VEHICLE SPEEDS IN SCHOOL ZONES

Charlie Saibel, Philip Salzberg, Richard Doane, and John Moffat

WASHINGTON TRAFFIC SAFETY COMMISSION
1000 S. Cherry Street, PO Box 40944
Olympia Washington 98504

ABSTRACT

The present study was conducted to answer two questions: do drivers slow down
for children in school zones, and what factors affect vehicle speeds in school zones
during times of school zone activity. The posted speed limit in school zones is 20
MPH, and there are various types of signs in use at different schools that inform
drivers of the school zone speed limit. The study was designed to examine the
effect of different types of school zone speed limit signs and the effect of different
speed limits on the roads approaching the schools. Vehicle speed data were
obtained using pneumatic-tube speed measurement devices that were placed in
the road in front of 40 elementary schools. The speed data were analyzed for the
30-minute critical time periods before the start of the school day and after the close
of school.

The findings showed that, overall, a substantial percentage of vehicles were
traveling at high speeds through school zones during the critical time periods;
about 12% exceeded 35 MPH. Schools situated on roads with 25 MPH approach

 speed limits were associated with slower vehicle speeds; less than 5 percent
exceeded 35 MPH, and the type of school zone sign had no effect on speeds.
School zones with faster approach speed limits (30-40 MPH) were associated with
higher speeds. Up to 30 percent of the vehicles exceeded 35 MPH during the
critical time periods at the start and end of the school day. However, “flashing
light” school zone signs were effective in slowing vehicles; average speeds were
5-7 MPH slower, and less than 4% exceeded 35 MPH.

The conclusions and recommendations were:
(1) A significant proportion of vehicles travel at high speeds through school zones
during times when children are likely to be in proximity to the road. Enhanced
enforcement of traffic laws in school zones would seem to be clearly warranted
by the study findings.
(2) Excessive speeds occurred predominately at schools with faster approach speed
limits. The siting of new school construction on roads with higher speed limits
should be discouraged.
(3) School zone speed limit signs with flashing lights were effective in slowing
vehicles. Obviously, there are higher costs associated with installation and
maintenance of such signs. Nevertheless, replacement of existing signs with
flashing light signs should be considered for schools located on roads with
higher speed limits.

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 VEHICLE SPEEDS IN SCHOOL ZONES

Charlie Saibel, Philip Salzberg, Richard Doane, and John Moffat

INTRODUCTION
The School Zone Safety Act was passed by the 1996 Washington Legislature and
directed the Washington Traffic Safety Commission (WTSC) to develop and
implement programs to improve safety in school zones. In response to the School
Zone Safety Act the WTSC initiated a study to measure vehicle speeds in school
zones. The study was designed to answer two questions: do drivers slow down for
children in school zones, and what factors affect vehicle speeds in school zones
during times of school zone activity. The posted speed limit in school zones is 20
MPH.

The data from an initial sample of 17 schools showed that just before the start of
school almost half of the vehicles were measured at under 25 MPH. However, more
than 25 percent of the vehicles were exceeding 30 MPH, and almost 9 percent were
traveling at more than 15 MPH over the posted speed limit of 20 MPH. In the time
period coinciding with school dismissal, about 50 percent of the vehicle speeds
were under 25 MPH. Again, more than a quarter exceeded 30 MPH, and almost 10
percent were measured at 35 MPH or faster.

The schools were grouped in various categories and analyzed for differences in
vehicle speeds. City population, average daily traffic, and the percent of students

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 who rode school buses did not affect speed compliance. However, there appeared
to be marked differences in vehicle speeds at schools with various types of school
zone speed limit signs, especially when the posted speed limit approaching the
school was 35 MPH.

These preliminary findings suggested that a substantial number of vehicles were
travelling through school zones at excessive speeds and posed a risk to students
walking to and from school. The data also suggested that the type of speed limit
sign and approach speed limit may be important factors. To examine more
precisely the effects of these two variables, another 23 schools were added to the
initial sample of 17 schools. These additional schools were selected so that the
final sample would provide a cross-section of the different types of signs and
approach speed limits found at elementary schools in Washington State.

METHOD
Sample.
There were 40 schools in the study sample, and all were elementary (K-6) schools.
Most were located in smaller to mid-size cities in Washington. Larger cities in the
sample included Tacoma and Spokane, but there were no Seattle schools in the
sample. The schools comprise a convenience sample; i.e., the schools were in
cities that a Washington State Department of Transportation employee would travel
to during his normal work schedule. The schools were selected to provide a mix of
the various types of school zone signs and approach speed limits found in
Washington State.

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 There were four types of school zone speed limit signs among the 40 schools:
1. Signs indicating specific times of the day for the 20 MPH speed limit (“time of
day” signs), for example, “Speed limit 20 MPH, 7:30 AM to 4:30 PM”. Signs with
no indication of specific times, i.e., the 20 MPH limit is in effect for all hours of
the day, were also included in this category.
2. Signs with yellow lights on the sign post that indicated the 20 MPH limit was in
effect when the lights were flashing, (“flashing light” signs).
3. Signs indicating the 20 MPH limit was in effect when children are present (“when
present” signs).
4. Signs which indicated that the 20 MPH limit was in effect when orange flags
were attached to the sign post (“when flagged” signs).

These sign types were categorized into groups by the posted speed limit
approaching the schools: approach speed of 25 MPH and approach speed of 35
MPH (30 MPH and 40 MPH speed limits were counted with the 35 MPH group).

There would have been eight groups in the study, but no schools were found with
“flashing light” signs and 25 MPH approach speeds. Also, only two schools were
found with “time of day” signs and 35 MPH approach speeds; they were excluded
from the data analysis of signs and approach speed limits. The final sample
consisted of 38 schools categorized into six groups. These groups were:


25 MPH approach speed and:
a) “time of day” signs (N=8),
b) “when present” signs (N=8),
c) “when flagged” signs (N=5).

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 

35 MPH approach speed and:
a) “flashing light” signs (N=6),
b) “when present” signs (N=5),
c) “when flagged” signs (N=6).

Procedure.
Vehicle speed data were obtained using pneumatic-tube speed measurement
devices. Two tubes were placed in the road several feet apart and were attached to
an automatic recording device that calculated the speed of each vehicle passing
over the tubes. The devices also recorded the time of day that vehicles passed
over the tubes. Speeds were recorded in 5 MPH intervals, and time of day was
recorded in 15-minute intervals. These 5 MPH intervals were the maximum
precision of measurement of the speed recording device. Before activating the
device, the installer used a radar gun to check accuracy, and calibration was done,
if necessary. The installer was a Washington Department of Transportation (DOT)
employee with prior experience in collecting vehicle speed data.

The speed measuring devices were placed inside the 20 MPH speed zones in front
of each school, and the tubes spanned one lane in one direction of travel. Data
collection took place during a one to five day period while school was in session.
The installer also recorded information about the speed limit approaching the
school zone and the type of school zone sign. After collection of the speed data
was completed, the schools were contacted to determine their time schedules for
the beginning and end of school on the specific days that the speed data were

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 collected. Early dismissal and late start days as well as school closure days were
noted and accounted for in the data analysis.

Data Analysis.
The data for each school were summed across all days that the speed
measurement device was in operation and entered onto a spreadsheet. Rows
displayed a 24 hour day in 15 minute intervals and columns displayed vehicle
speeds (in intervals ranging from 0-5 MPH to 56-60 MPH and 61+ MPH). Each cell
in the spreadsheet showed the number of vehicles counted in the particular speed
and time interval.

Time of day on the spreadsheets was matched with the different start and end
times for each school. Two “zero points” were set for each school to coincide with
the start and the end of the school day. The zero point in the morning was the 15
minute interval beginning at or just after the time school started. The zero point in
the afternoon was the 15 minute interval ending at or just before the time school
was dismissed. These zero points provided common time reference points for
schools with different time schedules.

Data for two critical time periods were extracted from the spreadsheets for each
school. These time periods were the ½ hour before the school-start zero point and
the ½ hour following the school-dismissal zero point. The zero points and critical
time periods were adjusted to account for days with late starts or early dismissal,
and days coinciding with school closures were excluded from the analysis.

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 Two dependent measures were used in the data analysis: mean (average)
vehicle speed and the percent of vehicles exceeding certain speeds. Because the
precision of measurement of the speed recording device was only 5 MPH
intervals, the mean was calculated using the formula for grouped frequency
distribution data. The midpoint of each speed interval (e.g., 22.5 for the 20-25
MPH interval) was multiplied by the number of vehicles in the interval, the
products summed and then divided by the total number of vehicles.
RESULTS
Vehicle speed data for all schools combined.
Figures 1 and 2 show overall average speeds for all 40 of the schools in the study.
Average speed slowed from over 35 MPH in the morning hours (Figure 1) to about
25 MPH just before the start of school. Speeds returned to about 30 MPH for the
rest of the morning. Figure 2 displays average speeds for the afternoon hours.
Average speeds of about 30 MPH dropped to under 25 MPH during the time when
students were leaving school, and then returned to between 30 and 35 MPH about
an hour later.

The critical time periods of 30 minutes before the start of school in the morning and
30 minutes after school dismissal in the afternoon were analyzed in detail. During
these times children are actively using school crosswalks, and the speed limit in
school zones is 20 MPH.

Figure 3 shows the percentage of vehicles travelling at various speeds during the
30-minute critical morning period. (Given that the precision of speed measurement
for the present data was 5 MPH intervals, we defined compliance with the school

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 zone speed limit as 25 MPH or less.) Most vehicles were measured at 25 MPH or
under. However, a substantial percentage of vehicles (45 percent) exceeded 25
MPH, 12 percent of the vehicles exceeded 35 MPH, and 4 percent were travelling at
41 + MPH.

Figure 4 shows the percentage of vehicles travelling at various speeds during the
afternoon critical time period. The distribution of vehicle speed was nearly identical
to that found in the morning speed data; 47 percent exceeded 25 MPH, 13 percent
of the vehicles exceeded 35 MPH, and 5 percent exceeded 40 MPH.
Effects of school zone speed limit signs and approach speed limits.
To compare the effectiveness of different types of school zone speed limit signs, the
schools (N=38) were grouped by type of sign and posted speed limit approaching
the school zone. The data for both the morning and afternoon 30-minute critical
time periods were combined, and the dependent variables examined were average
speed and the percentage of vehicles exceeding 35 MPH. These data were
analyzed using one-way ANOVAs with post-hoc tests of between group differences
(Student-Newman-Keuls test).

There was an overall significant difference between the six groups of schools in
average speed, F(5,32)=5.73. When the posted speed limit approaching the
schools was 25 MPH, vehicles were in substantial compliance with the 20 MPH
school zone limit, regardless of the type of sign. As shown in Figure 5, average
speeds ranged from 21 to 24 MPH for the “time of day”, “when children are
present”, and “when flagged” signs. None of these differences were statistically
significant. However, when the approach speed limit was 35 MPH, vehicles were

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 measured at significantly higher average speeds with “when children are present”
signs (30.3 MPH) and “when flagged” signs (28.3 MPH). In contrast, “flashing light”
signs were associated with significantly slower average speeds of 22.5 MPH.
Schools with “flashing light” signs had vehicle speeds that were not significantly
different than schools with 25 MPH approach speed limits.

Data for the percent of vehicles exceeding 35 MPH are summarized in Figure 6 and
show a pattern that is similar to the average speed data. The overall between
group difference was significant, F(5,32)=7.50. Few vehicles (less than 5 percent)
were measured at high speeds when the approach limit was 25 MPH, and there was
no significant difference between the “time of day”, “when present”, and “when
flagged” signs. However, a substantial and significantly greater percentage of
vehicles were travelling at high speeds in school zones with 35 MPH approach limits
and “when children are present” signs (30 percent) and “when flagged” signs (23
percent). In spite of higher approach speed limits, schools with “flashing light”
signs had significantly fewer vehicles travelling in excess of 35 MPH (3 percent).
This percentage was not significantly different than the schools with 25 MPH
approach speed limits.

DISCUSSION

Even though about half of the vehicles in the study were in compliance with
the 20 MPH speed limit in school zones, a major finding of this study was
that many vehicles were measured at speeds greater than 35 MPH. About
12-13 percent exceeded the school zone speed limit by 15 MPH or more,
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 and 4-5 percent were travelling at 40+ MPH. Vehicles travelling at these
speeds pose a substantial risk of injury to children who are in proximity to
traffic while going to or leaving school.

The above percentages represent an aggregate of all schools in the study.
When the data were analyzed by the type of school zone sign and speed
limit approaching the school, a different pattern of results emerged. Schools
situated on roads with slower approach speed limits were associated with
much slower vehicle speeds; less than 5 percent exceeded 35 MPH. In
addition, the type of school zone sign had no effect on vehicle speeds when
the approach speed limit was 25 MPH. These schools were located
predominately in residential neighborhoods and on non-arterial streets.

School zones with faster approach speed limits were associated with
substantially higher speeds during the ½ hour critical time periods just
before and just after school hours. However, “flashing light” school zone
signs were effective in slowing vehicles. The average speed at schools
with “flashing light” signs was about 5-7 MPH slower than at schools with
“when children are present” and “when flagged” signs. A 5 MPH decrease
in vehicle impact speed can mean the difference between a
minor/moderate injury vs. a major/fatal injury to a pedestrian. Using data
gathered by the Interdisciplinary Working Group for Accident Mechanics
(1986), Anderson et al (1997) estimated that the probability of a
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 pedestrian fatality rose from about 10 percent to about 60 percent, a sixfold increase, when vehicle impact speed increased from 23 mph to 28
mph.

Limitations of the study.

Whether the findings can be generalized to all schools in Washington and in
other States is uncertain because the schools included in the study were a
convenience sample and may not be completely representative of the entire
population of schools. An important omission is that no Seattle schools were
included. However, the sample can be considered a rough cross-section of
schools in small, medium, and larger cities. It is unlikely that the sample
was systematically biased in that the selection of schools was quasi-random
within the constraints of the travel itinerary of the DOT employee who
collected the data.

It is possible that the findings of this study could be confounded with other
factors that were not identified during data collection. Potential confounding
variables include changes in pavement textures in the approaches to the school
zones, other warning markers such as road bumps or rumble strips, or the
presence of law enforcement in the school zones.

The speed data reported in this study are from the 30 minute time periods
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 just before the start and after the end of school. We assume that students
were in proximity to the road and the school zone during these times, but we
could not independently verify this. The 30 minute periods were selected
based on discussions with school administrative personnel to identify school
start and end times.

Conclusions and Recommendations.


A significant proportion of vehicles were measured at excessive speeds through
school zones during times when children are likely to be in proximity to the road.
Enhanced enforcement of traffic laws in school zones would seem to be clearly
warranted by the findings of this study.



Excessive speeds occurred predominately at schools with faster approach speed
limits, while slower approach speed limits were associated with better speed
compliance in school zones. The siting of new school construction on roads with
higher speed limits should be discouraged.



School zone speed limit signs with flashing lights were effective in slowing
vehicles. Obviously, there are higher costs associated with installation and
maintenance of such signs. Nevertheless, replacement of existing signs with
flashing light signs should be considered for schools located on roads with
higher approach speed limits.

REFERENCES

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 Anderson, R.W.G., McLean, A.J., Farmer, M.J.B., Lee, B.H., & Brooks, C.G. (1997).
Vehicle travel speeds and the incidence of fatal pedestrian crashes. Accident
Analysis and Prevention 29 (5), 667-674.

Interdisciplinary Working Group for Accident Mechanics (University of Zurich and
Swiss Federal Institute of Technology ETH). (1986). The car-pedestrian collision:
injury reduction, accident reconstruction, mathematical and experimental
stimulation: head injuries in two wheeler collisions. Zurich: The Group.

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