Wesley E. Marshall

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.

Estimating Annual Average Daily Bicyclists: Error and Accuracy

Cities around the United States are investing in bicycle infrastructure, and to secure additional transportation funding, cities are reporting bicycle use and safety improvements. Data on bicyclist traffic volume is necessary for performing safety studies and reporting facility use. Meeting the need for data, available manual bicycle counting programs count cyclists for a few hours per year at designated locations. A key issue in the design of counting programs is determining the timing and frequency of counts needed to obtain a reliable estimate of annual average daily bicyclists (AADB). In particular, in which days of the week, hours of the day, and months of the year should counts be collected? And, most important to program cost, how many hours should be counted? This study used continuous bicycle counts from Boulder, Colorado, to estimate AADB and analyze the estimation errors that would be expected from various bicycle-counting scenarios. AADB average estimation errors were found to range from 15% with 4 weeks of continuous count data to 54% when only 1 h of data was collected per year. The study found that the most cost-effective length for short-term bicycle counts is one full week when automated counting devices specifically calibrated for bicycle counting are used. Seasons with higher bicycle volumes have less variation in bicycle counts and thus more accurate estimates.

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.

Bicyclist safety performance functions for a U.S. city

Efforts have intensified to apply a more evidence-based approach to traffic safety. One such effort is the Highway Safety Manual, which provides typical safety performance functions (SPFs) for common road types. SPFs model the mathematical relationship between frequency of crashes and the most significant causal factors. Unfortunately, the manual provides no SPFs for bicyclists, despite disproportionately high fatalities among this group. In this paper, a method for creating city-specific, bicycle SPFs is presented and applied to Boulder, Colorado. This is the first time a bicycle SPF has been created for a U.S. city. Such functions provide a basis for both future investigations into safety treatment efficacy and for prioritizing intersections to better allocate scarce funds for bicycle safety improvements. As expected, the SPFs show that intersections with higher bicyclist traffic and higher motorist traffic have higher motorist-cyclist collisions. The SPFs also demonstrate that intersections with more cyclists have fewer collisions per cyclist, illustrating that cyclists are safer in numbers. Intersections with fewer than 200 entering cyclists have substantially more collisions per cyclist.

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...

The metrics of street network connectivity: their inconsistencies

The concept of street connectivity has been gaining increasing appeal among researchers, planners, and planning authorities. In response, many connectivity metrics have been developed in an effort to understand better street network connectivity. This paper will study the effectiveness and consistency of three mainstream metrics – the Connectivity Index, Intersection Density, and Street Density – with respect to differences in study area and geometry. While these metrics are intended to be applied incrementally, this paper reveals that the metrics often fail to do this successfully. By controlling for many variables – including block size, block geometry, right-of-way size, network size, and network geometry – actual behaviors of these metrics deviate substantially from their intended behaviors. The metrics are non-linear functions of both study area and geometry and are ultimately inconsistent and unpredictable. In other words, each metric will yield inconsistent readings based upon the amount of area studied or the arrangement of the study boundary drawn. This has two major consequences: (1) the metrics will not produce the results desired as they are applied to incremental development; and (2) the metrics can be easily gamed by a developer privy to the information found within this paper. Neither of these outcomes is desirable in helping to better understand and potentially regulate street connectivity.

On-Street Parking

Abstract Purpose To overview the gamut of issues that on-street parking impacts in mixed-use centers including: parking demand, land use, vehicle speed, road safety, the pedestrian environment, and travel behaviors. Methodology/approach In addition to reviewing existing literature, the following two case studies are presented. The first study explores the impact in centers built before the advent of parking regulations as compared to more contemporary, conventional developments. The second study investigates how street design factors affected vehicle speeds and safety, based on a study of over 250 roads. Findings On-street parking typically: serves the highest demand; is efficient in terms of land use and cost; induces lower vehicle speeds; increases safety on low-speed streets; enhances walkability; and fosters less driving, more pedestrian activity, and increased vitality. Practical implications On-street parking is one piece of a larger puzzle of complementary factors that influence issues such as travel behavior and safety, and therefore, it is difficult to isolate. On-street parking plays a crucial role in helping create places that are walkable, require less parking, and have more vitality. 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. Originality/value of chapter Prevailing thought on the subject of on-street parking has shifted back-and-forth for generations, in part because most studies focus on one or two impacts. This chapter takes a more comprehensive approach in order to increase our understanding of on-street parking in mixed-use, commercial centers.

Large-scale tactical urbanism: the Denver bike share system

How did one of the most historically automobile-dependent US cities end up with the first large-scale modern US bike-sharing system? The story reveals that it is less about transportation demand planning and engineering and more about the principles of what has been termed tactical urbanism. The tactical urbanism movement revolves around the idea that temporary interventions can help one understand what interventions might work in a particular context and lay the foundation for more permanent ones. That the first large-scale bike-sharing system in the US began as such an intervention is quite remarkable and illustrative of the potential of this strategy to build public aspiration and political will via temporary, short-term empirical success. Beginning with preparations for the 2008 Democratic National Convention (DNC) in Denver, Colorado, this paper details what went into creating the temporary Freewheel!n bike-share system and how that was able to be leveraged into a permanent bike-sharing system, Denver B-Cycle, two years later. Consistent with the characteristics of tactical urbanism, the partnerships formed with the temporary DNC implementation served as a catalyst. The paper then identifies lessons that other cities can learn and apply in similar large-scale tactical urbanism interventions.