Degree Type


Date of Award


Degree Name

Master of Science


Civil, Construction, and Environmental Engineering


Civil Engineering

First Advisor

Peter T. Savolainen


Limited access facilities, such as freeways and expressways, are generally designed to the highest standards among public roads. Consequently, these facilities demonstrate crash, injury, and fatality rates that are significantly lower than other road facility types. However, these rates are generally elevated in the immediate vicinity of interchanges due to increases in traffic conflicts precipitated by weaving, merging, and diverging traffic. Given the extensive costs involved in interchange construction, it is important to discern the expected operational and safety impacts of various design alternatives. To this end, the objective of this study was to analyze the safety performance within the functional areas of interchanges. The study involves the integration of traffic crash, volume, and roadway geometric information using data from 2010 to 2014 from the state of Iowa in order to assess the relationships between these factors and frequency of crashes within the interchange functional area. Separate analyses were conducted for the freeway mainline and ramp connections. Safety performance functions (SPFs) were estimated for the interchange mainline and ramps using negative binomial regression models, and random effects models were estimated to account correlation in crash counts at the same location over time. The results from this study suggest that speed limit and interchange configuration have a significant impact on crash rates. Lower ramp advisory speeds (10 mph to 35 mph) were associated with fewer crashes on-ramps. Off-ramps were also associated with elevated crash risk in comparison to on-ramps and freeway-to-freeway ramps. Comparison SPF models were also developed using Iowa-specific data to relate the outcomes of these simple SPFs with Florida-specific SPFs and the national default SafetyAnalyst SPFs with varying results.


Copyright Owner

Akinfolarin O. Abatan



File Format


File Size

82 pages