Effect of Residential Street Speed Limit Reduction on Driving Speeds in Portland, Oregon

Findings This study analyzed the impacts on motor vehicle observed speeds following a residential speed limit reduction from 40.23 km/hr (25 mi/hr) to 32.19 km/hr (20 mi/hr) in Portland, OR that was accompanied by a public awareness and signage campaign. The study used before and after observations of vehicle speeds collected by pneumatic tube traffic counters. Overall, the analysis suggests that the reduction of posted speed limits to 32.19 km/h (20 mi/h) has resulted in lower observed vehicle speeds and fewer vehicles traveling at higher speeds. The reduction in the percentage of vehicles traveling above 48.28 km/h (30 mi/h) (-1.7%) and 56.33 km/h (35 mi/h) (-0.5%) are larger in magnitude than other speed metrics.

1. Was there a statistically significant change in mean speed, 85th percentile speed, and proportions of vehicles traveling greater than 40.23 km/hr (25 mi/hr), 48.28 km/hr (30 mi/hr), and 56.33 km/ hr (35 mi/hr) after the residential speed limit reduction was implemented and the accompanying public awareness and signage campaign?
2. What is the estimated effect of the speed limit change on observed vehicle speeds?
and after data were collected during weekdays and a few weekends, with the duration varying between 24-97 hours at each location.Recorded speeds of 0 km/hr or greater than 160.93 km/hr (100 mi/hr) were removed from the data prior to analysis for quality control.The excluded data, as a percentage of total observations, was consistent across the two periods (approximately 8%).After cleaning, 131,452 before and 82,768 after observations were available for analysis.All analyses were conducted using the disaggregate, or raw, speed data.
Available controlling factors included time-of-day, day of the week, vehicle classification, and data extracted from Portland's GIS database on physical and operational aspects of the roadway (curb-to-curb pavement width, number of lanes, presence of sidewalks, curb height, presence of parking signs, and pavement condition).A summary of available controlling factors is given in Table 1.

Findings
Descriptive statistics for the pooled data (all sites) and each site were computed to assess changes in common speed measures.Figure 2 shows mean speed changes using a bar plot, sorted by the magnitude of change.Changes in mean speed range from a decrease of 5.63 km/hr (3.5 mi/hr) to an increase of 3.86 km/hr (2.4 mi/hr).The average decrease was 2.25 km/hr (1.4 mi/hr) and the average increase 1.13 km/hr (0.7 mi/hr).Figure 3 • 40.23 km/hr (25 mi/hr) decreased by 0.5% • 48.28 km/hr (30 mi/hr) decreased by 1.7% shows the change in percentage of vehicles traveling faster than 48.28 km/hr (30 mi/hr).These changes range from a decrease of 29.5% to an increase of 4.4%.The average decrease was 3.8% and the average increase 0.8%.
A series of statistical models were developed to determine the effects of the speed limit reduction while controlling for other available factors.An indicator variable (1 if after reduction, 0 if before) was created to estimate the effects of the reduction.2. The estimate for the after-reduction indicator indicates an expected decrease in observed speed of approximately 1.0%, on average.The parameter is significant with well over 99% confidence.This expected decrease is about 3-times greater than the 0.30% observed by Hu and Cicchino (2020) in a similar study.
Model results align with expectations regarding vehicle speeds.Street width is associated with higher speed (Fitzpatrick et al. 2001).Curb height is likely associated with on-street parking and contributes to narrow available travel way (decrease in speed).Pavement quality and ride are likely to be related to vehicle speed, but the literature is sparse.Previous studies have also found that speeds were less on weekends (Bornioli et al. 2018;Giles 2004).
Binary logit model specifications for the three speed thresholds are shown in Table 3. Parameter estimates, in absolute value, increase as the speed thresholds increase.For the after reduction indicator, the change in odds increases in magnitude as the speed threshold increases.The parameter estimate for after the speed limit reduction suggests a 15.9% reduction in odds of observing speeds greater than 40.23 km/hr (25 mi/hr), a 33.6% reduction in odds of observing speeds greater than 48.28 km/hr (30 mi/hr), and a 49.6% reduction in odds of observing speeds greater than 56.33 km/hr (35 mi/hr).These results confirm the inference from the descriptive analysis; specifically, the percentage of vehicles traveling in the higher speed bins decreased after the reduction.

Table 1 .
Summary of Potential Controlling Explanatory Variables • 56.33 km/hr (35 mi/hr) decreased by 0.5% Effect of Residential Street Speed Limit Reduction on Driving Speeds in Portland, Oregon Findings a OILM defined as Oil-Macadam Pavement Streets

Table 2 .
Log-Linear Regression Model Specifications for Observed Speed Effect of Residential Street Speed Limit Reduction on Driving Speeds in Portland, Oregon Findings

Table 3 .
Binary Logit Model Specifications of Speeds Greater Than 40.23 km/hr, 48.28 km/hr, and 56.33 km/hr 40.23 km/hr = 25 mi/hr; 48.28 km/hr = 30 mi/hr; 56.33 km/hr = 35 mi/hr Effect of Residential Street Speed Limit Reduction on Driving Speeds in Portland, Oregon a estimated odds ratio b pavement condition index *