Frequent pedestrian deaths in Arizona have raised concerns about improving road safety. Many proponents of safer streets call for lowering traffic speeds. Collisions between motorized vehicles and pedestrians or cyclists at lower speeds are known to cause less severe injuries and fewer fatalities than collisions at higher speeds.

However, empirical evidence that would demonstrate that the lowering of speeds on specific urban roads results in improved pedestrian safety is either missing or inconclusive. Most research on the impact of speed limits on pedestrian safety is either based in rural settings or is cross-jurisdictional rather than before and after studies of the same urban road segments.

Equally relevant is the consideration that lowering speed limits alone is unlikely to alter driver behavior if it is not accompanied by introducing traffic calming devices or reducing lane widths.

The design speed of the road segment and not just the posted speed limit determines driver behavior. For instance, Haris County in Texas ended up reinstating higher speed limit on freeways after the County realized that lowered speed limits introduced in mid-2000 did not reduce operating speeds.

Whereas traffic safety, when compared to the total number of fatalities, has improved over the years, the same is not necessarily true for pedestrians. In the U.S., traffic deaths have declined by 14 per cent from 2007 to 2016. However, during the same time, pedestrian deaths increased by 27 per cent, reaching 6,000 in 2017.

The causes behind the increase in pedestrian fatalities in the U.S. are not known for certain. A recent report by the Governors Highway Safety Association though speculates that legalization of cannabis and increased prevalence of cellphones are correlated with the increase in pedestrian fatalities. Also, the pedestrian death toll is larger in Sunbelt states, including Arizona, California, Florida, and Texas.

Prof. Bhagwant Persaud of Ryerson University is an expert in safety engineering. He points out that when traffic calming devices are placed on some road segments, traffic might migrate to other non-calmed streets, thus shifting the problem from one part of the road network to another.

In the fall 2009, Montreal lowered speed limits from 50 km/h to 40 km/h on local streets. A retrospective analysis of the intervention showed that while occurrences of speeding (50 km/h) were reduced, the intervention was not effective with excessive speeding of 80 km/h or higher.

While traffic calming interventions are generally effective, their placement and type require due consideration. Prof. Dominique Lord, an expert in traffic safety at the Texas A&M University, cautions that traffic calming devices, such as curb extensions and road humps, may not be placed on arterial or collectors, but instead on local urban roads. He further suggests that 10-foot wide lanes are more effective in lowering speed limits than the 12-foot wide lanes.