5. PROGRAM EVALUATION (continued)

The descriptions and tables below present the speed results by road segment. Before turning to the results themselves, however, it is important to highlight several points with respect to the analyses.

There were five waves of speed data taken, but not all roads were included in each wave. Almost all Peoria sites had five measurements. In Phoenix , every site was included in Waves 1, 4, and 5. Waves 2 and 3 were done in Phoenix at only one location before and immediately after 3-D markings were installed. This permitted the assessment of the additive influence of these markings above any effect of the education and enforcement countermeasures previously deployed.

Wave 1 was in October 2002 before any countermeasures were implemented. The remaining waves were taken monthly in the succeeding months with Wave 5 in the February/March 2003 time period.

The measures used to examine speed were:

• Average speed on the road segment (“Mean Speed”). Using the mean was judged to be more illustrative for this research effort than the 85 th percentile speed that is commonly used in assessing traffic calming efforts because the present study was also utilizing several other measures of the highest speeds.
  
  
• Mean difference in speed from the Wave 1 (baseline) value. This measure indicates the change in mean speed from the value before the study. In addition to a statistically significant decline, changes were sought that would be meaningful for safety.
  
  
• Average speed above the speed limit (“Mean Above Limit”). This is the average of each vehicle's speed minus the prevailing speed limit. A reduction in this measure could indicate either a moderation of high speeds or both a change in high speeds and a lowering of the speeds of those driving below the limit. For example, in a 30 mph zone with four cars doing 25, 25, 40 and 50, this average would be 5 mph (-5, -5, 10, 20). If the speeds changed to 20, 25, 40, 45, the average would be 2.5 mph (-10, -5, 10, 15). This same value would result if the new speeds were 25, 25, 40, 40 (-5, -5, 10, 10).
  
  
• Average speed of those exceeding the speed limit (“Mean of Speeders”). This average excludes all vehicles going the speed limit or below. In the example above, the values would be 15 mph (10, 20), 12.5 mph (10, 15) and 10 mph (10, 10). This measure focuses in on the extent of excessive speed.
  
  
• The percentage of drivers going: the speed limit or less; over the limit but less than 7 mph over; 7 mph or more over the limit. This last category is the aberrant group that flouts the law and compromises safety. The cutoff of 7 mph was selected because both the expert panel and the involved Phoenix and Peoria police indicated that this was the threshold at which they became concerned and would typically issue tickets.

Basically, changes in the mean were used to assess whether the speed profile on a road segment had changed significantly while the other measures were included to assess the nature or profile of any speed change that did occur. This is consistent with the findings from the literature discussed earlier that indicate that the largest safety benefit would arise from eliminating the upper or fastest tail of the speed distribution.

Each of the 10 roadway studies is summarized in terms of these measures in the sections that follow.

The 84 th Avenue test segment is part of the Bell Park neighborhood. It is a typical residential street with a 25 mph speed limit. It received a moderate-to-high education effort consisting of many of the yard and street signs and some literature distributed locally by the neighborhood association. The police focused their special enforcement around the time of Wave 2, which was before any physical changes in the roadway. Prior to Wave 3, both speed tables and 3-D markings were installed. Speed was then measured in Waves 3, 4, and 5. The results are shown in Table 60⁹.

Table 60. Peoria 84 th Avenue Speed Results

The results on 84 th Avenue are quite dramatic. The initial drop in mean speed at Wave 3 (3.088 mph) is almost completely sustained two months later. Also, the drop is largely the result of elimination of the highest speeds. The percentage of those going 7+ mph over the limit drops from 8.8 percent to 2.5 percent , a 71.6-percent decrease. This is not unexpected from the installation of vertical treatments (the speed tables). Figure 8 displays the percentage of cars in the three speed categories (at or under the limit, less than 7 mph over the limit and 7 or more mph over the limit). It depicts the major drop in the faster vehicles in favor of those going at or below the speed limit.

Figure 8. Speed Classes on Peoria – 84 th Avenue

The number of cars counted in each wave declined somewhat on this test segment after the installation of the speed humps. This is not an unusual result when vertical treatments are installed. Some drivers simply do not want to contend with them and find other routes. The effect of this diversion, which reached 23.7 percent of the traffic between Waves 1 and 5 on this segment, represents a further reduction in risk for this specific roadway. If the diverted drivers speed on other roads, however, this safety benefit might be partially offset.

⁹Speed data are reported in this and subsequent tables to three decimal points. This has been done for convenience since this is the format of the output of the statistical routines employed. It is neither intended to imply that speeds were measured with this degree of precision nor that this level of precision was required to test statistical significance.

¹⁰ In this and all of the speed result tables positive mean differences indicate an increase in mean speed from baseline (wave 1) to the particular wave's value and negative results indicate a speed reduction.

¹¹ns= not significant; exact probabilities are shown in parenthesis for significant results with .000 indicating p < . 001

¹²Significance here and for mean of speeders is related to the Wave 1 value.