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Purpose>On-ramp merging areas are typical bottlenecks in the freeway network since merging on-ramp vehicles may cause intensive disturbances on the mainline traffic flow and lead to various negative impacts on traffic efficiency and safety. The connected and autonomous vehicles (CAVs), with their capabilities of real-time communication and precise motion control, hold a great potential to facilitate ramp merging operation through enhanced coordination strategies. This paper aims to present a comprehensive review of the existing ramp merging strategies leveraging CAVs, focusing on the latest trends and developments in the research field.Design/methodology/approach>The review comprehensively covers 44 papers recently published in leading transportation journals. Based on the application context, control strategies are categorized into three categories: merging into sing-lane freeways with total CAVs, merging into sing-lane freeways with mixed traffic flows and merging into multilane freeways.Findings>Relevant literature is reviewed regarding the required technologies, control decision level, applied methods and impacts on traffic performance. More importantly, the authors identify the existing research gaps and provide insightful discussions on the potential and promising directions for future research based on the review, which facilitates further advancement in this research topic.Originality/value>Many strategies based on the communication and automation capabilities of CAVs have been developed over the past decades, devoted to facilitating the merging/lane-changing maneuvers at freeway on-ramps. Despite the significant progress made, an up-to-date review covering these latest developments is missing to the authors’ best knowledge. This paper conducts a thorough review of the cooperation/coordination strategies that facilitate freeway on-ramp merging using CAVs, focusing on the latest developments in this field. Based on the review, the authors identify the existing research gaps in CAV ramp merging and discuss the potential and promising future research directions to address the gaps.

Open-access multilane highways have a significant share in the highway network of Pakistan and other developing countries in Asia. These highways have high access density, design inconsistency, and other operational characteristics that differentiate them from partially controlled-access multilane highways. This study identified significant factors affecting the mobility of open-access highways based on road users’ perceptions as well as field observations. An interview-based questionnaire survey from local respondents and an in-service road survey formed the database for the present study. Questionnaire survey results showed that heavy traffic was the most critical mobility influencing factor on open-access multilane highways, followed by road width and condition, whereas the result of multilinear regression revealed that the most significant variable was access density, followed by the flow and pedestrian crossings. However, it was concluded that controlling access density, preventing pedestrian crossings, and improving pavement condition will improve the mobility of open-access multilane highways.

Multilane highways are highways with two or more lanes in each direction that are either undivided or divided by medians or two-way left turn lanes. Free flow speed (FFS) was adjusted to a non-ideal road condition in the Highway Capacity Manual (HCM) from the early 1990s. The HCM methodology for multilane highway level of service analysis depends mainly on the FFS estimation. The main factors affecting FFS can be classified into five groups: driver, road, vehicle, environment, and traffic operations and control. The HCM assumes ideal pavement condition in estimating the FFS; however, this is not always the case, especially in developing countries. The main objectives of this paper are to investigate the effect of pavement condition on FFS and to develop models for estimation of FFS on suburban and rural multilane highways in Jordan.

We present a new model for traffic on a multilane freeway (with n lanes). Our basic descriptors are the car density ρ (in cars/mile), taken across all lanes in the freeway, and the average car velocity u (in miles/hour). The flux of cars across all lanes is given by $\\rho u = \\Sigma_{i=1}^n\\rho_{i}u_i$, where ρi is the car density in the ith lane, and ui the velocity of cars in the ith lane. We shall track only ρ and u and not what is going on in each individual lane. On such multilane freeways, one often observes distinct stable equilibrium relationships between car velocity and density. Prototypical situations involve two equilibria, $v = v_{1}(\\rho) > v = v_{2}(\\rho)$, $0 \\leq \\rho < \\rho_{max}$, where $v_{1}(\\cdot)$ and $v_{2}(\\cdot)$ are monotone decreasing and satisfy $v_{1}(\\rho_{max}) = v_{2}(\\rho_{max}) = 0$. The upper curve is typically stable for densities satisfying $0 \\leq \\rho \\leq \\rho_1$, whereas the lower curve is stable for densities satisfying $\\rho_2 \\leq \\rho \\leq p_{max}$. Our interest is in the situation where $0 < \\rho_2 \\leq \\rho_1 < \\rho_{max}$ and $v_{2}(\\rho_2) \\leq v_{1}(\\rho_1)$. In this paper we present a model that incorporates both equilibrium curves and a simple switching mechanism which allows cars to transit from one equilibrium curve to the other. This switching mechanism, when combined with the continuity equation, produces relaxation or self-excited oscillations in the system, and these oscillations are what interests us here.

One of the important issues affecting the accuracy of traffic analyses is heterogeneity in the vehicular traffic mix that composes a traffic stream. Heavy vehicles, which usually constitute the remaining smaller proportion of a traffic mix, are larger in dimension and often inferior to passenger cars in performance. In the US, the Highway Capacity Manual (HCM) provides passenger car equivalents (PCEs) for use in capacity and level of service analyses. This feature aims to shed light on some important issues critical to the understanding of the effect of heavy vehicles on traffic flow and, therefore, on the use of PCEs for heavy vehicles in traffic analyses. These issues are: 1. mechanism of heavy vehicles' effect, 2. equivalency criteria, 3. application type, and 4. heavy vehicle mix. PCE factors for heavy vehicles are an effective means to account for the presence of heavy vehicles in the traffic stream in performing traffic analyses.

The inapplicability of current methods for analyzing the operations of notconventional urban roundabouts (i.e. not referable to standard schemes) seriously hampers the assessment of their operational performances. Moreover, local constraints and the road network structure have produced lots of geometric layouts that make it hard to propose a framework to explain the performances of not-conventional roundabouts. For these intersections it is also hoped to have scientifically based methods to analyze operations with proper reliability. Starting from these considerations, this paper shows the conceptual path followed to analyze the operations of not-conventional roundabouts along an arterial of Palermo City. The examined intersections have suggested a theoreticexperimental approach that balances the need both to match field observations and to have a general criterion to determine performances. A case study application allows one to explain how to derive the analytical capacity model from field data. Moreover the comparison of results to those derived by models for conventional schemes informs us about the effect on capacity caused by a more realistic operational pattern such as that observed at multilane-largediameter roundabouts.

This study investigates automobile crash density, rate, type and severity on Iowa and Minnesota divided expressways. State expressways provide high speed and capacity of interstate highways without the expense of conforming to all features of interstate design standards, such as full access control and grade separation. Many states are quickly expanding their expressway networks with no empirical data to determine at what point safety deteriorates to an undesirable level, a lack the study attempts to address. Minnesota crash data was for 128 expressway segments during the period 1999-2001; Iowa data was for 108 segments during 1998-2000. The study statistically confirms the hypothesis that crash rate and severity increase with the increase in average daily traffic, and that appreciable safety benefit exists for converting expressways to full access control.

Transportation services both condition and are conditioned by the spatial pattern of population and activities within a region. In recent years various methods of analyzing and synthesizing transportation networks have been developed. In this paper, some optimizing methods are applied within the context of the system of cities of Central Canada. All of these models may be interpreted from two points of view. First they may be viewed as planning models, that is, models which generate normative transportation networks insofar as they maximize the attainment of a set of objectives subject to certain constraints. These models may also be used

This paper presents a generalized analysis of the effects of on-ramp time varying flow on unidirectional n-lane freeways. The analysis incorporates the speed-density relations for every lane, which are then used to set up separate continuity equations for every lane and are coupled by some reasonable lane-changing hypotheses. The paper assumes the same wave velocities (uniform freeway) for the perturbed flow in each lane of a freeway. Using the generalized n-lane analysis, the paper then predicts the relative propagation of density perturbations in time and space in different lanes of a two-lane, three-lane, and four-lane freeway. The corresponding relaxation distances from the on-ramp, where the initial perturbation had started, have been defined and compared for the two-lane, three-lane, and four-lane freeway. The above studies have then incorporated the effects of controlled ramp flow by suitable mathematical expressions that simulate controlled ramp flow.