to improve safety for highway facilities. Included in the HSM is a quantitative method for predicting crashes on the basis of recently developed scientific approaches. These predictive methods currently exist for three facility types: rural two-lane roads; rural multilane highways; and urban and suburban arterial highways. To enhance precision, each HSM predictive method should be calibrated for location conditions. This paper demonstrates the HSM calibration procedure for total crashes in Oregon. The research identified three critical data collection limitations on information about pedestrian volumes, minor road traffic volumes at rural locations, and minimum sample size for underrepresented crash locations. Most of the calibration factors for Oregon were determined to be considerably lower than the expected value of approximately 1, and this observation was attributed to Oregon crash reporting thresholds and procedures. The paper includes an evaluation of crash severity distribution methods and an assessment of the significance of collision type distributions on the overall predicted crashes.

Operating-Speed Model for Low-Speed Urban Tangent Streets Based on In-Vehicle Global Positioning System Data

Low-speed urban streets are designed to provide both access and mobility and to accommodate multiple road users, such as bicyclists and pedestrians. However, speeds on these facilities often exceed the intended operating speeds, as well as their design speeds. Several studies have indicated that the design speed concept, as implemented in the roadway design process in the United States, does not guarantee a consistent alignment promoting uniform operating speeds less than design speeds. A promising design approach to overcome these apparent shortfalls of the design speed approach is a performance-based design procedure with the incorporation of operating speeds. However, this approach requires a clear understanding of the relationships between operating speeds and various road environments. Although numerous previous studies have developed operating-speed models, most of these studies have concentrated on high-speed, rural two-lane highways. In contrast, highway designers and planners have little informat...

Evaluating Speed-Reduction Strategies for Highway Work Zones

Research was conducted to identify the potential of fluorescent orange sheeting, innovative message signs, and changeable message signs with radar for reducing speeds in highway work zones. The study investigated the effect of each strategy immediately after implementation (immediate effect) as well as several weeks after implementation (novelty effect). In addition to the overall effect of each strategy on all vehicles, the study included the effect on specific vehicle types during various lighting conditions. The researchers collected traffic data before, immediately after, and 2 or 3 weeks after implementation of each strategy (3 consecutive weeks for the changeable message sign). They collected data upstream of the temporary traffic-control zone, in the advancewarning area, and adjacent to the active work area. The researchers used various statistical tests to evaluate the significance of speed changes from phase to phase and adjusted vehicle speeds with the upstream speed changes over time. The study indicated that fluorescent orange sheeting and innovative message signs help reduce speeds at highway work zones (with diminished influence over time). Moreover, both strategies influence vehicle speeds more during the day than at night. Drivers of passenger vehicles tended to decrease their speeds more than did truck drivers. Changeable message signs with radar significantly reduced the vehicle speeds in the immediate vicinity of the sign and did not demonstrate a novelty effect.

Conflict Analysis for Double Left-Turn Lanes with Protected-Plus-Permitted Signal Phases

A conflict study evaluated double left-turn lanes with protected-plus-permitted signal phasing. The data collection team observed traffic behavior at four intersections in the metro Atlanta region. Included in the data set are three intersections operating under protected-plus-permitted signal phasing and a fourth intersection with a before and after examination of a protected-plus-permitted signal phase changed to protected-only phasing. The authors calculated the traffic rates for five conflict types and one traffic event unique to double left-turn movements. A statistically significant decrease in traffic conflicts was identified for the before and after comparison site. The study also identifies unique intersection geometry and traffic volumes at each site and compares traffic conflicts associated with the features. The results generally show that the enhanced capacity of double left-turn lane intersections operating under protected-plus-permitted signal phasing may occur at the expense of the safety of the traveling public.

Predicting single-vehicle fatal crashes for two-lane rural highways in Southeastern United States

The rural two-lane highway in the southeastern United States is frequently associated with a disproportionate number of serious and fatal crashes and as such remains a focus of considerable safety research. The Georgia Department of Transportation spearheaded a regional fatal crash analysis to identify various safety performances of two-lane rural highways and to offer guidance for identifying suitable countermeasures with which to mitigate fatal crashes. The fatal crash data used in this study were compiled from Alabama, Georgia, Mississippi, and South Carolina. The database, developed for an earlier study, included 557 randomly selected fatal crashes from 1997 or 1998 or both (this varied by state). Each participating state identified the candidate crashes and performed physical or video site visits to construct crash databases with enhance site-specific information. Motivated by the hypothesis that single- and multiple-vehicle crashes arise from fundamentally different circumstances, the research team applied binary logit models to predict the probability that a fatal crash is a single-vehicle run-off-road fatal crash given roadway design characteristics, roadside environment features, and traffic conditions proximal to the crash site. A wide variety of factors appears to influence or be associated with single-vehicle fatal crashes. In a model transferability assessment, the authors determined that lane width, horizontal curvature, and ambient lighting are the only three significant variables that are consistent for single-vehicle run-off-road crashes for all study locations.