Use and perceptions of a real time passenger information system

The use of high-technology systems in the transport sector has increased steadily over recent years. This paper outlines the development of vehicle monitoring and control systems and their use in the public transport arena. The paper shows how one such system, that operated by Datatrak Ltd., has been adapted to provide a real time passenger information system for the RiverBus Partnership in London.

1 The RiverBus service described in this article ceased operation in August 1993. The collapse of the RiverBus Partnership followed the financial difficulties surrounding Olympia and York, developers of Canary Wharf in London Docklands.

Passenger use and perception of the system is evaluated, based on surveys of RiverBus users. This provides an evaluation of the system, and highlights the importance of introducing such systems based on user information needs and as part of the total marketing package.     

Increasing the capacity of signalized intersections with separate left turn phases

A separate turn phase is often used on the approach leg to an intersections with heavy left turns. This wastes capacity on the approach because some of its lanes cannot discharge during its green phases. The paper shows that the problem can be eliminated by reorganizing traffic on all the lanes upstream of an intersection using a mid-block pre-signal. If drivers behave deterministically, the capacity that can be achieved is the same as if there were no left turns. However, if the reorganization is too drastic, it may be counterintuitive to drivers. This can be remedied by reorganizing traffic on just some of the available lanes. It is shown that such partial reorganization still increases capacity significantly, even if drivers behave randomly and only one lane is reorganized. The paper shows how to optimize the design of a pre-signal system for a generic intersection. It also identifies both, the potential benefits of the proposed system for a broad class of intersections, and the domain of application where the benefits are most significant.     

多种交通方式下的城市空间分配

从集计角度分析了运行多种交通方式的城市交通系统。利用目前已受关注的宏观基本图（简称MFD）,描述城市交通的整体特性。以旧金山某区为例,使用现有数据,通过仿真论证如何应用MFD监管真实交通网络。然后讨论了同一路网中不同交通方式间如何相互作用,以及如何将这些作用纳入包含多种交通方式路网的MFD。研究得到的两个主要结论是：1）证实了已有研究猜想——对市区拥挤区域限行,可提高包括被限行者在肉的所有出行者的机动性;2）公交方式提供专用道路空间可提高所有交通方式的可达性。     

Vehicle reidentification and travel time measurement on congested freeways

The paper presents an algorithm for matching individual vehicles measured at a freeway detector with the vehicles’ corresponding measurements taken earlier at another detector located upstream. Although this algorithm is potentially compatible with many vehicle detector technologies, the paper illustrates the method using existing dual-loop detectors to measure vehicle lengths. This detector technology has seen widespread deployment for velocity measurement. Since the detectors were not developed to measure vehicle length, these measurements can include significant errors. To overcome this problem, the algorithm exploits drivers’ tendencies to retain their positions within dense platoons. The otherwise complicated task of vehicle reidentification is carried out by matching these platoons rather than individual vehicles. Of course once a vehicle has been matched across neighboring detector stations, the difference in its arrival time at each station defines the vehicle’s travel time on the intervening segment.Findings from an application of the algorithm over a 1/3 mile long segment are presented herein and they indicate that a sufficient number of vehicles can be matched for the purpose of traffic surveillance. As such, the algorithm extracts travel time data without requiring the deployment of new detector technologies. In addition to the immediate impacts on traffic monitoring, the work provides a means to quantify the potential benefits of emerging detector technologies that promise to extract more detailed information from individual vehicles.     

On the capacity of isolated, curbside bus stops

The maximal rates that buses can discharge from bus stops are examined. Models were developed to estimate these capacities for curbside stops that are isolated from the effects of traffic signals. The models account for key features of the stops, including their target service levels assigned to them by a transit agency. Among other things, the models predict that adding bus berths to a stop can sometimes return disproportionally high gains in capacity. This and other of our findings are at odds with information furnished in professional handbooks.     

Assessing appropriate stiffness levels for spudcan foundations on dense sand

Before a jack-up can operate at a given location, a site-specific assessment of its ability to withstand a design storm during operation must be performed. During this assessment, the complex state of stress and strain under a spudcan is usually simplified to a value of foundation stiffness that is integrated as a boundary condition into the structural analysis. Soil stiffness is a critical parameter affecting the foundation and structural load distribution and displacements, and the jack-up natural period and dynamic response. The level of spudcan stiffness is an area of intense interest and debate. This paper assesses appropriate stiffness levels for numerical simulation. Utilising results from a detailed “pushover” experiment of a three-legged model jack-up on dense sand, the paper compares the experimental pushover loads and displacements on the hull and spudcans to numerical simulations using different assumptions of spudcan stiffness. These include pinned and encastré footings, linear springs and a force-resultant model based on displacement-hardening plasticity theory. Constant stiffness levels are shown to be inadequate in simulating the experimental pushover test. The non-linear degradation of stiffness associated with the latter force-resultant model is critical.     

On the allocation of city space to multiple transport modes

Abstract

This paper analyzes urban multimodal transportation systems in an aggregated way. To describe the aggregate behavior of traffic in cities, use is made of an idea that is now receiving some attention: the macroscopic fundamental diagram (MFD). We demonstrate through simulation how the MFD can be used to monitor and control a real network, in this case a portion of San Francisco, using readily available input data. We then show how different modes interact on the same network and discuss how these interactions might be incorporated into an MFD for multimodal networks. The work unveils two main results: first, it confirms recent results showing that restricting access to a city's congested areas can improve mobility for all travelers, including those who endure the restrictions; and second, that dedicating street space to collective transport modes can improve accessibility for all modes, even those from which space is taken away.     

Increasing the capacity of an isolated merge by metering its on-ramp

Measurements taken downstream of freeway/on-ramp merges have previously shown that discharge flow diminishes when a merge becomes an isolated bottleneck. By means of observation and experiment, we show here that metering an on-ramp can recover the higher discharge flow at a merge and thereby increase the merge capacity. Detailed observations were collected at a single merge using video. These data revealed that the reductions in discharge flow are triggered by a queue that forms near the merge in the freeway shoulder lane and then spreads laterally, as drivers change lanes to maneuver around slow traffic. Our experiments show that once restrictive metering mitigated this shoulder lane queue, high outflows often returned to the median lane. High merge outflows could be restored in all freeway lanes by then relaxing the metering rate so that inflows from the on-ramp increased. Although outflows recovered in this fashion were not sustained for periods greater than 13 min, the findings are the first real evidence that ramp metering can favorably affect the capacity of an isolated merge. Furthermore, these findings point to control strategies that might generate higher outflows for more prolonged periods and increase merge capacity even more. Finally, the findings uncover details of merge operation that are essential for developing realistic theories of merging traffic.     

Lane changing patterns of bane and benefit: Observations of an uphill expressway

A mechanism is unveiled by which congestion forms on a 3-lane, uphill expressway segment, and causes reductions in output flow. Vehicular lane-changing (LC) is key to the mechanism, particularly LC induced by speed disturbances (SDs) that periodically arise in the expressway’s median and center lanes. Early in the rush, when flow was relatively low in the shoulder lane, drivers readily migrated toward that lane to escape the oncoming SDs. The shoulder lane thus acted as a ‘release valve’ for the high vehicular accumulations created by the SDs, such that forced vehicular decelerations were short-lived. The release valve failed only later in the rush, when flow increased in the shoulder lane in response to rising demand. LC induced by the SDs thereafter became disruptive: the decelerations they imposed spread laterally, and a persistent queue formed in all lanes. Long-run output flow dropped each day by 4-11% once the queue engulfed the base of the incline, and impeded vehicle ascent.Subtle details of this mechanism became visible by examining thousands of vehicle trajectories that were extracted from a series of eleven roadside video cameras. Though these trajectories were collected from only a single day, we suspect that the findings can be generalized to other days at the present site, and to other sites. This is because: (i) conspicuous features of the mechanism were repeatedly observed in loop detector data that were measured over many days at the site; (ii) these macroscopic features are consistent with observations previously made at other sites; and (iii) the more subtle details unveiled by the trajectories are compatible with a general theory of multi-lane traffic.     

Dual influences on vehicle speed in special-use lanes and critique of US regulation

We verify that slow speeds in a special-use lane, such as a carpool or bus lane, can be due to both, high demand for that lane and slow speeds in the adjacent regular-use lane. These dual influences are confirmed from months of data collected from all freeway carpool facilities in the San Francisco Bay Area. Additional data indicate that both influences hold: for other types of special-use lanes, including bus lanes; and for other parts of the world.The findings do not bode well for a new US regulation stipulating that most classes of Low-Emitting Vehicles, or LEVs, are to vacate slow-moving carpool lanes. These LEVs invariably constitute small percentages of traffic; e.g. they are only about 1% of the freeway traffic demand in the San Francisco Bay Area. Yet, we show: that relegating some or all of these vehicles to regular-use lanes can significantly add to regular-lane congestion; and that this, in turn, can also be damaging to vehicles that continue to use the carpool lanes. Counterproductive outcomes of this kind are predicted first by applying kinematic wave analysis to a real Bay Area freeway. Its measured data indicate that the site selected for this analysis stands to suffer less from the regulation than will others in the region. Yet, we predict: that the regulation will cause the site’s people-hours and vehicle-hours traveled during the rush to each increase by more than 10%; and that carpool-lane traffic will share in the damages. Real data from the site support these predictions. Further parametric analysis of a hypothetical, but more generic freeway system indicates that these kinds of negative outcomes will be widespread. Constructive ways to amend the new regulation are discussed, as are promising strategies to increase the vehicle speeds in carpool lanes by improving the travel conditions in regular lanes.