published journal article
conference paper
published journal article
conference paper
conference paper
Taxicabs are certainly the most popular type of on-demand transportation service in urban areas because taxi dispatching systems offer more and better services in terms of shorter wait times and travel convenience. However, a shortage of taxicabs has always been critical in many urban contexts especially during peak hours and taxis have great potential to maximize their efficiency by employing shared-ride concept. There are recent successes in real-time ridesharing projects that are expected to bring substantial benefits on energy consumption and operation efficiency, and thus it is essential to develop advanced vehicle dispatch algorithms to maximize occupancy and minimize travel times in real-time. This paper investigates how taxi services can be improved by proposing shared-taxi algorithms and what type of objective functions and constraints could be employed to prevent excessive passenger detours. Hybrid Simulated Annealing (HSA) is applied to dynamically assign passenger requests efficiently and a series of simulations are conducted with two different taxi operation strategies. The simulation results reveal that allowing ride-sharing for taxicabs increases productivity over the various demand levels and HSA can be considered as a suitable solution to maximize the system efficiency of real-time ride sharing.
published journal article
published journal article
Introduction Individual- and neighborhood-level social determinants of health (SDOH) have been assessed separately in pregnancy, but their relationship to one another remains uncertain. We investigated the intersectionality of three neighborhood-level SDOH measures with three individual-level SDOH measures. This was done to examine the concomitant experiences of multiple SDOH in pregnancy. Methods A secondary analysis of data from the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-To-Be. We assessed three neighborhood-level SDOH measures using geocoded participant home addresses in the first trimester at the census-tract level: (1) high socioeconomic disadvantage (in tertiles) by the 2015 Area Deprivation Index, (2) inadequate food access by the USDA Food Access Research Atlas, and (3) low walkability by the EPA National Walkability Score. We assessed three individual-level SDOH measures: low household income, lower educational attainment, and Medicaid insurance. We examined the combinations of these three neighborhood SDOH and three individual SDOH measures by graphical visualization and using statistical tests to assess overall differences in the distribution of these measures. Results Of 9588 nulliparous individuals, adverse neighborhood-level SDOH [high socioeconomic disadvantage (28%), inadequate food access (24%), and low walkability (66%)] and adverse individual-level SDOH [low household income (19%), lower educational attainment (23%), and Medicaid insurance (33%)] were common in early pregnancy. Six percent of individuals lived in a community with all three adverse neighborhood-level SDOH measures. Of those living in a community with at least two neighborhood-level SDOH measures, 23% lived in areas with inadequate food access and low walkability, 19% with high socioeconomic disadvantage and low walkability, and 1% with high socioeconomic disadvantage and inadequate food access. Overall, 23% lived in a community with no adverse neighborhood-level SDOH, and among this group, 88% had no adverse individual-level SDOH. There were significant differences in adverse individual-level SDOH based on whether individuals lived in a community with all three adverse neighborhood-level measures [low household income (39%), lower educational attainment (44%), Medicaid (55%)], any two measures [low household income (22%), lower educational attainment (27%), Medicaid (37%)], or only one measure [low household income (14%), lower educational attainment (17%), Medicaid (27%)] (p < 0.001 for all). Conclusion Among nulliparous individuals in early pregnancy, the frequency of adverse individual-level SDOH was generally higher when they lived in communities with more adverse neighborhood-level SDOH. Future approaches that identify and classify the multifaceted and multilevel nature of structural determinants as they relate to pregnancy outcomes are needed.
working paper
We examine spatial patterns and their changes during the 1970s for the Los Angeles region, by estimating monocentric and polycentric density functions for employment and population. Downtown Los Angeles is clearly identified as the statistical monocentric center of the region, and it is the most consistently strong center in the polycentric patterns. Polycentric models fit statistically better than monocentric models, and there was some shift in employment distribution toward a more polycentric pattern. These findings verify the existence of polycentricity in Los Angeles and demonstrate for the first time that employment and especially population follow a polycentric pattern based on exogenously defined employment centers. The results confirm that both employment and population became more dispersed during the 1970s.
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Oct 1993
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Jun 1996
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Oct 1995
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Oct 1995
working paper |
Mar 1997
working paper
The physical and perceptual dimensions of commuting travel impedance were again found to have stressful consequences in a study of 99 employees of two companies. This quasi-experimental replication study, which focuses here on home environment consequences, investigated the effects of physical impedance and subjective impedance on multivariate measures of residential satisfaction and personal affect in the home. Both sets of residential outcome measures were found to be significantly related to the two impedance dimensions. As predicted, gender was a significant moderator of physical impedance effects. Females commuting on high physical impedance routes were most negatively affected. Previously found subjective impedance effects on negative home mood, regardless of gender, were strongly replicated with several methods and were buttressed by convergent results with objective indices. The theoretical conjecture that subjective impedance mediates the stress effects of physical impedance was supported for the personal affect cluster but only for one variable in the residential satisfaction cluster. Traffic congestion has increased in metropolitan areas nationwide, and commuters, families, and organizations are absorbing associated hidden costs. The results are reviewed in terms of our ecological model, and the moderating effects of gender are discussed in terms of choice and role constraints.
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May 1990
working paper |
Sep 1991
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Nov 1991
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Sep 1991
working paper |
Mar 1989
working paper
This report presents the results of an evaluation of the Transportation System Management plan employed during the 1984 Los Angeles Summer Olympics. The Summer Olympics presented Los Angeles area transportation planners with an unprecedented challenge: to manage the circulation of an expected 1.2 million visitors, 6 millions spectators, and nearly 25,000 athletes, media, and Olympic family within a regional transportation system which had reached capacity in many areas. Owing to the lack of both funds and time, capital improvements to meet the anticipated increase were not feasible. Rather, Los Angeles transportation planners had no choice but to develop and implement the most ambitious transportation management program ever attempted.Caltrans District 7, in conjunction with several local transportation agencies and the Los Angeles Olympics Organizing Committee, invested two years of effort in the development of a viable and effective traffic management plan for the 1984 Summer Olympics. From a traffic management perspective, the Los Angeles Summer Olympics were an unqualified success. With few exceptions, major traffic problems failed to materialize, and, for the first time in the recent history of the Olympics, not one group of spectators got stranded and missed an event.The Los Angeles Olympics provided a unique opportunity to test the effectiveness of transportation system management under extreme conditions. The apparent success of the experiment merits close analysis, both in order to identify what worked and what did not, and to determine whether lessons learned from the experience can provide guidelines for future transportation policy decisions.