Development of an Operating Speed Prediction Model and Investigation of the Relationship between Speed Variability and Crash Frequency: A Case Study of Residential Urban Streets in the City of Saskatoon
Date
2021-08-16
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-2104-9183
Type
Thesis
Degree Level
Masters
Abstract
This research focuses on two topics of major interest in highway engineering: predicting operating
speeds and investigating the relationship between speed and safety for urban roads. Speed is a
foundational parameter in highway safety, highway design and operational analysis. However,
appropriate speed levels to meet satisfactory operational targets might negatively affect safety. In
fact, higher speed levels can reduce travel time and its costs, but high speeds could potentially lead
to higher collision frequency. The study was based on speed and collision data collected for 140
residential streets as part of neighbourhood traffic reviews (NTRs) by the City of Saskatoon
between 2016 and 2018.
In the first part of the thesis, covering operating speed prediction models, i.e., models used by
highway engineers to design and build roads that are expected to generate desired (operating)
speed, are developed. In urban residential areas, operating speeds often exceed intended design
speeds, as well as posted speeds, creating speeding concerns among residents. One of the reasons
for this is a lack of performance-based design procedures that incorporate expected operating
speeds during the planning and design stages of urban roadways. Therefore, this study seeks to
understand the relationship between operating speeds and various road characteristics, though
operating speed prediction models, with the goal of promoting performance-based roadway design
procedures. Since 85th percentile speed is commonly used to model operating speeds, 85th percentile
free-flow speed prediction models were developed. Multiple linear regression models
were developed using fixed and then mixed effects to account for unobserved heterogeneity
(unmeasured differences) among neighbourhoods. The results show that segment length,
pedestrian crossing density, bus stop density, tree density, traveled-way width, on-street parking,
centerline (marking) presence and school zone are highly associated with operating speeds and
these effects are subsequently discussed. Moreover, the overall goodness of fit of the mixed-effects
model showed the importance of accounting for unmeasured heterogeneity at the neighbourhood
level.
In the second part of the thesis, the relationship between speed on residential urban streets (in
particular, speed variability among vehicles) and road safety in the form of predicted crash
frequency is investigated. More research is, in fact, needed to demonstrate whether vehicles
travelling slower or faster than mean traffic speed are more often involved in collisions than
vehicles travelling close to the mean speed. The coefficient of variation (CV) of speed was
computed for this research as a mediator variable, a variable which is predicted by roadway and
traffic characteristics to predict crash frequency. This latter modelling was conducted using
regression models within a path analysis framework, a methodology that can describe both direct
and indirect dependencies among a set of variables. Within path analysis, a relationship between
an independent and a dependent variable can be direct or mediated by a third factor. The results
showed that CV was positively related to crash frequency (sites with higher speed variation
showed lower safety levels) and the relationship was found to be statistically significant.
Moreover, the interrelationship among roadway and traffic factors and crash frequency was also
analyzed, providing a better understanding of the indirect effect of independent predictors of CV
on crash frequency. Overall, the results could be particularly important in the context of using
speed-related variables as surrogate measures of safety, which would allow assessment of safety
levels for urban residential streets by measuring speeds without waiting for collisions to occur.
Description
Keywords
Operating speed prediction, Speed variability and safety, Surrogate Safety measures, Path modelling
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering