Norman Garrick

Does street network design affect traffic safety

Negative binomial regression models were used to assess the effect of street and street network characteristics on total crashes, severe injury crashes, and fatal crashes. Data from over 230,000 crashes taking place over 11 years in 24 California cities was analyzed at the U.S. Census Block Group level of geography. In our analysis we controlled for variables such as vehicle volumes, income levels, and proximity to limited access highways and to the downtown area. Street network characteristics that were considered in the analysis included street network density and street connectivity along with street network pattern. Our findings suggest that for all levels of crash severity, street network characteristics correlate with road safety outcomes. Denser street networks with higher intersection counts per area are associated with fewer crashes across all severity levels. Conversely, increased street connectivity as well as additional travel lanes along the major streets correlated with more crashes. Our results suggest that in assessing safety, it is important to move beyond the traditional approach of just looking at the characteristics of the street itself and examine how the interrelated factors of street network characteristics, patterns, and individual street designs interact to affect crash frequency and severity.

Effect of Street Network Design on Walking and Biking

The objective of this research was to investigate whether a relationship existed between street network characteristics and the transportation modes selected in a neighborhood. Factors such as street characteristics, vehicle volumes, activity levels, income levels, and proximity to limited-access highways and the downtown area were controlled for. The results suggested that all three of the fundamental characteristics of a street network—street connectivity, street network density, and street patterns—were statistically significant in affecting the choice to drive, walk, bike, or take transit. Both increased intersection density and additional street connectivity were generally associated with more walking, biking, and transit use. Street patterns with gridded street networks, which tended to have a higher-than-average street connectivity and a much higher street network density, were associated with much more walking and biking. These results suggested that street network patterns were extremely important for encouraging nonautomobile modes of travel. As the United States begins to focus on reducing vehicle miles traveled as a strategy to combat carbon production and cut energy use, it is increasingly imperative that this relationship between the built environment and mode choice be accounted for in the planning and design of the transportation system.

Reassessing On-Street Parking

The ongoing debate about the merits and drawbacks of on-street parking offers few definitive answers because comprehensive research in this area has been lacking. The goal is to develop a better understanding of the gamut of issues related to on-street parking, ranging from parking demand and the pedestrian environment to less researched topics such as the efficiency of land use. In addition, the basic question of safety is addressed in a more precise way than previously by taking into account actual vehicle speeds and crash severity levels. The investigation points to on-street parking as crucial in benefiting activity centers on numerous levels. Users of the downtowns consistently valued these land-efficient on-street parking spaces over and above off-street surface lots and garages. Low-speed streets with on-street parking also had the lowest fatal and severe crash rates of any road category in the study of 250 Connecticut roadway segments. Part of the reason is that the presence of parking had a measurable effect on vehicle speeds. On-street parking is not purely a device to be used in the right environment; rather, it is a tool to help create that right environment. On-street parking should be more commonly used but especially in situations in which the road is part of the destination and the intent is to cause drivers to slow down. Results suggest that these places are safer, are more walkable, require less parking, and have more vitality.

Quantifying the Economic Domain of Transportation Sustainability

The lens of sustainability refocuses the perception of transportation and allows a look beyond its accustomed role of providing vehicular mobility to the broader impacts of transportation on the environment, society, and the economy. As the understanding of transportations function evolves beyond throughput and capacity, sustainability can be used as an organizing principle for transportation planning to promote livable communities. To fully understand and integrate the ideas of sustainability with transportation, the proper metrics and performance measures need to be developed and adopted. This study demonstrated how the theoretical concepts and definitions of transportation sustainability could be transformed into a practical metric for assessing the performance of the U.S. transportation system in terms of sustainability. The study focused on characterizing and measuring the economic aspect of sustainability in relation to transportation. The analysis was carried out for surface transportation at the statewide level and took into consideration the degree of urbanization of states. The final results described the relationship between urbanity, mode share, and the economic aspects of transportation sustainability. On the basis of this assessment, the best-performing states in terms of the economic aspects of transportation sustainability were more urban and had lower automobile mode shares.

The Applicability of Space Syntax to Bicycle Facility Planning

With the emergence of bicycles as an increasingly viable form of urban transportation comes the need for improved design and planning tools. Existing methods for evaluating bicycle facilities and for prioritizing their construction and maintenance are reviewed. Two components are necessary for such an analysis: one for assessing the quality of the segments that make up the network, and one for assessing the overall network itself. Space syntax analysis is evaluated as a tool for network assessment on the basis of its potential to predict patterns of travel over different network configurations. The theory behind space syntax is evaluated and then tested by using data from the city of Cambridge, Massachusetts. A good model for predicting bicycle volumes within a network can be constructed by using only census data and the space syntax measure “choice.” Unlike existing bicycle suitability measures, space syntax describes the importance of segments to the connectivity or completeness of the network.

Research Article: Evidence on Why Bike-Friendly Cities Are Safer for All Road Users

Biking is increasingly being recognized as a highly sustainable form of transportation. Consequently, a growing number of American cities have seen tremendous growth in bicycle travel, in part because many cities are also investing resources into improving bicycling infrastructure. Aside from the environmental advantages, there is now growing evidence to suggest that cities with higher bicycling rates also have better road safety records. This study attempts to better understand this phenomenon of lower fatality rates in bike-oriented cities by examining 11 years of road safety data (1997–2007) from 24 California cities. The analysis included accounting for crashes across all severity levels, as well as for three classes of road users: vehicle occupants, pedestrians, and bicyclists. Additionally, we looked at issues of street and street network design to help determine the role that these features might play in affecting both bicycling rates and road safety outcomes. Overall, cities with a high bicycling rate among the population generally show a much lower risk of fatal crashes for all road users when compared to the other cities in our database. The fact that this pattern of low fatality risk is consistent for all classes of road users strongly suggests that the crashes in cities with a high bicycling rate are occurring at lower speeds. This agrees with the finding that street network density was one of the most notable differences found between the safer and less safe cities. Our data suggest that improving the streets and street networks to better accommodate bicycles may lead to a self-reinforcing cycle that can help enhance overall safety for all road users.

Parking at Mixed-Use Centers in Small Cities

The debate about parking has shifted in the past decade, as some places attempt to move from conventional development patterns to urban centers modeled on new urbanism and smart growth concepts. There is now less focus on providing sufficient parking to meet demand and more on ensuring that the issue of parking does not undermine the creation of vibrant places. The goal for the presented project is to understand parking and parking provision better as it relates to smaller cities and towns with mixed-use centers. Specifically addressed are how a dense, walkable, mixed-use center affects parking supply and demand and how mixed-use centers compare with centers designed along more conventional lines. These questions were addressed by conducting case study assessments of six sites in New England. In general, the three mixed-use study sites provided much less parking per square foot than the conventional control sites. The study sites thrived by making much more efficient use of land for parking. The study sit...

Influence of Parking Policy on Built Environment and Travel Behavior in Two New England Cities, 1960 to 2007

Over the past 40 to 50 years, most American cities have experienced significant increases in automobile use. Now, to offset increasing energy use and greenhouse gas emissions, many are contemplating measures to reduce automobile use. This study examined Hartford, Connecticut, and Cambridge, Massachusetts, which exhibited an increase and a decrease in automobile use, respectively, between 1960 and 2007. It is hoped that these cities provide lessons in how to successfully reduce automobile travel. The study focused on the cumulative effects of historical policy decisions over decades on parking provisions and changes in travel behavior. The results of this analysis suggest that parking policy affects incremental changes in parking provision that may greatly influence gradual changes in automobile use over time.

THREE-DIMENSIONAL VISUALIZATION APPROACH TO ILLUSTRATING ESTHETIC CONCEPTS FOR HIGHWAY DESIGN

Realistic and dynamic visual models were developed that can be used in a teaching context for illustrating concepts that affect the appearance and fit of a highway alignment. The models consist of short video clips of the highway taken from the perspective of a vehicle’s windscreen. These video images of the roadway are coupled with graphs showing the corresponding vertical and horizontal alignment of the roadway. The users of these models can simulate a drive over the selected highway segment and visually assess the appearance and continuity of the alignment. They can then study the accompanying alignment graphs to evaluate how different combinations of vertical and horizontal elements affect the appearance and continuity of the alignment. Each of the video clips was selected to illustrate either a good or bad example of a specific concept affecting alignment design. Some of the concepts illustrated include the coordination of vertical and horizontal alignment and consistency in alignment development. The video clips were developed using images and data from the Connecticut Department of Transportation’s photologging system.

New Concepts for Context-Based Design of Streets and Highways

Street and highway designers are increasingly being challenged to develop context-based design solutions that support the development of places that are more livable or that are compatible with meeting goals for sustainable development. However, conventional design standards have not yet caught up to this new design paradigm and so do not fully support the needs of designers working in this new environment. The evaluation of the AASHTO-based approach to design in this paper shows the two main areas of concern that must be addressed in developing a more coherent and context-based approach to design. Specifically, the issues of (a) how to define context better and (b) how to design for appropriate operations (including speed) need to be addressed. In addition, there is a clear need for a comprehensive and coherent design framework that ties together the urban (or place) function and the mobility function of streets and highways and that takes into account the full context for the design, including multimodal accommodation and full integration into the context.