Measurement biases in panel data

The objective of this paper is to examine reporting errors in panel data obtained from multi-day travel diaries. A distinction is made between within and between wave biases. The former leads to an increase in under-reporting associated with the number of days the diary is kept. The latter is related to the number of waves respondents have been participating, so-called panel experience. These biases imply that observed mobility changes between waves are partly due to reporting errors: without controlling for them, changes in mobility can not be inferred from the data. An important cause of these measurement errors is the increase in the number of days on which no trips at all were reported. In addition, shorter trips and less complex chains are more susceptible to underreporting. The methodology used in this paper provides a means of dealing with these problems. Attrition is taken into account by a rather simple measure. The paper concludes with a number of suggestions for sample and survey design.     

Lifestyles and transportation needs of the elderly in Los Angeles

This study examined the trip-making behavior of persons over 65 years of age residing in Los Angeles County. To date, the major shortcoming of most research concerned with the transportation needs of the elderly is that the aged have been treated as a homogeneous group without recognizing the various lifestyles of the senior population. The two most easily distinguishable groups of elderly persons within the county are those residing in the inner-city and those around the urban fringe. Although many elderly are still found in gray ghettos there is currently a trend toward a more suburban generation of retirees. Four areas were selected within the county that reflected these two living patterns of the elderly. A comparative analysis of trip patterns and socio-economic data was completed. It was found that the suburban elderly are characterized as (1) having higher incomes, (2) residing predominantly in single-family units and (3) being reliant upon automobile transportation. As is typical of a large proportion of those over 65 years of age, many of the suburban elderly cannot drive. Studies of the transportation needs of this group indicate that without transit alternatives they may well become society's most transit-deprived segment. On the other hand, while the inner-city elderly may have a wider range of transportation options, they also have a distinct economic disadvantage which preludes their accessibility to opportunities. An analysis of modal choices, trip purposes and the frequency of travel exhibited many similarities and some significant differences in the travel behavior as well as problems of these two groups.In addition to investigating travel patterns a survey of taxi-cab use on weekends and weekdays was undertaken. Elderly persons represent a substantial proportion of taxi-cab patrons. Their use of taxis is further indicative of the change that occurs in mobility patterns upon reaching retirement. This mode of transportation is presently the only type of demand-responsive service available to the senior population in some parts of the county.This report was produced as part of a program of Research and Training in Urban Transportation sponsored by the Urban Mass Transportation Administration of the U.S. Department of Transportation. The results and views expressed are the independent products of university research and are not necessarily concurred in by the Urban Mass Transportation Administration of the Department of Transportation.     

A Method for Evaluating Marine Protected Area Management

An assumption underlying the growing support for marine protected areas (MPAs) is that they meet conservation goals and provide economic benefits to fisheries and ecotourism. However, support for MPAs will be at risk if managers cannot assess whether various MPA objectives are being fulfilled. Current approaches to MPA management emphasize the need to evaluate performance criteria; however, there is little consensus on criteria and their evaluation. We propose a marine protected area evaluation model (MPAEM), based on and modified from a multidisciplinary approach used to assess the sustainability of fisheries, called rapid appraisal of fisheries (Rapfish). The application of the MPAEM was explored in a pilot study of 20 MPAs located in different regions of the world. Results indicate that the MPAEM can be used to evaluate MPA management effectiveness. However, the manner in which the evaluation attributes are scored and ways of engaging user groups should be explored before MPAEM can become part of the day-to-day management of MPAs.     

Integrated Marine Planning for Cocos (Keeling), An Isolated Australian Atoll (Indian Ocean)

The Cocos (Keeling) Islands are located on an isolated atoll in the Indian Ocean and have a strong Malay culture owing to the translocation of a substantial plantation workforce in the early 19th century. The atoll became one of Australia's External Territories in 1955. Meeting the marine resource needs of the atoll residents within an Australian legislative system by formulating an integrated marine management plan presented several challenges. Key factors in drafting a culturally meaningful plan were a commitment to understanding and accommodating the cultural and subsistence needs of the community, innovative communication strategies, and a simple approach to management. Development of the plan highlighted the limitations of current Australian marine planning legislation in cross-cultural settings and for sustainable development.     

Mapping minibuses in Maputo and Nairobi: engaging paratransit in transportation planning in African cities

ABSTRACT

Often called paratransit because of their flexible stops, schedules and routes, minibuses make up the bulk of public transport in African cities. Despite their ubiquity and importance, these systems are poorly understood by transportation planners who tend to focus on large-scale urban infrastructure projects such as highways, commuter rail or bus rapid transit systems. The assumption within much of this planning is that these minibus systems are barriers to change and will become at most secondary “feeder” buses within large-scale projects, but structured plans detailing this vision are lacking. This paper argues that frequent failure to collect data and value important paratransit systems as a critical part of transportation in their own right is deeply problematic from the point of view of equity, access and inclusive and effective planning. We ask whether the growing number of bottom up mapping projects of minibus systems can disrupt this status quo. By comparing two mapping projects, Digital Matatus in Nairobi and the Mapa Dos Chapas in Maputo, we find that inclusive, collaborative mapping can help render these minibuses more visible in planning and provoke more grounded and inclusive “planning conversations” on multi-modal integration, passenger information and minibus upgrading, all key but relatively marginalised aspects of creating accessible, low emission, high quality and safe public transport in African cities.     

Developing MPA Effectiveness Indicators: Comparison Within and Across Stakeholder Groups and Communities

Shared expectations about marine protected areas (MPAs) might be expected to enhance their effectiveness. This study determined whether members of local communities had common or divergent views about the objectives and indicators of MPA effectiveness. In the central Philippines we used unconstrained and constrained methods to interview people in five stakeholder groups (elders, fishers, leaders, women, and youth) associated with each of 10 community-based no-take MPAs that had been protected between 0 and 21 years. Respondents identified multiple MPA objectives, the most frequently mentioned of which was restoring and sustaining fish populations. The results suggest that MPA age accounts for the greatest level of variation across communities. When unconstrained, progress in enforcement was identified as a key indicator among respondents from younger MPAs while respondents from older MPAs identified increased fish abundance and catch. Although consensus can be reached more easily under constrained conditions, the results derived from unconstrained methods are more revealing of the needs and aspirations of communities. These are likely to contribute more to the search for appropriate management interventions, particularly given that MPAs evolve over time. Hence, the development of MPA effectiveness indicators must be an iterative community-informed process to ensure timely and relevant management interventions.     

Th-derived particulate organic carbon fluxes in the northern Barents Sea with comparison to drifting sediment trap fluxes

Large-volume sampling of ²³⁴Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. ²³⁴Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m^− 2 d^− 1 in 2003 and 47.1 ± 30.6 mmol C m^− 2 d^− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m^− 2 d^− 1 in 2003 and 17.3 ± 11.4 mmol C m^− 2 d^− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although ²³⁴Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of ²³⁴Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/²³⁴Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of ²³⁴Th may yield relatively high, and possibly inaccurate POC/²³⁴Th ratios. Hence, POC fluxes derived from ²³⁴Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.     

Modeling bus bunching using massive location and fare collection data

Bus bunching is a well-known phenomenon for operators, users and regulators of high-frequency bus services. Bus operations are usually affected by increasing differences in the time intervals (headways) between consecutive buses. The effect of this variability is that buses tend to group into bunches of two or more, which severely affects the quality of service and the operational efficiency. The aim of this paper is to analyze which factors are associated to the phenomenon, using massive data from high-frequency services available in Santiago (Chile) and common-route services in Gatineau (Canada). The data is obtained from the bus GPS and AFC systems and are processed to obtain headways between buses. Using data from one week, we develop models to explain the variation of the continuous and discrete indicators of bus bunching as a function of variables related to the operation, variables related to the demand structure, and variables related to the infrastructure. Some of the factors that contribute to increase bus bunching are: stops located toward the end of the route, high scheduled frequency, irregular bus dispatch headways, non-homogeneous fleet, high demand, and high variability of demand. The results are useful for the design of quality indexes to measure bunching in bus operations, and for the design and operation of bus routes, taking into consideration the potential bus bunching problems.     

Theme Issue 30(2): "Economics of Marine Protected Areas"

Most Florida manatees depend on localized warm-water refuges in the southern two-thirds of Florida to survive winter; about 60% use outfalls from 10 power plants, whereas 15% use 4 natural warm-water springs. Future availability of these refuges is in doubt; most of these power plants may be retired within the next 20 years and groundwater withdrawals for human use threaten natural springs. This article examines possible effects on manatees from losing major warm-water refuges and alternative management actions. Because of manatee site-fidelity patterns, plant retirements may increase cold-stress-related deaths and significantly decrease manatee abundance. A forward-looking management strategy is urgently needed before decisions are made to retire plants now used by large numbers of manatees. Possible management alternatives include: gradually weaning manatees off plant outfalls, maintaining the flow of springs now used by manatees, enhancing access to suitable warm-water springs now little used or unused by manatees, constructing new non-industry dependent warm-water refuges, and creating new thermal basins to retain warm-water pockets able to support overwintering manatees.