Safety effects of street lighting on roadway segments: Development of a crash modification function

Objective: Nighttime crashes are overrepresented on the U.S. highway system. Roadway lighting, which provides additional visibility by supplementing vehicle headlights, has been identified as an effective countermeasure to improve nighttime safety. However, the existing literature does not provide a thorough understanding of the effects of street lighting photometric characteristics on nighttime crash occurrence on roadway segments. This study aimed to investigate the relationship between lighting photometric measures and nighttime crash risk on roadway segments and develop a crash modification function/factor (CMF).

Methods: The research team collected horizontal illuminance data on 440 roadway segments between 2 successive signalized intersections in Florida for 2012–2014 and matched 4 years of nighttime and daylight crash data (2011–2014). Random parameter negative binomial models were estimated for both nighttime and daylight crash frequencies. The expected night-to-day crash odds ratio, as an equivalent of CMF, was derived from the fitted models with the correction of estimation variances. The confidence intervals (CIs) of the developed CMF were estimated using the Cox method.

Results: The coefficient of the mean of horizontal illuminance is significantly negative in the nighttime model. The coefficients of the standard deviation of horizontal illuminance are significantly positive and normally distributed in both the nighttime and daylight models. The significance of the standard deviation in the daylight model captures the confounding effects—a high standard deviation correlates with high traffic exposures, poor safety design standards, and low maintenance quality. The CMF based on the expected daylight-to-day odds ratio was developed as an exponential function of the increments and the increment squares of the mean and the standard deviation of horizontal illuminance. Its 95% CIs indicate that the CMF is almost significant over the whole range. Other significant variables contributing to nighttime crash risk include annual average daily traffic, truck percentage, segment length, access density, undivided roads, and urban/city limits.

Conclusions: Horizontal illuminance characteristics have a significant impact on nighttime crash risk on roadway segments. An increase in the mean of horizontal illuminance, indicating an improvement in average lighting level, tends to decrease nighttime crash risk; an increase in the standard deviation, representing a poor uniformity of lighting pattern on a roadway segment, is more likely to raise nighttime crash risk. Because the 2 measures are strongly correlated in a low mean range (<0.44 fc), the 2 photometric measures need to be considered together to interpret the safety effects of lighting patterns. The standard deviation shows better performance in measuring lighting uniformity on a roadway segment than the traditional ratios (max-to-min and mean-to-min). However, a new photometric measure is needed to capture the true lighting pattern influencing driver vision at night.     

The performance evaluation of WinOSPM model for urban street canyons of Nantes in France

Air quality modelling is primarily the quantative approach. It is more difficult as it demands input data accuracy, uncertainties and the efficient methodologies to judge the extent of models accuracy. As a result, model validation has to be regarded as an integral part of the modelling process. Furthermore, models are often validated on a limited number of testcases therefore, appropriate evaluation procedure must be implemented to ensure these models will be applicable for various conditions. The study presented here was carried out to evaluate theWinOSPM (Preliminary version of windows based Operational Street Pollution Model) for air pollutants viz. CO, NO, NO₂, NO_x and C₆H₆ for three street canyons of Nantes (France) and for the three base years 1999, 2000, and 2001. Each street canyon selected for this study has typical and unidentical features. The rue de Strasbourg and Boulevard Victor Hugo have many building exceptions whereas rue Crébillon has not any. Application of the model above to the three street canyons revealed that WinOSPM could be used in the case when measurements are not available. This was justified from the results at rue Crébillon. The special interest was in the benzene modelled values as its content in fuel has been targeted to reduce to 1% for the years 2000 and onwards (from its 5% until the year 1999). The 50 to 70% reduction in the benzene concentrations is found for both the years i.e. in 2000 and 2001. This has further justified that air quality models are useful and interesting tools in optimising emission reduction strategies. Moreover, it is also the new pollutant added to the measurement campaign of Air Pays de la Loire (APL) for the city of Nantes. For benzene weekly averages are estimated from the hourly-modelled values for all the streets and compared with that of measurements. They are found in excellent agreement with each others. For other pollutants annual means and percentiles were compared. The statistical analysis was done to evaluate the models performance using index of agreement and correlation coefficient. The index of agreement (d) and correlation coefficient (r) for all the streets show that estimated concentration levels are in good agreement with that of measurements. From the index of agreements, it can be inferred that model has very less potential for errors. The models sensitivity to building-exceptions was also tested for the rue de Strasbourg. Results did not reflect this feature very well. It is perceived that the influence of this feature might have been suppressed in averaging the annual hourly values. This influence is apparently seen in hourly average time series variations. Finally, WinOSPM model was found a simple but very useful model. It could very well represent the detailed flow and dispersion conditions in urban streets.     

Effects of building aspect ratio, diurnal heating scenario, and wind speed on reactive pollutant dispersion in urban street canyons

A photochemistry coupled computational fluid dynamics (CFD) based numerical model has been developed to model the reactive pollutant dispersion within urban street canyons, particularly integrating the interrelationship among diurnal heating scenario (solar radiation affections in nighttime, daytime, and sun-rise/set), wind speed, building aspect ratio (building-height-to-street-width), and dispersion of reactive gases, specifically nitric oxide (NO), nitrogen dioxide (NO₂) and ozone (O₃) such that a higher standard of air quality in metropolitan cities can be achieved. Validation has been done with both experimental and numerical results on flow and temperature fields in a street canyon with bottom heating, which justifies the accuracy of the current model. The model was applied to idealized street canyons of different aspect ratios from 0.5 to 8 with two different ambient wind speeds under different diurnal heating scenarios to estimate the influences of different aforementioned parameters on the chemical evolution of NO, NO₂ and O₃. Detailed analyses of vertical profiles of pollutant concentrations showed that different diurnal heating scenarios could substantially affect the reactive gases exchange between the street canyon and air aloft, followed by respective dispersion and reaction. Higher building aspect ratio and stronger ambient wind speed were revealed to be, in general, responsible for enhanced entrainment of O₃ concentrations into the street canyons along windward walls under all diurnal heating scenarios. Comparatively, particular attention can be paid on the windward wall heating and nighttime uniform surface heating scenarios.     

动态风场及交通流量下街道峡谷内污染物扩散模拟

根据现场实测数据,应用标准k-ε模型研究了动态风场及交通流量下三维街道峡谷内的污染物扩散规律,数值模拟利用CFD软件FLUENT,其中动态风场和车流量变化信息通过用户自定义编程实现.结果发现,动态风场下空气在街道内部不断经历膨胀和压缩的过程,街道峡谷内部流场形态时刻都在变化;当风速由大变小时,空气膨胀出街谷,流型呈近似椭圆形分布;当风速由小变大时,空气压缩在街谷内部,流型呈近似圆形分布.风速的不断变化引起街谷内、外大气的压缩和膨胀过程,这种过程能够改善街谷内污染物的扩散情况.背风面行人高度处,动态来流下的平均污染物浓度要比定常来流下低17.7%;迎风面行人高度处,动态来流下的平均污染物浓度要比定常来流下低27.1%.动态环境下污染物浓度的分布和峰值由风场和车流量变化共同决定.     

可持续发展模型在小城镇生态规划中的应用——以温州市瓯海区娄桥街道为例

构建了由3个准则层和20个指标的评价体系,结合娄桥街道生态规划中拟订的近期、中期、远期规划目标年指标值,采用加权求和计量模型,从经济、生态环境、社会进步的指数和可持续发展4个方面,对娄桥街道基准年和各规划目标年的可持续发展度进行分析研究。     

Evaluation of the QUIC-URB fast response urban wind model for a cubical building array and wide building street canyon

This paper describes the QUIC-URB fast response urban wind modeling tool and evaluates it against wind tunnel data for a 7 × 11 cubical building array and wide building street canyon. QUIC-URB is based on the Röckle diagnostic wind modeling strategy that rapidly produces spatially resolved wind fields in urban areas and can be used to drive urban dispersion models. Röckle-type models do not solve transport equations for momentum or energy; rather, they rely heavily on empirical parameterizations and mass conservation. In the model-experiment comparisons, we test two empirical building flow parameterizations within the QUIC-URB model: our implementation of the standard Röckle (SR) algorithms and a set of modified Röckle (MR) algorithms. The MR model attempts to build on the strengths of the SR model and introduces additional physically based, but simple parameterizations that significantly improve the results in most regions of the flow for both test cases. The MR model produces vortices in front of buildings, on rooftops and within street canyons that have velocities that compare much more favorably to the experimental results. We expect that these improvements in the wind field will result in improved dispersion calculations in built environments.     

街区污染物扩散的数值研究

对街区峡谷内交通污染扩散规律的研究是城市空气质量预测的主要内容之一。应用现代流体力学数值模拟方法进行大量算例的计算和分析,研究了城市街区峡谷污染物二维扩散的规律。包括比较各种条件下污染物的扩散浓度,考察来流风速、街区特征比例和日照等因素对扩散的影响,通过研究流场及湍流强度分析其扩散的动力学原因,为改进工程预测模式提供依据。     

不同结构形状的街道峡谷内污染物扩散

针对不同的城市街道峡谷结构形状,通过求解二维不可压缩N-S方程和K-ε湍流模型方程及污染物对流扩散方程,数值模拟了街道峡谷内的流场及机动车排放污染物浓度场,从而说明了街道峡谷的结构是影响街道峡谷内污染气体扩散的主要因素之一。     

Influence of cetane improvers on the air quality in an urban street canyon

The concentration distributions of NO_x, PM, HC and CO in an urban street canyon have been estimated using a two-dimensional air quality numerical model based on the k– turbulent model and the atmospheric convection diffusion equation when various cetane improvers were used in diesel fuels. A wind vortex can be found within the street canyon, and the pollutants emitted from the bottom of the street canyon tend to follow the course of the wind field, moving circularly. The addition of cetane improvers can improve the air quality in a street canyon, all of the pollutants were found to decrease with increasing centane number.     

餐饮产业集群发展探析--以安徽省芜湖市为例

分析了我国餐饮产业的特点与面临的问题,提出了餐饮业集群发展的战略选择.以安徽省芜湖市凤凰美食街为例,从餐饮产业集群发展形成的背景、区位依托、构成要素以及竞争优势等方面对餐饮业的集群发展进行了实证分析.