Free-flow Travel Time Research Articles

Estimating the effect of turn penalties and traffic congestion on measuring spatial accessibility to primary health care

Geographic variations in spatial accessibility to public resources, such as health care services, raise important questions about the efficiencies and inequities of the processes that determine where these services are located. Spatial accessibility can be measured many different ways, but many of methods in use today involve some measure of travel cost (in time or distance). In this study we explore a simple methodological question: how much are models of spatial accessibility influenced by the precise metric of travel cost? We address this question by comparing spatial accessibility to primary care physicians for two different methods of calculating travel cost (in time) on a street network: free-flow travel time and congested with turn penalties travel time—which augments free-flow travel times with the burden of traffic congestion and traffic intersection controls. We consider the effect of these two metrics of travel cost on a gravity-based measure of spatial accessibility to primary health care services in Edmonton, Alberta, Canada. Our results suggest that while travel times between locations of demand and locations of primary care providers greatly differ based on how travel cost is calculated, the gravity-based measure of spatial accessibility provides similar information for both travel cost metrics. Using congested with turn penalties travel time can be an onerous addition to the analysis of spatial accessibility, and is more useful for measuring absolute travel time rather than modeling relative spatial accessibility.

The morning commute problem with heterogeneous travellers: the case of continuously distributed parameters

We study the morning commute problem with a heterogeneous travelling population whose early/late arrival penalty parameters are continuously distributed. Following Arnott et al.'s (1988) [Schedule delay and departure time decisions with heterogeneous commuters, Transportation Research Record, 1197, 56–67] model, where the ratio of the value of early schedule delay (VESD) over the value of late schedule delay (VLSD) is assumed to be constant across the population, we first derive the user-optimal travel profiles and the corresponding total travel time (TTT) for a one-to-one network with a single route. We show that although every commuter is better off if the bottleneck capacity is enlarged, commuters with high values of early arrival penalty (EAP) benefit more than those with low values. In addition, the homogeneity assumption overestimates the queuing delay and thus the TTT. Then we extend our analysis to a network with two routes, a freeway and an arterial road (AR) connecting a single origin–destination (O–D) pair. In this case, we find that the travellers who weigh schedule delay more will first shift to the AR, prompted by an increase in total demand. However, the critical travel demand at which travellers start to use the AR remains the same if the common EAP for homogeneous users is equal to the expected value of EAP distribution for heterogeneous users. Interestingly, there exists a critical value of EAP parameter, such that all the travellers whose EAP is less than it choose the freeway, and those whose EAP is larger than it choose the AR. Sensitivity analysis on TTT is performed with respect to freeway capacity, AR capacity and free-flow travel time on the AR. The results indicate that enlarging freeway (AR) capacity will always reduce the TTT of the whole network and the TTT of the AR (freeway), and increase the demand share of the freeway (AR). We also show that every commuter is better off if either the freeway capacity or the AR capacity is enlarged. Finally, we provide numerical examples to demonstrate the considerable differences in flow patterns and network performances between a homogeneous and a heterogeneous population.

The impact of traffic images on travel time valuation in stated-preference choice experiments

We develop a simple but modern stated-preference (SP) choice experiment to analyse the impact of traffic images on the estimated value of travel time savings (VTTS). A random subsample of survey respondents view computer-generated images that depict traffic conditions for congested and free-flow portions of hypothetical travel alternatives. The remaining respondents receive otherwise-identical surveys that only verbally describe the attributes of those alternatives. From surveys with images, the estimated VTTS is $7.40 per hour for congested travel time and $5.70 for free-flow travel time, implying a ‘congestion premium’ of 30%. From surveys without images, however, congested and free-flow VTTS estimates are identical at $5.90 per hour. Our results indicate that traffic images, however rudimentary, can substantially influence travel-time valuation, thus compelling further research on incorporating them into SP experimental design.

Assessment of Dynamic Speed Limit Management on Metropolitan Freeways

This article evaluates the impacts of dynamic speed limit strategies in metropolitan freeways. An objective function is defined in order to balance the benefits (e.g., reduction of congestion, pollutant emissions and fuel consumption, or increase in traffic safety) and costs (e.g., increase of free flow travel times). This function is generic in its nature, and could be used to assess any type of traffic management strategy. In particular, it is applied to the Barcelona's freeways dynamic speed limit management case study. The results suggest that the social profitability of dynamic speed limit (DSL) management in metropolitan freeways is limited. The main reason for this is the reduced effect of this kind of policy, if isolated in application, in alleviating congestion. It is emphasized that the absolute magnitude of the profitability is highly sensitive to the value given to each one of the social costs. The profitability of the policy would improve if the value of traffic externalities increased in relation to the marginal cost of travel times. A sensitivity analysis with respect to this last cost is presented. The potential synergies of applying DSL strategies jointly with other active traffic management policies should be considered as an issue for further research.

Real-time Arterial Performance Measurement Using BRT Probe Data and Signal Timing Data

AbstractReal-time arterial travel time estimation has been a very challengeable task due to the complexity of urban street traffic behaviors and lack of high resolution and high quality data. However, travel time information is considered as one of the most important tools to manage arterial flow. This study developed a series of data processing methods to estimate arterial travel time and LOS based on real-time BRT bus data and signal timing data. BRT buses have a few advantages over local buses to serve as probe vehicles to measure arterial performance. A few data processing procedures were introduced in this paper to estimate the arterial travel time by converting bus trajectory to general traffic travel time. The arterial travel time is estimated by adding the bus queuing delay, average traffic signal waiting time, and free-flow travel time. To verify the effectiveness of the developed method, a field test was conducted. The Valley Transportation Authority (VTA) BRT buses together with Intelli Drive corridor were selected as the data sources. The testing results demonstrate that the proposed method estimated arterial travel time and level of service well based on BRT probe data and traffic signal data. The root mean square error (RMSE) and root mean square percentage error (RMSPE) for the estimation of travel time are 49 seconds and 9%, respectively. The accuracy for the estimation of LOS is 73%.

Modeling and solving continuous-time instantaneous dynamic user equilibria: A differential complementarity systems approach

This paper is the second of a two-part research wherein we undertake a mathematically rigorous investigation of the continuous-time dynamic user equilibrium (DUE) problem using the recently introduced mathematical paradigm of differential complementarity systems (DCSs). Based on the thorough study of continuous-time single-destination point-queue models in the previous part, we first extend this special case to multiple destinations respecting the First-In–First-Out property of travel flows. A DCS with constant time delay is then introduced to formulate the continuous-time model of instantaneous dynamic traffic equilibria (IDUE) with a fixed demand profile. We develop a time decomposition scheme based on link free flow travel times to convert the delay DCS to a series of DCSs without time delays that are solved by a numerical time-stepping method. We provide rigorous numerical treatment of the time-decomposed IDUE model, including solvability of the discrete-time complementarity problems and convergence of the numerical trajectories to a continuous-time solution. We present numerical results to validate the IDUE on a small network and also on the Sioux Falls network.

Model for Location of Medical Facility and Evaluation of Vulnerability and Accessibility of Road Network

This study proposes a method of evaluating the robustness of a road network against a natural disaster and aims to determine the optimal allocation of medical facilities if road network degradation occurs because of severe natural disasters. Connectivity vulnerability and accessibility are important concepts in evaluating the strength and reliability of road networks. This paper first introduces a new network evaluation measure, the distinct paths–based accessibility index (DPAI), which is based on the concepts of both connectivity vulnerability and accessibility; considers the number of distinct paths between a certain origin–destination pair, free-flow travel time, and the number of opportunities in an area; and develops a solution algorithm. A medical facility location model that optimizes the allocation of a medical facility to each centroid according to the DPAI is then formulated, and the results are applied to a real network. Although the optimal allocation of a medical facility varies according to the minimum DPAI, the robustness of an actual network may be improved by reallocating medical facilities. Moreover, the allocation results depend on the value of the importance placed on the number of distinct paths. These results imply that it is critical to set the parameter on the importance of the number of distinct paths by considering risks and geographical features in the subject area.

Discretised link travel time models based on cumulative flows: Formulations and properties

In the research area of dynamic traffic assignment, link travel times can be derived from link cumulative inflow and outflow curves which are generated by dynamic network loading. In this paper, the profiles of cumulative flows are piecewise linearized. Both the step function (SF) and linear interpolation (LI) are used to approximate cumulative flows over time. New formulations of the SF-type and LI-type link travel time models are developed. We prove that these two types of link travel time models ensure first-in-first-out (FIFO) and continuity of travel times with respect to flows, and have other desirable properties. Since the LI-type link travel time model does not satisfy the causality property, a modified LI-type (MLI-type) link travel time model is proposed in this paper. We prove that the MLI-type link travel time model ensures causality, strong FIFO and travel time continuity, and that the MLI-type link travel time function is strictly monotone under the condition that the travel time of each vehicle on a link is greater than the free flow travel time on that link. Numerical examples are set up to illustrate the properties and accuracy of the three models.

Private toll roads: Competition under various ownership regimes

Interest is growing in private toll roads as an alternative to public free-access road infrastructure. Private toll roads have gained favour for various reasons, including a dearth of public funds for road construction and maintenance, increasing traffic congestion, and growing acceptance of the user-pay principle in general, and road pricing in particular. This paper focuses on allocative efficiency of private toll roads. The model features one origin and one destination linked by two parallel routes that can differ in capacity and free-flow travel time. Congestion takes the form of queueing. Prospective travellers decide whether to drive, and if so on which route and at what time. Three private ownership regimes are considered: (1) a private road on one route and free access on the other, (2) a private roads duopoly, and (3) a mixed duopoly with a private road competing with a public toll road. Private toll roads are generally found to enhance allocative efficiency (measured by social surplus) relative to free access. The efficiency gain is greater when both routes are tolled, tolls are varied over time to eliminate queueing, and when no private road has a dominant fraction of total capacity. Paradoxically, mixed duopoly can be less efficient than a private duopoly. Price leadership by a public toll road avoids this possibility, although leadership typically yields little additional efficiency gain.

Simulating travel reliability

We present a simulation model designed to determine the impact on congestion of policies for dealing with travel time uncertainty. The model combines a supply side model of congestion delay with a discrete choice econometric demand model that predicts scheduling choices for morning commute trips. The supply model describes congestion technology and exogenously specifies the probability, severity, and duration of non-recurrent events. From these, given traffic volumes, a distribution of travel times is generated, from which a mean, a standard deviation, and a probability of arriving late are calculated. The demand model uses these outputs from the supply model as independent variables and choices are forecast using sample enumeration and a synthetic sample of work start times and free flow travel times. The process is iterated until a stable congestion pattern is achieved. We report on the components of expected cost and the average travel delay for selected simulations.

The full cost of intercity highway transportation

In this paper we review the theoretical and empirical literature on the cost structure of the provision of intercity highway transportation and specify and estimate our own cost functions. We develop a full cost model which identifies the key cost components and then estimate costs component by component: user costs, infrastructure costs, time and congestion costs, noise costs, accident costs, and pollution costs. The total long run average cost is $0.34 per vehicle km traveled. The single largest cost category is freeflow travel time. While the marginal cost of infrastructure is higher than its average cost, indicating that new construction is increasingly expensive, the marginal cost of driving (user fixed and variable costs) is less than the average cost, indicating that by increasing travel the user can spread his fixed cost of a vehicle over more trips without penalty.

A variational inequality formulation of the Dynamic Traffic Assignment Problem

This paper deals with the modelling of the Dynamic Traffic Assignment Problem (DTAP) for analyzing the day time varying flows of urban transportation networks. During the past two decades, many models have been proposed in the literature, but some of them are based on heuristic concepts, and most of them incorporate important limitations. In this paper, we propose a Dynamic Traffic Assignment Model which is mainly based on the following assumption: the time spent by a vehicle on a link may be decomposed into a fixed travel time and a waiting time. The fixed travel time is the free flow travel time, after which vehicles are put in the link exit queue, until there is room for them to proceed their trip. We show that this model leads to a network structure (a temporal expansion of the base network, including the queues), and then formulate the DTAP as a network equilibrium problem over the expanded network. The mathematical formulation is achieved through a variational inequality where the variables are the path (or link) flows over the space-time expanded network. Numerical results show that the model may handle large networks, so it may be used in practice to analyze the traffic congestion moves in real cities, as well in space (physical links) as in day time.

Link travel time functions for traffic assignment

In this paper a probabilistic model developed by Cowan describing flow characteristics on a single lane of traffic is adopted to produce link travel time/flow relationships appropriate for use on some links in traffic assignment. The resulting model parameters relate directly to the mean and variance of free flow travel time allowing their values to be estimated without the need of extensive flow/travel time data as is required to calibrate empirically based models.

ESTIMATING THE CUMULATIVE ARRIVAL CURVES OF COMMUTE TRIPS AT BOTTLENECKS IN A SINGLE CORE CITY

Many highway problems arise from bottleneck congestion. Regarding highway commute traffic, this analysis proposes a model which estimates the cumulative arrival curves at bottlenecks around a single core during the morning peak, given commuters' home locations and work schedules. It is necessary, therefore, to identify when and which bottleneck each commuter passes. Each commuter is assumed to have a common form of travel cost function which consists of a static cost of free flow travel time and a time-dependent cost due to bottleneck congestion. Commute trips are assigned spatially and temporally, so as to establish an equilibrium. The model is applied to the Metropolitan Expressway Network, and the estimated cumulative arrival curves reasonablly fit the observed.

Formulating and solving the network design problem by decomposition

The optimal transportation network design problem is formulated as a convex nonlinear programming problem and a solution method based on standard traffic assignment algorithms is presented. The technique can deal with network improvements which introduce new links, which increase the capacity of existing links, or which decrease the free-flow (uncongested) travel time on existing links (with or without simultaneously increasing link capacity). Preliminary computational experience with the method demonstrates that it is capable of solving very large problems with reasonable amounts of computer time.

Implanted high value resistors : Kenneth R. Whight, Peter Blood and Keith H. Nicholas. Solid-St. Electron.19, 1021 (1976)

In the research area of dynamic traffic assignment, link travel times can be derived from link cumulative inflow and outflow curves which are generated by dynamic network loading. In this paper, the profiles of cumulative flows are piecewise linearized. Both the step function (SF) and linear interpolation (LI) are used to approximate cumulative flows over time. New formulations of the SF-type and LI-type link travel time models are developed. We prove that these two types of link travel time models ensure first-in-first-out (FIFO) and continuity of travel times with respect to flows, and have other desirable properties. Since the LI-type link travel time model does not satisfy the causality property, a modified LI-type (MLI-type) link travel time model is proposed in this paper. We prove that the MLI-type link travel time model ensures causality, strong FIFO and travel time continuity, and that the MLI-type link travel time function is strictly monotone under the condition that the travel time of each vehicle on a link is greater than the free flow travel time on that link. Numerical examples are set up to illustrate the properties and accuracy of the three models.