Debbie A. Niemeier

Measuring Accessibility: An Exploration of Issues and Alternatives

Accessibility is an important characteristic of metropolitan areas and is often reflected in transportation and land-use planning goals. But the concept of accessibility has rarely been translated into performance measures by which policies are evaluated, despite a substantial literature on the concept. This paper is an attempt to bridge the gap between the academic literature and the practical application of such measures and provide a framework for the development of accessibility measures. Issues that planners must address in developing an accessibility measure are outlined, and two case studies suggestive of the range of possible approaches are presented.

Near-Roadway Air Quality: Synthesizing the Findings from Real-World Data

Despite increasing regulatory attention and literature linking roadside air pollution to health outcomes, studies on near roadway air quality have not yet been well synthesized. We employ data collected from 1978 as reported in 41 roadside monitoring studies, encompassing more than 700 air pollutant concentration measurements, published as of June 2008. Two types of normalization, background and edge-of-road, were applied to the observed concentrations. Local regression models were specified to the concentration-distance relationship and analysis of variance was used to determine the statistical significance of trends. Using an edge-of-road normalization, almost all pollutants decay to background by 115-570 m from the edge of road; using the more standard background normalization, almost all pollutants decay to background by 160-570 m from the edge of road. Differences between the normalization methods arose due to the likely bias inherent in background normalization, since some reported background values tend to underpredict (be lower than) actual background. Changes in pollutant concentrations with increasing distance from the road fell into one of three groups: at least a 50% decrease in peak/edge-of-road concentration by 150 m, followed by consistent but gradual decay toward background (e.g., carbon monoxide, some ultrafine particulate matter number concentrations); consistent decay or change over the entire distance range (e.g., benzene, nitrogen dioxide); or no trend with distance (e.g., particulate matter mass concentrations).

Travel to work and household responsibility: new evidence

A persistent empirical finding in the research on travel patterns is that women tend to engage in shorter work commutes than men. Research evaluating the degree to which this gender differential in commuting may be explained by the division of labor in the household has produced decidedly mixed findings. This paper presents a critical review of the methods and results of recent research on the gender differential in commuting along with the associated implications for the household responsibility hypothesis (HRH). While all of the articles reviewed are informative and unique, not all conclusions are well supported. The paper then tests the HRH using the 1990 NPTS travel data. The new evidence confirms that women continue to exhibit shorter commute times and distances than men and provides support for the HRH. The paper concludes with a summary of findings and recommendations for further research.

Accessibility: an evaluation using consumer welfare

This study explores the worth consumers place on mode-destination accessibility for the AM journey to work trip. To accomplish this, a multinomial mode-destination choice model is estimated and the denominator of the specified logit model is used as an estimate of mode-destination accessibility. To improve the interpretability of this measure, compensating variation is then applied to convert the mode-destination accessibility to units of dollars per AM journey to work trip. The model is estimated using travel survey data from the Puget Sound Region in Washington state. It is reasonable to assume, for example, that the worth placed on mode-destination accessibility varies by mode, by destination, and by market segment (e.g., low income, high income). Less intuitive, however, are the magnitude and direction of these variations. This paper presents a methodological approach, followed by an empirical evaluation, for examining the worth of journey to work mode- destination accessibility. The results have important policy implications and also provide a mechanism for incorporating a monetary value for accessibility in future cost-benefit analyses.

BICYCLE JOURNEY-TO-WORK: TRAVEL BEHAVIOR CHARACTERISTICS AND SPATIAL ATTRIBUTES

The relationship between the demographic attributes and spatial clustering of individuals making a weekday bicycle journey-to-work commute and their commuting travel time is explored. The study uses data from a 1993 bicycle-intercept survey distributed in Seattle, Washington, in which individual bicycle-travel behavior characteristics were collected. The data include socioeconomic information, such as age, gender and income. The results indicate that these three factors may play unexpected roles in the length of bicycle commuting travel times for the journey-to-work trips. This study also suggests that separated bicycle paths play an integral part in the overall bicycle transportation network. Statistical analysis also indicated that cyclists traveling primarily on separated paths tend to make significantly longer trips.

Integrating vehicle design and human factors: Minimizing elderly driving constraints

With a projected rise in the number of elderly, most of whom have also relied primarily on the private automobile for their mobility, it is likely that future adaptations in vehicle design will be linked in some part to the physical infirmities often faced by the elderly. This paper offers a bridge between medical research on the physical impairments of the elderly and automobile design and driving safety. We describe recent findings on the driving-related physical and cognitive impairments faced by the elderly. We then propose two major types of vehicle design and infrastructure adaptations: (1) modifications for private vehicles, and (2) intelligent technology and support services for private vehicles, which can help to minimize the driving-related effects of these impairments. For example, we present a range of modest vehicle design adaptations for components such as seats and doorways, handles, knobs, and steering wheels, and seat belts. We find that many of these improvements can be made to standard passenger vehicles with little additional design effort, and that the adaptations should also increase overall vehicle marketability. Finally, we argue that while most, if not all, of our proposed adaptations would be made to largely benefit the elderly, they will nevertheless support and improve driving across all age groups.

Duration of trip-making activities by men and women

The purpose of this paper is to present the results of a survival analysis for the duration of particular trip-making activities based on sex. Specifically, this study investigates the duration of those activities related to household and family support shopping, personal business, and free time and how these durations vary between men and women. It was found that there were no significant differences in the survival curves (i.e., durations) of free-time or personal business activities; this suggests that men and women spend approximately similar amounts of time on these activities, although it is not known if the activities themselves are similar (for example, banking versus getting gas). Alternatively, sex was found to be a very significant indicator of the duration of household and family support shopping activities. In the model specification, assuming all variables except sex are the same, it was found that women were 1.32 times more likely than men to spend a longer period of time in a household and family support shopping activity. Additionally, it was found that women are 1.33 times more likely than men to have a longer household and family support activity duration if the activity is nested in the journey to work trip.

How Much Can Vehicle Emissions Be Reduced?: Exploratory Analysis of an Upper Boundary Using an Emissions-Optimized Trip Assignment

The development of a trip-assignment model designed to systematically compute emissions-optimized traffic flows for carbon monoxide is explored. Then the exploratory theoretical emissions-optimized tripassignment model is used to estimate the maximum carbon monoxide emissions reductions under varying congestion levels on a hypothetical network. The experimental results indicate moderate reductions in system-level vehicle emissions under emissions-optimized trip assignment compared with the conventional time-dependent user-equilibrium and system-optimized models. They suggest a possible method for approximating an upper boundary of expected emissions reductions using the standard travel-demand model with a modified trip-assignment objective function. The approximate boundary is useful as an estimate of emissions reductions that might be possible under optimal conditions (e.g., drivers choose emissions-optimized routes, fully implemented intelligent transportation systems, etc.). In other words, the upper boundary suggests that current traffic optimization strategies may be insufficient to fully remedy air quality problems created by mobile-source emissions.

An exploratory analysis comparing a stochastic driving cycle to California's regulatory cycle

Abstract As the fundamental building block of the emissions estimation process, a driving cycle needs to be representative of real-world driving behavior. The driving cycle construction method becomes crucial for generating a representative driving cycle. In this paper, we revisit the Unified Cycles (i.e., the LA92 driving cycle) construction method. The California Air Resources Boards Unified Cycle used a “microtrips” approach, a speed–acceleration frequency distribution plot, and a quasi-random selection mechanism to build the driving cycle. There is concern that the Unified Cycle does not reflect the true driving patterns due to the identified flaws in the construction methodology. Treating a driving trace as a stochastic process, we construct a new driving cycle (LA01) with the same driving data originally used to build the Unified Cycle. We then compare the two driving cycles with the sample data set with respect to the durations and intensities of the modal events. The new driving cycle is found to better replicate the modal events observed in the sample data. A comparison of average road power values between the sample data, LA01, and the Unified Cycle also confirms the effect of fine-scale driving on emissions. These differences result from the different construction approaches and can be expected to affect emissions inventory estimation.

Regional driving characteristics, regional driving cycles

Abstract The Environmental Protection Agency’s (EPA) MOBILE6 driving cycles were developed by combining chase car data collected in three cities: Baltimore, Spokane, and Los Angeles, and then organizing the data by facility and level of service (LOS) to eliminate regional variability in driving. EPA’s approach to driving cycle construction presumes that regional driving variability is insignificant when controlling for facility type and LOS. In this study we re-visit the issue of regional driving variability and its potential impact on emissions using driving data recently collected in the Bay Area, Sacramento and Stanislaus, California. We begin by examining regional driving characteristics for four types of driving conditions: un-congested and congested freeway driving, and un-congested and congested arterial driving. The results suggest regional similarities in terms of the average speeds and accelerations, but marked differences in frequency, duration, and intensity of both steady state and acceleration modal events. A one-way ANOVA analysis indicates that regional driving variability exists even after controlling for LOS and facility type. We also show how these regional differences can result in driving cycles with significantly different compositions of modal events (i.e., cruise, idle, acceleration and deceleration) using a new method for constructing driving cycles. An examination of the cycles with respect to steady state driving and acceleration/deceleration modal events confirms that regional driving differences are sufficiently large enough to result in important driving cycle differences, which may also translate into important regional variability in vehicle emissions estimation.