Ana Tsui Moreno

Use of speed profile as surrogate measure: Effect of traffic calming devices on crosstown road safety performance

Urban road safety management is usually characterized by the lack of sufficient, good quality crash data and low budgets to obtain it even though many traffic accidents occur there. For example, 54 percent of road crashes in Spain take place in urban areas, and 10 percent of urban fatal crashes occur on crosstown roads, which are rural roads that traverse small communities. Traffic calming measures (TCMs) are often implemented on these parts of rural roads that traverse small communities in order to reduce both the frequency and severity of crashes by lowering speeds, but evaluation of their effectiveness has been limited. The objective of this study was to develop a methodology using continuous speed profiles to evaluate the safety effectiveness of TCMs on crosstown roads as part of an integrated system in the absence of historical data. Given the strong relationship between speed and crash experience, safety performance can be related to speed. Consequently, speed can be used indirectly as a surrogate safety measure in the absence of crash and speed data. Two indexes were defined in this study as surrogate safety measures based on the continuous speed profile: Ra and Ea. Ra represents the absolute accumulated speed variations relative to the average speed and is inversely related to accumulated speed uniformity; and Ea represents the accumulated speed variations above the speed limit and is directly related to accumulated speeding. Naturalistic data were collected using GPS trackers for 12 scenarios with different TCM spacings. Then, the indexes were applied to individual observed speed profiles (individual analysis) as well as the operating speed profile (global analysis). The values obtained from individual and global analysis were statistically different. Spacing lower than 110m, which was found optimal from previous research, did not allow drivers to modify their speeds as the accumulated speed uniformity was quite similar regardless of the average operating speed; and, accumulated speeding was also minimized. Consequently, the scenarios where the TCMs were implemented according to the technical criteria presented a better design quality. On the other hand, age and gender differences did not seem to affect the average speeds or the accumulated speed uniformity and accumulated speeding.

Operational Effectiveness of Passing Zones Depending on Length and Traffic Volume

Most studies on two-lane highway operations have focused on the percentage of following vehicles or the adjustment of the Highway Capacity Manual (HCM) procedure to local data. The HCM proposes the length of no-passing zones as a model parameter; however, the distribution and characteristics of passing zones are not addressed. In fact, only a few studies on the expected number of passes in a passing zone have been carried out. This research presents an analysis of the effectiveness of passing zones in terms of their length and traffic volume. Data were collected from four passing zones on a rural highway in Spain. The two-way traffic volumes ranged from 100 to 900 vehicles per hour (vph), and the passing zone lengths ranged from 265 to 1,270 m. More than 1,600 passing maneuvers were recorded. The operational effectiveness of the passing zones was obtained from the passing frequency and the passing rate. The results indicated that the longer the passing zone, the higher the passing frequency; however, the results stabilized with lengths above 1,100 m. Balanced flows with two-way traffic volumes between 600 and 700 vph optimized the number of passes. Nevertheless, the increase in the passing frequency with the traffic volume was lower than with the increase in following vehicles. The results were validated with data from another 12 passing zones. Finally, HCM adjustments based on the percentage of no-passing zones did not reliably represent the effectiveness of passing zones. Therefore, the effectiveness of every passing zone should be considered, and adjustment factors should be modified to maximize the passing opportunities for traffic volumes between 600 and 700 vph.

Daytime and Nighttime Passing Maneuvers on a Two-Lane Rural Road in Spain

Passing is one of the most complex driving maneuvers performed on two-lane rural roads and has important effects on road safety and traffic operation. Passing is affected by driving behavior, road geometry, traffic volume, and traffic composition as well as external factors. Research was developed to compare the passing process under daytime and nighttime conditions. An experimental method was designed to collect video data of passing maneuvers on a two-lane rural road segment located near Valencia, Spain. Two methods were used: (a) external observations of four passing zones with six video cameras and (b) an instrumented vehicle equipped with video cameras and laser rangefinders, driven slightly below the operating speed along a segment of the same road so it would be passed by other vehicles. A total of 291 maneuvers were observed, up to 20% of which were at night. Macroscopic analysis results indicated that approximately 17% of passes were at night, even though passing frequency and passing demand decreased at night. Also, the behaviors of individual drivers who passed other vehicles were different at night and during the day. Maneuvers limited by the presence of an opposing vehicle were performed more quickly at night, even if the accepted gaps were longer. In this case, a more difficult perception of distances to opposing vehicles and of vehicle speeds explained the differences. In contrast, maneuvers limited by sight distance (without a visible opposing vehicle) were slower at night. This observation matched a traditional hypothesis: passing at night is safer because the headlights of an opposing vehicle allow a driver to anticipate the vehicles position before it becomes visible.

Sight distance standards based on observational data risk evaluation of passing

The paper presents an application of a reliability analysis for evaluating the risk associated with passing sight distance (PSD) standards in terms of the expected probability of noncompliance. Calculation of PSD is required to determine where drivers can safely execute passing maneuvers. Traditional PSD standards are based on deterministic, theoretical models, which are calibrated with conservative percentile values for uncertain design inputs to account for uncertainty. The PSD standards do not provide information about the risk of deviation from them. Reliability analysis is a technique that is based on limit state design and that accounts for the propagation of variability from input random parameters to the design outputs. A total of 1,098 passing maneuvers were observed on several two-lane highways in Spain; two data collection methodologies were used: external observations and an instrumented vehicle. The most significant factors affecting PSD were impeding-vehicle speed, passing-vehicle acceleration, and headways between impeding and passing vehicles. A uniform acceleration model described the passing-vehicle trajectory. The characterized input parameters and the passing model were used to perform a reliability analysis. The results showed the probability of noncompliance in different scenarios, defined as the proportion of cases in which the required PSD would exceed the available sight distance. American and Spanish PSD standards were evaluated. Geometric design standards presented a probability of noncompliance of about 0.15, whereas some marking standards had a probability of noncompliance exceeding 0.85. These standards may be associated with higher risk levels if they are followed by drivers. As well, PSD risk levels were not consistent for different design speeds, since they underestimated operating speed at some locations.

Benefits of Metering Signals at Roundabouts with Unbalanced Flow: Patterns in Spain

Unbalanced flow patterns may be a problem in roundabouts, even at medium demand levels. One single approach can cause queues on the downstream approaches; the average delay can increase greatly, and the roundabout can fail. This operational problem can be lessened by regulating traffic with metering signals, one of the most cost-effective measures used in Australia, the United Kingdom, and the United States. Even though some Spanish roundabouts exceed capacity, the potential benefit to local conditions has not been studied. Traffic microsimulation was used to analyze capacity improvements and operational performance on roundabouts with metering signals. A field study characterized gap acceptance behavior on conditions close to capacity. Vissim was calibrated and validated, and a signal control logic was implemented in its VisVAP module. The capacity from traffic microsimulation represents the traffic demands observed in the field more accurately because of the variable follow-up headways. In contrast, the U.S. Highway Capacity Manual 2010 underestimates capacity for almost all observed conditions. Almost 400 combinations of design and control parameters for metering signals were required to obtain the optimal (location and timing) model with the lowest average delay. Then, traffic demand scenarios were varied with the optimal metering system. The percentage benefit, calculated as overall average delay, could be up to 60% depending on the combination of controlling traffic demand and conflicting traffic flow. Results allow users to determine the need for metering and quantify the potential benefit from its application to one-lane roundabouts.

Population Synthesis Handling Three Geographical Resolutions

In this paper, we develop a synthetic population as the first step in implementing an integrated land use/transport model. The model is agent-based, where every household, person, dwelling, and job is treated as an individual object. Therefore, detailed socioeconomic and demographic attributes are required to support the model. The Iterative Proportional Updating (IPU) procedure is selected for the optimization phase. The original IPU algorithm has been improved to handle three geographical resolutions simultaneously with very little computational time. For the allocation phase, we use Monte Carlo sampling. We applied our approach to the greater Munich metropolitan area. Based on the available data in the control totals and microdata, we selected 47 attributes at the municipality level, 13 attributes at the county level, and 14 additional attributes at the borough level for the city of Munich. Attributes are aggregated at the household, dwelling, and person level. The algorithm is able to synthesize 4.5 million persons in 2.1 million households in less than 1.5 h. Directions regarding how to handle multiple geographical resolutions and how to balance the amount and order of attributes to avoid overfitting are presented.

Design Criteria for Minimum Passing Zone Lengths: Operational Efficiency and Safety Considerations

Passing zones are provided to improve operational efficiency of two-lane highways. Minimum passing zone lengths of 120 m were established by FHWA and AASHTO. Some studies indicate that lengths may need to be increased, but no changes have been recommended, pending further research. The objective of this study was to develop design and marking criteria for minimum passing zone lengths, with traffic operational efficiency and safety taken into consideration. First, a traffic microsimulation was conducted with AIMSUN software. The calibration and validation included the observation of 1,750 passing maneuvers in Spain. Results indicated that passing zones shorter than 250 m added little to operational efficiency. Second, a reliability analysis was applied. The analysis quantified the probability that a passing maneuver was completed beyond the end of the passing zone (noncompliant passing maneuver). Then the number of noncompliant passing maneuvers was calculated. Traffic flow and passing zone length were contributing factors. Findings from the analysis indicated that the minimum passing zone length should be increased to a minimum of 275 m for high traffic volumes, 300 m for medium traffic volumes, and 350 m for low traffic volumes. With these lengths, noncompliant passing maneuvers decrease. The marginal increase in the minimum length of passing zones can potentially improve safety without significantly reducing operational efficiency. The results can be used by practitioners to establish minimum passing zone length on the basis of hourly volumes and level of risk.

Development and Validation of Speed Kidney, a New Traffic-Calming Device

Traffic calming has been widely applied to urban areas in recent decades to moderate speed and traffic volume. Because most measures used are physical, a vehicle must modify its path and speed. A benefit of traffic calming is improved safety from speed reduction and traffic volume reduction. However, traffic calming has disadvantages, such as emergency response delays, traffic diversion, noise, vibrations, damage to vehicles, and discomfort. Common traffic-calming devices do not consider discomfort for drivers who achieve an appropriate calmed and safe speed. Consequently, it was necessary to develop a new traffic-calming device to moderate speed and to minimize the disadvantages of the previous devices. The Highway Engineering Research Group of the Polytechnic University of Valencia, Spain, has invented, designed, and developed a new traffic-calming device, Speed Kidney. This paper provides the objectives of the Speed Kidney, a description of the new traffic-calming device, and its geometric design. The technological development of the device is described, as are the main results from preliminary tests, implementation on a campus street, and implementation on a controlled test track. Implementation criteria are also given. The Speed Kidney is a functional, feasible, sustainable, and safe solution for traffic calming.

Shared Autonomous Vehicles Effect on Vehicle-Km Traveled and Average Trip Duration

Intermediate modes of transport, such as shared vehicles or ride sharing, are starting to increase their market share at the expense of traditional modes of car, public transport, and taxi. In the advent of autonomous vehicles, single occupancy shared vehicles are expected to substitute at least in part private conventional vehicle trips. The objective of this paper is to estimate the impact of shared autonomous vehicles on average trip duration and vehicle-km traveled in a large metropolitan area. A stated preference online survey was designed to gather data on the willingness to use shared autonomous vehicles. Then, commute trips and home-based other trips were generated microscopically for a synthetic population in the greater Munich metropolitan area. Individuals who traveled by auto were selected to switch from a conventional vehicle to a shared autonomous vehicle subject to their willingness to use them. The effect of shared autonomous vehicles on urban mobility was assessed through traffic simulations in MATSim with a varying autonomous taxi fleet size. The results indicated that the total traveled distance increased by up to 8% after autonomous fleets were introduced. Current travel demand can still be satisfied with an acceptable waiting time when 10 conventional vehicles are replaced with 4 shared autonomous vehicles.

2 + 1 Highways: Overview and Future Directions

Most of the rural transportation system is composed of two-lane highways, and many of them serve as the primary means for rural access to urban areas and freeways. In some highways, traffic volumes can be not high enough to justify a four-lane highway but higher than can be served by isolated passing lanes, or can present high number of head-on collisions. In those conditions, 2 + 1 highways are potentially applicable. This type of highway is used to provide high-performance highways as intermediate solution between the common two-lane highway and the freeway. Successful experiences reported in Germany, Sweden, Finland, Poland, or Texas (US) may suggest that they are potentially applicable in other countries. The objective of this white paper is to provide an overview of the past practice in 2 + 1 highways and discuss the research directions and challenges in this field, specially focusing on, but not limited to, operational research in association with the activities of the Subcommittee on Two-Lane Highways (AHB40 2.2) of the Transportation Research Board. The significance of this paper is twofold: (1) it provides wider coverage of past 2 + 1 highways design and evaluation, and (2) it discusses future directions of this field.