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For a variety of environmental, health, and social reasons, there is a pressing need to reduce the automobile dependence of American cities. Bicycles are well suited to help achieve this goal. However, perceptions of rider safety present a large hindrance toward increased bicycle adoption. These perceptions are largely influenced by the design of our current road infrastructure, including the crossing distances of large intersections. In this paper, we examine the role of intersection crossing distances in modifying rider behavior through the construction of a novel dataset integrating street widths and probable trip routes from Chicago’s Divvy bikeshare system. We compare real trips to synthetic trips that are not influenced by the width of intersections and exploit behavior differences that result from the semi-dockless nature of the bikeshare system. Our analysis reveals that bikeshare riders do avoid large intersections in limited circumstances; however, these preferences appear to be heavily outweighed by the relative spatial positions of origins and destinations (i.e., the urban morphology of Chicago). Our results suggest that specific infrastructural investments such as protected intersections could prove feasible alternatives to reduce the perception and safety concerns associated with large road barriers and enhance the attractiveness of non-motorized mobility.

 

In confined stratified basins, wind forcing is an important mechanism responsible for the onset and generation of internal waves and seiches. Previous observations have also found that gravity currents in stratified environments can also initiate internal waves. We conducted a series of laboratory experiments to investigate the generation of internal motions due to such dense gravity currents on an incline entering a two-layer stratification, focusing in particular on the interaction between the onset of internal motions and topography and diapycnal mixing due to breaking internal waves. The baroclinic response of the ambient stratification to the gravity current is found to be analogous to a system forced by a surface wind stress, and the response as characterized by a Wedderburn-like number was found to be linearly proportional to the initial gravity current Richardson number. The generated internal motions are characterized as having a low-frequency internal surge and higher-frequency progressive internal waves. The overall mixing efficiency of the breaking internal wave was calculated and found to be low compared with similar previous studies. 

Neighborhoods are the building blocks of cities, and thus significantly impact urban planning from infrastructure deployment to service provisioning. However, existing definitions of neighborhoods are often ill suited for planning in both scale and pattern of aggregation. Here, we propose a generalized, scalable approach using topological data analysis to identify barrier-enclosed neighborhoods on multiple scales with implications for understanding social mixing within cities and the design of urban infrastructure. Our method requires no prior domain knowledge and uses only readily available building parcel information. Results from three American cities (Houston, New York, San Francisco) indicate that our method identifies neighborhoods consistent with historical approaches. Additionally, we uncover a consistent scale in all three cities at which physical isolation drives neighborhood emergence. However, our methods also reveal differences between these cities: Houston, although more disconnected on larger spatial scales than New York and San Francisco, is less disconnected at smaller scales. 

A series of laboratory experiments were conducted to investigate the characteristics of a dense gravity current flowing down an inclined slope into a quiescent two-layer stratification. The presence of the pycnocline causes the gravity current to split and intrude into the ambient at two distinct levels of neutral buoyancy, as opposed to the classical description of gravity currents in stratified media as being either a pure underflow or interflow. The splitting behaviour is observed to be dependent on the Richardson number ( $Ri_{\unicode[STIX]{x1D70C}}$ ) of the gravity current, formulated as the ratio of the excess density and the ambient stratification. For low $Ri_{\unicode[STIX]{x1D70C}}$ , underflow is more dominant, while at higher $Ri_{\unicode[STIX]{x1D70C}}$ interflow is more dominant. As $Ri_{\unicode[STIX]{x1D70C}}$ increases, however, we find that the splitting behaviour eventually becomes independent of $Ri_{\unicode[STIX]{x1D70C}}$ . Additionally, we have also identified two different types of waves that form on the pycnocline in response to the intrusion of the gravity current. An underflow-dominated regime causes a pycnocline displacement where the speed of the wave crest is locked to the gravity current, whereas an interflow-dominated regime launches an internal wave that moves much faster than the gravity current head or interfacial intrusion.