71 research outputs found
Flower roundabouts and double-lane roundabouts: a comparative performance analysis
AbstractIn the paper, a choice criterion between flower roundabouts and double-lane roundabouts is proposed, focusing on operational benefits that can derive from one scheme over the other, and outlining a general framework for benefit-cost analysis. In order to assess operational benefits of innovative roundabouts over modern roundabouts, a comparative analysis was made. Capacity was estimated using gap-acceptance models. In detail, assuming the dichotomic shifted negative exponential distribution to model headways in circulating streams, the Hagring formula was adjusted to obtain entry capacity estimations at roundabout approaches where entering vehicles face one or two conflicting flows. Based on the control delay, the suitability domains and indifference areas were constructed. Thus, a sensitivity analysis to changes in traffic demand for operational benefits of flower roundabouts over double-lane roundabouts was carried out and discussed. At last, evidence for new installations and conversion of existing roundabouts can be found
The Terminal Oxidase Cytochrome bd Promotes Sulfide-resistant Bacterial Respiration and Growth
Hydrogen sulfide (H2S) impairs mitochondrial respiration by potently inhibiting the heme-copper
cytochrome c oxidase. Since many prokaryotes, including Escherichia (E.) coli, generate H2S and
encounter high H2S levels particularly in the human gut, herein we tested whether bacteria can sustain
sulfide-resistant O2-dependent respiration. E. coli has three respiratory oxidases, the cyanide-sensitive
heme-copper bo3 enzyme and two bd oxidases much less sensitive to cyanide. Working on the isolated
enzymes, we found that, whereas the bo3 oxidase is inhibited by sulfide with half-maximal inhibitory
concentration IC50=1.1±0.1μM, under identical experimental conditions both bd oxidases are
insensitive to sulfide up to 58μM. In E. coli respiratory mutants, both O2-consumption and aerobic
growth proved to be severely impaired by sulfide when respiration was sustained by the bo3 oxidase
alone, but unaffected by ≤200μM sulfide when either bd enzyme acted as the only terminal oxidase.
Accordingly, wild-type E. coli showed sulfide-insensitive respiration and growth under conditions
favouring the expression of bd oxidases. In all tested conditions, cyanide mimicked the functional
effect of sulfide on bacterial respiration. We conclude that bd oxidases promote sulfide-resistant O2-
consumption and growth in E. coli and possibly other bacteria. The impact of this discovery is discussed
Passenger Car Equivalents for Heavy Vehicles at Roundabouts. A Synthesis Review
Passenger Car Equivalents (PCEs in the following) are used to transform a mixed fleet of vehicles into a fleet of equivalent passenger cars and to analyze capacity and level-of-service of roads and intersections. Most roundabouts guidelines propose constant values for PCEs but a single PCE value can result improper under heterogeneous traffic conditions. PCEs should be vary with traffic and road conditions and consequently PCEs applied to undersaturated traffic conditions can overestimate the heavy vehicle effect or be not sensitive to the traffic level or characteristics of heavy vehicles. Compared to other at-grade intersections, the interaction between the operational performances of the heavy vehicles and the geometric features at roundabouts can produce significant impacts on the heavy vehicle paths and traffic operations due to the curvilinear nature of the roundabout design. Literature presents various methods of estimation to obtain PCE values for heavy vehicles. The focus of this paper is to review statistical methods and traffic simulation studies based on microscopic approaches used to calculate PCEs for heavy vehicles driving roundabouts. Effects on capacity and estimates of PCEs based on models currently employed in roundabout analysis are also compared. The results obtained in this study aim at providing an overview of the existing knowledge concerning the estimation of PCEs at roundabouts and can represent a guideline for transportation engineers in planning, design and capacity analysis of roundabouts that operate under conditions of mixed traffic
Surrogate Measures of Safety at Roundabouts in AIMSUN and VISSIM Environment
This paper addresses issues on road safety analysis through microscopic traffic simulation models. The Surrogate Safety Assessment Model
(SSAM) was applied to read vehicle trajectory files generated by two microsimulators and then calculate surrogate measures of safety. Since safety assessment of any road entity can provide different results based on the microsimulator which is used, the main objective of this research was to estimate the safety performance of three roundabout layouts and compare the conflict events simulated by AIMSUN and VISSIM. The two micro-simulators were used to build the calibrated models of the roundabouts, each fitting the corresponding empirical capacity function. The
results provided insights on how to set the SSAM filters in order to have a comparable frequency of conflicts by simulation and examine conditions under which a safety analysis could be independent of the software used
Estimating the safety performance function for urban unsignalized four-legged one-way intersections in Palermo, Italy
Starting from consideration that urban intersections are sites with promise for safety and operational improvements, the paper describes the steps taken to develop a crash predictive model for estimating the safety performance of urban unsignalized intersections located in Palermo, Italy. The focus is on unsignalized four-legged one-way intersections widespread in Italian downtowns. The sample considered in the study consist of 92 intersections in Palermo, Italy. For the study were collected crashes occurred in the sites during the years 2006-2012, geometric design and functional characteristics and traffic flow. Results showed that data were overdispersed and NB1 distributed. In order to account for the correlation within responses Generalized Estimating Equations (GEE) were used under different working correlation matrices
Estimating the Safety Performance Function for Urban Unsignalized Four-Legged One-Way Intersections in Palermo, Italy
Starting from consideration that urban intersections are sites with promise for safety and operational improvements, the paper describes the steps taken to develop a crash predictive model for estimating the safety performance of urban unsignalized intersections located in Palermo, Italy. The focus is on unsignalized four-legged one-way intersections widespread in Italian downtowns. The sample considered in the study consist of 92 intersections in Palermo, Italy. For the study were collected crashes occurred in the sites during the years 2006-2012, geometric design and functional characteristics and traffic flow. Results showed that data were overdispersed and NB1 distributed. In order to account for the correlation within responses Generalized Estimating Equations (GEE) were used under different working correlation matrices
Flower Roundabouts And Double-Lane Roundabouts: A Comparative Performance Analysis
In the paper, a choice criterion between flower roundabouts and double-lane roundabouts is proposed, focusing on operational benefits that can derive from one scheme over the other, and outlining a general framework for benefit-cost analysis. In order to assess operational benefits of innovative roundabouts over modern roundabouts, a comparative analysis was made. Capacity was estimated using gap-acceptance models. In detail, assuming the dichotomic shifted negative exponential distribution to model headways in circulating streams, the Hagring formula was adjusted to obtain entry capacity estimations at roundabout approaches where entering vehicles face one or two conflicting flows. Based on the control delay, the suitability domains and indifference areas were constructed. Thus, a sensitivity analysis to changes in traffic demand for operational benefits of flower roundabouts over double-lane roundabouts was carried out and discussed. At last, evidence for new installations and conversion of existing roundabouts can be found
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