Joseph E. Hummer

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.

Operational Analysis of Uninterrupted Bicycle Facilities

The popularity of bicycles in North America is growing. As the popularity of bicycles has increased, so has the physical network of separate bicycle facilities and designated bicycle lanes in many locations. As a consequence of this growth, there is a demand for more information about bicycle operations on these facilities. Unfortunately, the state of knowledge regarding bicycle operations in the United States currently lags far behind that of motor vehicles and pedestrians. The international research that has been conducted to date regarding bicycle operations on uninterrupted facilities is thoroughly reviewed, and recommended procedures for the operational analysis of uninterrupted bicycle facilities are outlined. The recommended procedures are based on the concept of “frequencies of events” involving a bicyclist and other bicyclists or facility users. Events are defined as bicycle maneuvers required by a bicyclist on a facility, including passings (same-direction encounters) and meetings (opposite-direction encounters). The frequency of events for an uninterrupted bicycle facility is related to the service volumes of bicycles using or projected to be using the facility and does not have to be observed directly. The proposed procedures are, therefore, recommended based not only on their theoretical substance but also on their ease of use by practitioners.

Effect of Pedestrians on Capacity of Signalized Intersections

In Chapter 9 of the 1994 update to the 1985 Highway Capacity Manual, the operational and planning analysis of signalized intersections is discussed. The methodology for saturation flow rate estimation does not consider all elements of the interaction between pedestrians and turning vehicles. This study describes this interaction for left and right turns using a conflict-zone-occupancy approach. A conflict zone is a portion of an intersection, typically in the crosswalk, in which pedestrians and vehicles compete for space. Conflict-zone occupancy, defined as the fraction of the effective green period during which pedestrians occupy a conflict zone, provides the basis for a rational adjustment to saturation flow. This study details the results of a multiregional data collection effort that confirms the validity of the conflict-zone-occupancy approach. In addition, this study describes the effect of geometric constraints, as reflected in the number of receiving lanes versus the number of turning lanes, on turning-vehicle saturation flow. After consideration of signalized intersection phasing and turn protection, one can calculate saturation flow adjustment factors reflecting the effect of pedestrians on lane groups containing vehicles turning left (fLpb) or right (fRpb).

Operational and safety effects of U-turns at signalized intersections

In an effort to balance vehicular safety, mobility, and access, many transportation officials favor the use of raised median cross sections on highways. However, this decision leads to much controversy from those opposed to the resulting lack of direct driveway access. One of the issues in this controversy is the effect of increased U-turns at adjacent intersections. The purpose of this research was 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 signalized intersections. Regression analysis of saturation flow data showed 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 was opposed by protected right-turn overlap from the cross street. The safety analysis involved a set of 78 intersections. Fifty-four sites were chosen randomly, and 24 sites were selected on the basis of their reputation as U-turn problem sites. Although the group of study sites was 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 3-year study period. 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.

EFFECT OF BICYCLES ON CAPACITY OF SIGNALIZED INTERSECTIONS

Although much is known about the operation of signalized intersections, little or no empirical research has been conducted regarding the effect of bicycles on signalized intersection capacity. The purpose of this study was to accurately quantify the effects of bicycles on signalized intersection capacity through the videotaping of several intersections that had significant bicycle traffic. Through the videotaping of intersections in Davis, California, and Gainesville, Florida, a relationship was determined between bicycle volumes and the percent of the green phase during which bicycle traffic occupies a conflict zone between bicycles and right-turning motor vehicles. It was also determined that one can ascertain the total net occupancy due to pedestrians and bicycles by taking the overlapping effects between bicycles and pedestrians into account. Using this total occupancy due to bicycles and pedestrians, one can calculate a saturation flow adjustment factor (fRph) that reflects the reduction in saturation flow, and ultimately lane group capacity, for lane groups containing vehicles making permissive right turns in the presence of bicycles and pedestrians. The proposed procedure yields lower saturation flows and capacities than the current Highway Capacity Manual (HCM) procedure. In other words, on the basis of empirical data, when combined with pedestrian effects, the impact of bicycles on the saturation flow of lane groups containing right-turning vehicles is probably more detrimental than previously believed, and the capacities of intersections with significant bicycle and pedestrian traffic may be overestimated by using the current HCM procedures.

TRAVEL TIME COMPARISONS BETWEEN SEVEN UNCONVENTIONAL ARTERIAL INTERSECTION DESIGNS

Signalized intersections on high-volume arterials are often congested during peak periods, causing a decrease in through movement efficiency on the arterial. Much of the vehicle delay incurred at conventional arterial intersections is caused by high left-turn demand. Unconventional intersection designs attempt to reduce intersection delay and travel times by rerouting left turns away from the main intersection. Seven unconventional designs—the quadrant roadway intersection, median U-turn, superstreet median, bowtie, jughandle, split intersection, and continuous flow intersection designs—that could apply to a wide range of standard, four-leg intersections are compared. Previous comparisons of intersection delay and travel time between conventional designs and these unconventional designs have been piecemeal and have largely used hypothetical volumes. Simulation experiments were conducted using turning movement data from seven existing intersections of varying sizes to compare the travel time of conventional and unconventional designs fairly. Optimum cycle lengths were used for each design, and a number of factors were held constant to keep the comparisons fair. Off-peak, peak, and peak-plus-15-percent volume levels were examined. The results from the simulations showed that at each intersection one or more unconventional designs had lower total travel times than the conventional design. Whereas most of the unconventional designs showed improvement in one or more scenarios, the quadrant roadway intersection and the median U-turn designs consistently produced the lowest travel times. When considering the design of high-volume intersections like those tested, engineers should seriously consider quadrant roadway intersection and median U-turn designs where rights-of-way are available.

Analyzing System Travel Time in Arterial Corridors with Unconventional Designs Using Microscopic Simulation

Recent studies have identified the median U-turn (MUT) design as having potential for improving intersection stopped delay at isolated intersections. This study was an effort to quantify the reductions in travel time and other critical traffic operation measures of effectiveness for the MUT and super-street median (SSM) geometric designs over a system of signals compared with the traditional two-way left-turn lane (TWLTL) design. The key function of the MUT design is removal of all left-turn movements at signalized intersections, creating two-phase signal operations and increased progression opportunities. The SSM design allows perfect progression of through traffic in both directions because signals on both sides of the arterial can operate independently. The analysts constructed models of a typical suburban arterial corridor near Detroit, Michigan, in CORSIM according to a fixed external-node coordinate system and fixed origin-destination volumes. These allowed an equitable systemwide comparison of measures of effectiveness between the arterial designs. An analysis of variance determined the importance of arterial geometry related to total system time, average stops per vehicle, and average speed. Results showed that the MUT and SSM designs improved system travel time and average speed compared with the TWLTL design. Analyzing geometry as a function of the time of day showed that the MUT and SSM alternatives significantly reduced system travel time and increased average speeds during the a.m. and p.m. peak hours and showed very similar results compared with the TWLTL design during off-peak conditions. The authors recommend that engineers analyzing alternatives for arterials similar to that tested consider the MUT and SSM designs.

Modeling the impact of spatial relationships on horizontal curve safety

The curved segments of roadways are more hazardous because of the additional centripetalforces exerted on a vehicle, driver expectations, and other factors. The safety of a curve is dependent on various factors, most notably by geometric factors, but the location of a curve in relation to other curves is also thought to influence the safety of those curves because of a drivers expectation to encounter additional curves. The link between an individual curves geometric characteristics and its safety performance has been established, but spatial considerations are typically not included in a safety analysis. The spatial considerations included in this research consisted of four components: distance to adjacent curves, direction of turn of the adjacent curves, and radius and length of the adjacent curves. The primary objective of this paper is to quantify the spatial relationship between adjacent horizontal curves and horizontal curve safety using a crash modification factor. Doing so enables a safety professional to more accurately estimate safety to allocate funding to reduce or prevent future collisions and more efficiently design new roadway sections to minimize crash risk where there will be a series of curves along a route. The most important finding from this research is the statistical significance of spatial considerations for the prediction of horizontal curve safety. The distances to adjacent curves were found to be a reliable predictor of observed collisions. This research recommends a model which utilizes spatial considerations for horizontal curve safety prediction in addition to current Highway Safety Manual prediction capabilities using individual curve geometric features.

QUALITY OF SERVICE FOR INTERRUPTED-FLOW PEDESTRIAN FACILITIES IN "HIGHWAY CAPACITY MANUAL" 2000

The objective of the research described in this paper was to develop the basis for revised operational analysis procedures for transportation facilities with pedestrian users where flow is interrupted by traffic control devices. The paper commences with some background information on pedestrian walking speeds at signalized crossings and on pedestrian noncompliance at these locations. Then both new and revised level-of-service (LOS) tables are provided for analyzing various types of interrupted-flow pedestrian facilities. Results are detailed of a review and synthesis of American and international literature as part of a Federal Highway Administration study of pedestrian and bicycle facilities conducted by North Carolina State University between 1995 and 1998. The year 2000 edition of the U.S. Highway Capacity Manual (HCM) will incorporate, either directly or indirectly, most of the revised and synthesized information described in this paper. The research team recommends that the HCM include some background information that will be helpful for analysts timing signals and performing other operations. The authors of this paper recommend assumed crosswalk walking speeds of 1.2 m/s for most areas and 1.0 m/s for crosswalks serving large numbers of older pedestrians. Another important recommendation was to use delay to pedestrians as the basis for level of service at signalized and unsignalized street crossings. The recommended threshold for LOS F at signalized crossings is 60 s or more of delay per pedestrian, whereas at unsignalized crossings it is 45 s or more of delay per pedestrian.

Curve Collisions: Road and Collision Characteristics and Countermeasures

Horizontal curves are relatively dangerous portions of roadway networks. Agencies optimizing the use of safety funds should be aware of characteristics of the collisions on those segments. However, few previously published articles attempted to characterize collisions on horizontal curves. This article describes an effort that characterized collisions reported to be on curves in North Carolina using the Highway Safety Information System. More than 51,000 North Carolina (NC) collisions on two-lane road curves were compared to collisions on all two-lane roads and on all roads. In doing so we gained a perspective of how well various curves perform relative to other road areas. We investigated numerous two-lane curve-collision types. Those most overrepresented included: collisions on grades, rural, severe injury or fatal, fixed object (particularly tree, ditch, and embankment), overturn, off-peak hours (particularly during darkness on unlighted roads), weekend, holiday periods, and wet, icy, or snowy pavement. The analysis also revealed that there were few short roadway segments (of 0.1- to 1-mile length) with more than 10 reported curve collisions in 3 years. This article provides recommendations from the literature to treat overrepresented collision types on horizontal curves. Based on the analysis findings, agencies should target countermeasures for the most common and overrepresented collision types.