Xinwu Qian

Characterizing Urban Dynamics Using Large Scale Taxicab Data

Understanding urban dynamics is of fundamental importance for the efficient operation and sustainable development of large cities. In this paper, we present a comprehensive study on characterizing urban dynamics using the large scale taxi data in New York City. The pick-up and drop-off locations are firstly analyzed separately to reveal the general trip pattern across the city and the existence of unbalanced trips. The inherent similarities among taxi trips are further investigated using the two-step clustering algorithm. It builds up the relationship among detached areas in terms of land use types, travel distances and departure time. Moreover, human mobility pattern are inferred from the taxi trip displacements and is found to follow two stages: an exponential distribution with short trips and a truncated power law distribution for longer trips. The result indicates that the taxi trip may not fully represent human mobility and is heavily affected by trip expenses and the urban form and geography.

A Graph-Based Approach to Measuring the Efficiency of an Urban Taxi Service System

Taxi service systems in big cities are immensely complex due to the interaction and self-organization between taxi drivers and passengers. An inefficient taxi service system leads to more empty trips for drivers and longer waiting time for passengers and introduces unnecessary congestion on the road network. In this paper, we investigate the efficiency level of the taxi service system using real-world large-scale taxi trip data. By assuming a hypothetical system-wide recommendation system, two approaches are proposed to find the theoretical optimal strategies that minimize the cost of empty trips and the number of taxis required to satisfy all the observed trips. The optimization problems are transformed into equivalent graph problems and solved using polynomial time algorithms. The taxi trip data in New York City are used to quantitatively examine the gap between the current system performance and the theoretically optimal system. The numerical results indicate that, if system-wide information between taxi drivers and passengers was shared, it is possible to reduce 60%-90% of the total empty trip cost depending on different objectives, and one-third of all taxis required to serve all observed trips. The existence of destructive competition among taxi drivers is also uncovered in the actual taxi service system. The huge performance gap suggests an urgent need for a system reconsideration in designing taxi recommendation systems.

Time-of-Day Pricing in Taxi Markets

For a regular weekday in New York City, the number of taxi trips at 8 P.M. may be 10 times greater than that at 5 A.M., while passengers are charged under the same pricing scheme. Motivated by temporally non-stationary demand and supply in the taxi market, the time-of-day (TOD) pricing scheme for taxi industry is framed to vary trip cost dynamically over time, so that total market revenue is maximized. Temporal market dynamics is modeled as a semi-Markov process, which captures leftover of drivers, spillover of passengers, and restoration of drivers in service along the time horizon. The TOD pricing scheme is therefore formulated as discrete time stochastic dynamic programming with the goal to find the optimal sequence of price multipliers. The approximate dynamic programming (ADP) approach is introduced to solve the curse of dimensionality. Numerical experiments are conducted using New York City taxi trip data to illustrate the effectiveness of TOD price in real-world taxi market. The results suggest that TOD price may increase daily market revenue by over 10% using the ADP approach. Our experiments also show that TOD price may be even more effective if sudden surges in demand take place in the market.

Best practices for maximizing driver attention to work zone warning signs.

10 mph. • Changeable message signs (CMS) are ideal for short-term and mobile work zones due to their portability. For CMS displaying text messages, innovative use of fonts and content may achieve greater speed reduction. Graphic-aided CMS are found to be more legible and recognizable compared with text-message CMS. The location of CMS has significant impact on the effectiveness of this technology; placing in advance of a work zone is recommended. • The presence of police cars may reduce car speeds by 4.4 mph and truck speeds by 5 mph (in a 45 mph speed zone). High-speed drivers are found to be more affected by police enforcement. Despite the effectiveness of CMS, “halo” effects and high costs are known to be the two main drawbacks of the technology. • Implementations of variable speed limit (VSL) resulted in reduced speed variance, increased speed compliance, and improved traffic throughput and Joint transportation research program