Urban Gridlock

Chicago Gridlock

Gridlock is a fact of life in urban areas. Why is that? A new study explores the characteristics of urban gridlock, to better understand the condition and ways to ease congestion. From Transportation Research Part C: Emerging Techonologies, "Urban network gridlock: Theory, characteristics, and dynamics" by Hani S. Mahmassani, Meead Saberi, and Ali Zockaie tackles the issue. 

This study explores the limiting properties of network-wide traffic flow relations under heavily congested conditions in a large-scale complex urban street network; these limiting conditions are emulated in the context of dynamic traffic assignment (DTA) experiments on an actual large network. The primary objectives are to characterize gridlock and understand its dynamics. This study addresses a gap in the literature with regard to the existence of exit flow and recovery period. The one-dimensional theoretical Network Fundamental Diagram (NFD) only represents steady-state behavior and holds only when the inputs change slowly in time and traffic is distributed homogenously in space. Also, it does not describe the hysteretic behavior of the network traffic when a gridlock forms or when network recovers. Thus, a model is proposed to reproduce hysteresis and gridlock when homogeneity and steady-state conditions do not hold. It is conjectured that the network average flow can be approximated as a non-linear function of network average density and variation in link densities. The proposed model is calibrated for the Chicago Central Business District (CBD) network. We also show that complex urban networks with multiple route choices, similar to the idealized network tested previously in the literature, tend to jam at a range of densities that are smaller than the theoretical average network jam density. Also it is demonstrated that networks tend to gridlock in many different ways with different configurations. This study examines how mobility of urban street networks could be improved by managing vehicle accumulation and redistributing network traffic via strategies such as demand management and disseminating real-time traveler information (adaptive driving). This study thus defines and explores some key characteristics and dynamics of urban street network gridlocks including gridlock formation, propagation, recovery, size, etc.

The full paper can be found here.

Rebalancing and Bikeshare

The mythical #DivvyRed

Is this the summer of Bikeshare? Divvy Bikes in Chicago launched last month. CitiBikes in New York City launched around Memorial Day. Any time now Bay Area Bike Share will be launching in San Francisco and on then Peninsula. 

The issue of having bikes where people want them is a perennial issue for bikeshare systems. "Rebalancing" is the act of moving inventory around to match demand and travel patterns. This map provides realtime visualizations of the demand of bikeshare systems around the world. Researchers are working on solving the rebalancing problem

A new article from EURO Journal on Transportation and Logistics works to develop a model for rebalancing. "Static repositioning in a bike-sharing system: models and solution approaches" by Tal Raviv, Michal Tzur, and Iris A. Forma, looks at how rebalancing or repositioning can help bikeshare systems.

Bike-sharing systems allow people to rent a bicycle at one of many automatic rental stations scattered around the city, use them for a short journey and return them at any station in the city. A crucial factor for the success of a bike-sharing system is its ability to meet the fluctuating demand for bicycles and for vacant lockers at each station. This is achieved by means of a repositioning operation, which consists of removing bicycles from some stations and transferring them to other stations, using a dedicated fleet of trucks. Operating such a fleet in a large bike-sharing system is an intricate problem consisting of decisions regarding the routes that the vehicles should follow and the number of bicycles that should be removed or placed at each station on each visit of the vehicles. In this paper, we present our modeling approach to the problem that generalizes existing routing models in the literature. This is done by introducing a unique convex objective function as well as time-related considerations. We present two mixed integer linear program formulations, discuss the assumptions associated with each, strengthen them by several valid inequalities and dominance rules, and compare their performances through an extensive numerical study. The results indicate that one of the formulations is very effective in obtaining high quality solutions to real life instances of the problem consisting of up to 104 stations and two vehicles. Finally, we draw insights on the characteristics of good solutions.

The full paper can be found here

Crowding in transit: How does it effects on riders, operations and demand.

SCRTD Crowded Bus Stop RTD_1131_13

Crowded bus stops and subway stations, which beget crowded buses and trains, are a part of riding transit. ITS Berkeley researchers are exploring how this effects rider attitudes

A new article from Transportation Research Part A: Policy and Practice examines this issue. In "Crowding in public transport systems: Effects on users, operation and implications for the estimation of demand," researchers from Chile and Australia look at the effects of crowding on speed, waiting times, travel time reliability, and route choice. 

The effects of high passenger density at bus stops, at rail stations, inside buses and trains are diverse. This paper examines the multiple dimensions of passenger crowding related to public transport demand, supply and operations, including effects on operating speed, waiting time, travel time reliability, passengers’ wellbeing, valuation of waiting and in-vehicle time savings, route and bus choice, and optimal levels of frequency, vehicle size and fare. Secondly, crowding externalities are estimated for rail and bus services in Sydney, in order to show the impact of crowding on the estimated value of in-vehicle time savings and demand prediction. Using Multinomial Logit (MNL) and Error Components (EC) models, we show that alternative assumptions concerning the threshold load factor that triggers a crowding externality effect do have an influence on the value of travel time (VTTS) for low occupancy levels (all passengers sitting); however, for high occupancy levels, alternative crowding models estimate similar VTTS. Importantly, if demand for a public transport service is estimated without explicit consideration of crowding as a source of disutility for passengers, demand will be overestimated if the service is designed to have a number of standees beyond a threshold, as analytically shown using a MNL choice model. More research is needed to explore if these findings hold with more complex choice models and in other contexts.

The full article can be found here

Metro ExpressLanes preliminary report

Harbor Freeway Transitway

Last week the Los Angeles County Metropolitan Transportation Autority, or LA Metro, released the preliminary data from the ExpressLanes program. ExpressLanes is a demonstration project with Metro and Caltrans that implemented toll lanes on I-10 and I-110 in conjunction with improved transit and carpool options along those corridors. 

While the demonstration period is not yet over, there have already been noticable increases in transit ridership and vanpools along the corridor. To explore more of the data and figures, the full report can be found here

Bottlenecks and the evening commute.

Motion

During the evening commute there are often bottlenecks as people try to get home on fixe routes with finite capacity. Vickrey's "Congestion Theory and Transport Investment" (1969) decribes the problem of commuters trying to pass the bottleneck. A recent paper, "The evening commute with cars and transit: Duality results and user equilibrium for the combined morning and evening peaks" by Eric Gonzales and Carlos Daganzo tackles the commute problem looking at both the evening and morning commute, since mode travel decisions are often made based upon the travel needs for the whole day. 

The paper then considers both the morning and evening peaks together for a single mode bottleneck (all cars) with identical travelers that share the same wished times. For a schedule penalty function of the morning departure and evening arrival times that is positive definite and has certain properties, a user equilibrium is shown to exist in which commuters travel in the same order in both peaks. The result is used to illustrate the user equilibrium for two cases: (i) commuters have decoupled schedule preferences in the morning and evening and (ii) commuters must work a fixed shift length but have flexibility when to start. Finally, a special case is considered with cars and transit: commuters have the same wished order in the morning and evening peaks. Commuters must use the same mode in both directions, and the complete user equilibrium solution reveals the number of commuters using cars and transit and the period in the middle of each rush when transit is used.

The whole paper can be found here

#BARTSTRIKE

 

At midnight July 1 2013, after failed negotiations between BART and its two main unions, BART workers went on strike. The strike has disrupted transportation throughout much of the Bay Area-  increasing commute times and traffic congestion. Many commuters are turning to the ferries, casual carpool, and rideshare. The more adventurous have opted for helicopters or yachts. While there has been the predicted mix of frustration, criticism, and selfpromotion on Twitter via #BARTstrike, it's still too early to gague the real impact of the strike on transportation. Some projections estimate the econmic impact to be $73 million a day as well as 16 million pounds of carbon. Some clues might be gleaned from the recently published, Subways, Strikes, and Slowdowns: The Impacts of Public Transportation on Traffic Congestion. Using data from the 2003 transit worker strike in Los Angeles, researchers show that transit relieves traffic congestion

Air-traffic demand and capacity during bad weather

Landing Incheon Airport

What should airlines and air traffic controllers do to structure flights when airspace is reduced due to bad weather? A new paper from the recent aviation themed Transportation Research Record (no. 2325) examines that question. In "Mechanisms for Equitable Resource Allocation When Airspace Capacity Is Reduced," researchers from University of Maryland explore how carriers can prioritize flights. 

During bad weather and under other capacity-reducing restrictions, FAA uses various initiatives to manage air traffic flow to alleviate problems associated with imbalanced demand and capacity. A recently introduced alternative concept to airspace flow programs is the collaborative trajectory options program, in which aircraft operators are allowed to submit sets of alternative trajectory options for their flights, with accompanying cost estimates. It is not clear that these sets of alternative trajectory options can be generated or evaluated quickly enough to respond to flow programs that arise unexpectedly or that the program is intended to be folded into a formal resource allocation mechanism. This research proposes (a) a meaningful, yet simple, way for carriers to express some preference structure for their flights that are specifically affected by flow programs and (b) a resource allocation mechanism that will improve system efficiency and simultaneously take these airline preferences into account. The results are compared with the events that could occur if an airspace flow program were run by using a ration-by-schedule approach, with or without the opportunity for carriers to engage in swaps among their own flights.

The full paper can be found here

Closed 6/7 through 6/14.

sorry bus

Sorry! The library will be closed June 7 through June 14. We'll reopen on June 17. Enjoy the sunshine and go explore the world.

Is the stereotype of of electric vehicle drivers changing?

Special parking for Ram Plug-in Hybrid Electric Vehicle (PHEV)

That's the question a new article from Transportation Researc Part F asks. In "Electric vehicle drivers’ reported interactions with the public: Driving stereotype change?", researchers from Oxford Brookes University interviewed drivers of electric vehicles about their perceptions of the general public. They found that the stereotype is in a state of flux as the market shifts. It is also shown that the drivers are important as ambassors for electric vehicles taking hold with the rest of the general public. 

Library Closed 5/24 through 5/27

We will be closed Friday, May 23 through Monday, May 27 for the Memorial Day Holiday. If you are going to bicycle and barbeque this weekend, please don't mix them like this guy. Unsafe at any speed. 

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