ITE Journal March 2018 - 43

Traffic Speed Impacts
We investigated the road diet's impact on speeding by looking
at two measures of "speeders," the number of vehicles traveling
5 mph (8 km/h) or more over the speed limit and the number
of vehicles traveling 10 mph (16 km/h) or more over the speed
limit. These metrics were chosen to look directly at the number of
"problem" drivers per se-the people traveling too fast for safety.

All-Day Impacts
Looking at all-day effects (Table 2), the road diet successfully
reduced the number of speeders along the road diet segment,
especially the number of 10+ mph speeders, which fell by 525
vehicles, a 60-percent drop.
In contrast to the road diet locations, the three other street
types all saw the number of speeders increase, with the increases
being fairly large-double digits-in percentage terms. The change
was most pronounced on the major streets. The number of 5+ mph
speeders went up 24 percent, an increase of 375 vehicles over the
day. For the 10+ mph speeders, the percentage increase was even
greater (43 percent), though the number of additional speeding
vehicles was lower (111 vehicles).

Impacts by Time of Day

Source: Nixon et al., 2017.3

For almost every street type, the percentage change in the number
of speeders was greater during the peak hours than for the all-day
counts, and the change was usually more extreme in the AM peak
than the PM peak (Table 2). The only exception to this pattern
was for the neighborhood streets, where the percent increase in
PM-peak-hour speeders was lower than the all-day increase.

Options for Presenting the Data
A key objective of this study was to identify methods to present
the study findings so that readers could easily grasp the meaning
and significance of data. In addition to the tables shown above, we
developed two different methods to present findings graphically
that emphasize the change from pre- to post-diet conditions.
Figure 2 presents the numbers of 10+ mph speeders as a set of
three line and bar graphs. One line graph shows the number of
speeders pre and post-diet, while the other line graph shows the
percentage of all vehicles that were speeding in both time periods. The
area between the yellow and blue lines represents the change from one
time period to the next. The bar graph makes it easier to understand
the change from one time period to the next, with increases in
speeders above the y-axis (zero line) and decreases below that line.
Although this paper does not discuss detailed findings about
impacts at individual counter locations, we include Figure 3 as an
example of a method to show in map form the change 10+ mph
speeders at each counter. This presentation permits readers to
easily identify spatial patterns in data.

Figure 2. Vehicles traveling 10+ mph above the speed limit, pre- and
post-road diet, by hour of the day, Lincoln Avenue Locations. Note: Y-axis
scales differ for each graph.

Recommendations for Evaluating Speed and
Volume Impacts from Road Diets
This section provides recommendations for how city staff and
researchers can design road diet evaluations to effectively assess
speed and volume impacts. These recommendations flow from
our analysis of City of San José data, as well as from our review
of other road diet evaluations. First, suggested approaches to
designing the data collection plan are discussed, followed by
suggestions for the data analysis approach.
w w w .i t e.or g

M arch 2018

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Table of Contents for the Digital Edition of ITE Journal March 2018

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