1. The document analyzes trends in hourly traffic crashes between 2005-2010 in the Chicago region, finding that fatal and serious crashes decreased 31-46% while regional vehicle miles traveled (VMT) only fell 3.1%. 2. Late night/early morning hours experienced the largest VMT decreases, up to 18%, and have the highest fatal crash rates per VMT. However, late night freeway fatality rates remained similar over the study period. 3. Serious crashes on non-freeways were more common during morning and evening peaks, but weekends after midnight had the highest serious crash rate per VMT and number of hourly fatalities. The late night non-freeway fatality rate was up

1. Hour-of-the-Week Crash Trends between the Years 2005-2010 for the
Chicago, Illinois Region
Parry Frank
Transportation Systems Analyst
Chicago Metropolitan Agency for Planning (CMAP)
233 South Wacker Drive, Suite 800, Chicago, IL 60606
Tel: 312-386-8762
Email: pfrank@cmap.illinois.gov
Word Count is 4,461
Submitted to 2012 ITE Midwestern District Conference and
TRB 4th Urban Street Symposium
June 24-27, 2012
Chicago, Illinois.

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ABSTRACT
Serious and fatal crash rates per VMT, from 2005 to 2010, varied by the hour of the week for the
Chicago, Illinois region. A review of the trends in this variation over six years reveals the effects
of the safety programs and can inform the deployment of safety enhancement resources. Over
this period, the fatal and serious traffic crashes declined between 31% and 46% on the freeways
and other roads while the regional VMT only fell by 3.1%. The largest proportional decrease in
VMT occurred in the late night/early morning hours (up to 18%) which have the highest fatal
crash rate per VMT. These times have the lowest hourly VMT rates. The hourly serious and
fatal crash rates per VMT have fallen for most times of the day, but on freeways, the late-night,
weekend hours have a similar fatality rate per VMT for both periods in the study. Serious
crashes on the non-freeway roads are more likely to occur during the morning and evening peak
periods, but the highest serious crash rate per VMT occurs in the post-midnight hours of the
weekend. The highest number of hourly fatalities, as well as the highest rate per VMT, takes
place during the post-midnight hours of the weekend. For 2009/2010, at the extremes, the highest
late-night hourly fatality rate per VMT on the non-freeways is 68 times greater than the lowest
hourly fatality rate. The ratio of late-night fatal crashes to serious crashes is more than twice the
daytime ratio.

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INTRODUCTION
There has been a 36% reduction in the annual number of fatal traffic crashes and traffic crashes
with serious injuries that occur in the Chicago Illinois region between the years 2005 and 2010.
The change in crashes per hour varies greatly throughout the week. The drastic reduction has
taken place on both the freeways and the other roads in the region (non-freeways). This analysis
examines how the annual serious and fatal traffic crashes in the Chicago region are distributed
over the 168 hours in a week and the relationship between crash frequencies and vehicle miles of
travel (VMT). The Chicago region used for this analysis includes Cook County, DuPage
County, Kane County, Kendall County, Lake County, McHenry County and Will County. The
City of Chicago is within Cook County.
The Illinois Department of Transportation (IDOT), Illinois State Police, the City of
Chicago and numerous municipalities, have prioritized enhancing safety on the roads in Illinois.
They have used a combination of education, enforcement and enhancement of physical structures
and they been very successful. The safety of the automobile fleet has also increased across the
analysis period. Emergency response may have become more timely and effective.
This analysis will show how VMT and serious or fatal traffic crashes are distributed for
each hour of the week and how these values have changed between 2005 and 2010. The crash
rates per vehicle mile of travel will be analyzed to determine which time periods in the week
have the highest crash rates and how these may have changed. This analysis places additional
emphasis on the late-night crash rates and travel patterns because these have the highest fatality
rates per VMT.
DATA
The majority of the analysis compares 2005 data to 2010 data. The records analyzed in this
report include crashes that are categorized as serious crashes, which involve an incapacitating
injury, and crashes that resulted in a fatality. For the sections that examine fatal crashes,
multiple years were aggregated to make a larger dataset. The fatal crash analysis combines fatal
crashes from 2005 and 2006, and compares them to the combined fatal crashes from 2009 and
2010.
The crash data for this analysis was compiled by the IDOT Division of Traffic Safety
from crash reports that originated with local and State police departments. The analysis is based
on individual geocoded crash records.
The annual VMT data was collected from IDOT publications. The hourly factoring of
the annual VMT was prepared by the author based on station location data supplied by IDOT and
also Traffic.com.
The analysis uses an estimate of hourly traffic volumes that was derived from a sample of
traffic counts from various locations in the region. On the freeway system, there were 464
traffic sensor locations that had sufficient data available for both 2005 and 2010, and these were
used to estimate the hourly traffic volumes. For the analysis of the arterials, collectors and local
roads (the non-freeways), there were 36 permanent sensor locations with hourly data for 2005
and 2010.
The analysis is split between the freeways and non-freeways. The arterials, collectors
and local roads which make up the non-freeways could not be analyzed individually due to the
limitations in the available crash and traffic volume data.
All of the analysis in this report is the sole responsibility of the author.

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ANALYSIS
The following table (TABLE 1) includes data for 2005 through 2010 and shows the combined
total of fatal or serious crashes, or just the fatal crashes that occurred each year. The table also
shows the crash rate per 100,000,000 VMT. Nearly seven percent of serious and fatal crashes
involve a fatality and so the serious crashes are the dominant factor. Over the five years of this
analysis, on all of the roads combined, there was a 36 % reduction in the number of fatal crashes
and also in the combined number of fatal or serious crashes.
The percentage of decrease in the serious or fatal crash rates per 100,000,000 VMT, as
shown in TABLE 1, was slightly less than the percentage of reduction in fatal or serious crashes
because of the decline in regional VMT. The rates of serious and fatal crashes per VMT are
shown for the freeways and non-freeways in TABLE 2. The crash rate decreased for both the
freeway system and the non-freeway roads in the Chicago region between 2005 and 2010. The
rates of change were different on the two categories of roads in this analysis.
The non-freeways experienced a remarkable 34% reduction in the total number of fatal
crashes over the analysis period, but the rate of decrease was even greater on the freeways, where
the reduction in fatal crashes was 46%.
In the analysis of all serious and fatal crashes, the combined total of crashes was reduced
36% on the non-freeway roads, which is similar to the reduction in the fatal crash rate for those
roads. For these roads, the fatal crashes represented 6.2% of all the fatal and serious crashes in
2005 and for 2010 they increased slightly to 6.4% of the crashes.
On the freeways, there was a reduction of 31.6% in the combined total of fatal and
serious crashes. This is similar to the reduction on the non-freeway roads, but is much lower than
the 46% reduction in fatal crashes on the freeways. For the freeways, the fatal crashes
represented 10.8% of all the fatal and serious crashes in 2005. Fatal crashes were reduced to
8.5% of the crashes in 2010. Of the serious or fatal crashes on either road category, the fatal
crashes on the freeways were reduced the greatest relative amount (by percentage).
On average for all the years 2005 through 2010, about 15.4% of the fatal crashes took
place on the freeways, but only about 10.0% of the serious and fatal crashes took place on the
freeways. The characteristics of travel on the freeways, make crashes, while less frequent, more
likely to involve a fatality when they do occur.
TABLE 1 Total Serious and Fatal Crashes, Crash Rate per 100 Million VMT, and Crash Rates per Mile of Roadway for
the Chicago Region (2005-2010)
Year
Total VMT
(1,000,000)
Fatal and
Serious
Crashes
VMT Rate
for Fatal
and
Serious
Crashes
Fatal
Crashes
Fatal
Crash
Rate per
100
Million
VMT
Freeway
Miles
Total
Road
Milage
Fatal and
Serious
Crash
Rate per
Mile of
Freeway
Fatal and
Serious
Crash
Rate per
Mile of
Roadway
for Non-
Freeway
Fatal
Crash
Rate per
Mile of
Freeway
Fatal
Crash
Rate per
Mile of
Roadway
for Non-
Freeway
2005 60,561 8,643 14.27 579 0.96 451 24,439 2.06 0.32 0.222 0.020
2006 59,207 8,631 14.58 552 0.93 451 24,519 1.53 0.33 0.171 0.020
2007 59,313 7,436 12.54 484 0.82 465 24,565 1.46 0.28 0.140 0.017
2008 58,832 6,058 10.30 410 0.70 462 24,625 1.40 0.22 0.130 0.014
2009 58,927 5,777 9.80 365 0.62 462 24,760 1.29 0.21 0.145 0.012
2010 58,758 5,540 9.43 370 0.63 462 24,790 1.37 0.20 0.117 0.013

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TABLE 2 also shows the general trend for the VMT on the freeways and non-freeways.
The freeway VMT has increased by 1.3% while the freeway system has expanded by 2.9%. The
non-freeways have a 4.9% reduction in travel and the system has had an increase of 1.4% in
length. These are much smaller reductions than the relative changes in serious and fatal crashes.
In total, there have been 30.6% to 46.7% reductions in the serious and fatal crash rates per VMT.
Since the decrease in serious and fatal crashes greatly exceeds the decline in VMT, the fall in the
number of fatal and serious crashes is not due to a simple reduction in total vehicle miles of
travel.
Serious crashes and fatal crashes per mile of road on the freeways and non-freeways are
shown in Table 1. The freeways only account for 1.9% of the center-line mileage in the region,
but each center-line has many lanes. Depending on the year, the rate of fatal or serious crashes
per center-line mile of freeways is between six and seven times as high as the rate on non-
freeways. The fatal crash rate per center-line mile varies between nine and thirteen times the rate
for non-freeway roads.
Based on this general relationship, physical safety improvements and additional
enforcement on the freeway system would seem to be more effective due to the concentration of
serious and fatal crashes per center-line mile of facility, compared to the rate on the non-
freeways. The non-freeway roads may be more suited to system wide programs. For instances
where the arterials, collectors or local roads have known safety issues at specific locations,
physical safety improvements and additional enforcement could also be effective strategies for
improving safety.
The two preceding tables show that the total number of serious and fatal crashes has been
declining significantly from 2005 through 2010. This is true for both the freeways in the Chicago
region and also on the non-freeways. The rates for fatal crashes are declining more quickly on
the freeway compared to the non-freeways. It is also apparent that the decline in crashes is not
due to a simple reduction in total vehicle miles of travel.
TABLE 2 Vehicle Miles of Travel and Serious or Fatal Crash Rates per 100 Million VMT for the Chicago Region (2005-
2010)
Year
Total VMT
(1,000,000)
Freeway
VMT
(1,000,000)
Non-
Freeway
VMT
(1,000,000)
Fatal and
Serious
Crashes
on
Freeways
Freeway
Fatal and
Serious
Crash
Rate per
100
Million
VMT
Fatal and
Serious
Crashes
on Non-
Freeways
Non-
Freeway
Fatal and
Serious
Crash
Rate per
100
Million
VMT
Fatal
Crashes
on
Freeways
Freeway
Fatal
Crash
Rate per
100
Million
VMT
Fatal
Crashes
on Non-
Freeway
Non-
Freeway
Fatal
Crash
Rate per
100
Million
VMT
2005 60,561 18,886 41,675 927 4.91 7,716 18.51 100 0.53 479 1.15
2006 59,207 18,198 41,009 691 3.80 7,940 19.36 77 0.42 475 1.16
2007 59,313 18,275 41,038 680 3.72 6,756 16.46 65 0.36 419 1.02
2008 58,832 18,590 40,242 647 3.48 5,411 13.45 60 0.32 350 0.87
2009 58,927 18,899 40,028 595 3.15 5,182 12.95 67 0.35 298 0.74
2010 58,758 19,135 39,623 634 3.31 4,906 12.38 54 0.28 316 0.80
Change 250 -2,052 -293 -1.60 -2,810 -6.13 -46 -0.25 -163 -0.35
1.3% -4.9% -31.6% -32.5% -36.4% -33.1% -46.0% -46.7% -34.0% -30.6%

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Distribution of VMT by Time of Day
There are direct relationships between the VMT in a region and the number of crashes. In
general, additional VMT results in more crashes. The crash rate per VMT varies by the time
period. While the VMT may have only changed by 3.1% in this analysis period, the travel in
individual hours may have increased or decreased by much more than 3.1%. Large changes in
VMT for different hours of the week could be associated with a large change in the number of
serious or fatal crashes for each hour.
The following graph (FIGURE 1) displays the estimated VMT for each hour in a week
(the smooth lines used in this analysis are a generalization of the 168 individual data points).
The distribution is shown for the years 2005 and 2010 and is divided into the travel on the
freeway system and non-freeway system (collectors, arterials and local roads). The hourly VMT
profiles were based on 36 permanent sensors on the non-freeways and 474 sensors on the
freeway system that were well functioning during 2005 and 2010.
The patterns are similar for each year in the analysis. During the weekdays, on both the
freeways and non-freeways, there are two peak periods, with the evening peak being of longer
duration than the morning peak. The Saturday and Sunday travel patterns are similar to each
other, each having a single peak just after midday. The Saturday midday peak on the non-
freeways has more VMT than is experienced during the weekdays at midday. The hourly non-
freeway VMT is usually much larger than the freeway hourly VMT, but during the early
morning hours, the volume on both systems is about equal.
The following graph (FIGURE 2) shows the relative change in hourly VMT from 2005 to
2010 for each hour of the week on each type of road in this analysis. This graph shows that
change in VMT fit a repetitive pattern.
In general, there was almost always more VMT during each hour of the week on the non-
freeways for 2005 compared to 2010. Excluding Thursday, the difference in the VMT per hour
FIGURE 1 Hourly distribution of vehicle miles of travel throughout the Week for 2005 and 2010 on the freeways and
non-freeways in the Chicago region.

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rate was increasingly greater from about 6:00 PM (18:00 on the graph) to midnight, when the
difference in VMT between 2005 and 2010 peaked. From midnight to about 6:00 AM, the
hourly VMT reversed its previous increase and seems to stabilize from 6:00 AM until the
following evening at 6:00 PM. With the exception of Thursday, the midnight VMT was between
15% and 20% higher in 2005 than in 2010. Thursday traffic has the same initial rise in traffic
after 6:00 PM for 2005 data, but after midnight there is a sharp decrease, and by 4:00 AM the
VMT is grater in 2010 than in 2005.
The overall VMT rate per hour for the two years is more similar for the freeways than for
the non-freeways. Similar to the nighttime trends for the non-freeways, the freeways had more
VMT per hour in 2005 than in 2010 from about 6:00 PM until the early morning, but the 2005
VMT was about ten percent higher at midnight compared to 2010. In contrast to the nightly
reduction of VMT, from 6:00 AM Saturday through Sunday evening, there was more VMT on
the freeway system for each hour in 2010 than there was in 2005. The freeways experienced a
growth in VMT during the daytime on the weekdays. This reflects a redistribution of VMT
between the late night hours and the daytime hours. For 2010, a higher proportion of the VMT
takes place in the peak periods compared to the late-night period. For the same amount of VMT
for 2005 and 2010, 2010 would have more daytime peak travel and less late-night travel. This is
important if the rate of serious or fatal crashes is significantly different during various hours of
the week.
FIGURE 2 Relative changes in VMT by hour of the week between 2005 and 2010 for the freeways and non-freeways in the
Chicago region.

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Distribution of Serious or Fatal Crashes by Hour of the Week
The time distribution throughout the week for fatal and serious crashes is shown in the following
graph (FIGURE 3). This graph shows that the non-freeways have a much larger share of the
serious and fatal crashes than do the freeways. This is true for 2005 and 2010. The daily rhythm
of the crash frequencies on the non-freeways is similar to the general traffic patterns Monday
through Friday with two peaks of fatal or serious crashes that coincide with the rush hour VMT
peaks. The evening crash peak is of greater height and duration than the general VMT profile for
the PM peak. For the non-freeways, the late-night serious or fatal crash rate per hour Monday
through Friday is never much more than one-half the evening peak crash rate and is usually
much less. Nearly every hour in the week has fewer fatal or serious crashes in 2010 on the non-
freeways than in 2005.
The following graph (FIGURE 4) shows the freeway serious or fatal crashes in more
detail. There were generally fewer serious or fatal crashes for most time periods in 2010
compared to 2005. Contrary to this relationship, the hourly serious or fatal crash rate for Sunday
late-night was higher in 2010 than in 2005. The freeways behave differently than the non-
freeways in that the highest serious or fatal crash frequencies occur in the morning or evening
peaks as well as in the late-night, post-midnight period. It is significant to note that the largest
number of serious or fatal crashes on the freeways take place after midnight on Saturday and
Sunday nights. This was the case for 2005 as well as in 2010.
FIGURE 3 Serious or fatal crashes on the freeways and non-freeway roads in the Chicago region for 2005 and 2010.

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Distribution of Fatal Crashes by Hour of the Week
The hour-of-the-week trends for serious crashes compared to fatal crashes are different from
each other on the non-freeways for 2005 and 2010 (FIGURE 5). For both years, the highest
frequencies of fatal crashes for the entire week take place after midnight, during the Saturday and
Sunday early morning hours. This is not the rate per VMT, but the actual number of fatal
crashes, during a period with relatively low VMT. For both periods in the analysis, in the early
part of the week, the time with the highest frequencies of fatal crashes tends to be the evening or
late evening peak, less often the morning peak or mid-day period had the highest daily rate.
Comparing 2005 to 2010, the hourly rate of fatal crashes is nearly always lower for the 2010 data
and the range of the hourly fatal crash rates is lower and smaller for the 2010 data compared to
the 2005 data.
FIGURE 4 Hourly distribution of serious or fatal crashes throughout the week for 2005 and 2010 on the freeways in the
Chicago region.
FIGURE 5 Fatal crashes on the non-freeway roads by hour of the week in the Chicago region (average of 2005 and 2006
compared to the average of 2009 and 2010).

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The distribution of fatal crashes on the freeway system is shown in FIGURE 6. For both
2005 /2006 and 2009/2010 data, the Friday, Saturday and Sunday late evening and early morning
periods have the greatest frequencies of fatal crashes. Comparing 2005/2006 trends to
2009/2010, the early part of the week has had a general reduction in the number of fatalities.
The later part of the week, has had some shifting between portions of the day, but the count of
fatalities is similar between 2005 and 2010.
Serious or Fatal Crash Rates per Vehicle Mile of Travel
The previous sections have shown that the rate of serious and fatal crashes per hour varies
throughout the hours in a week as does the VMT associated with each hour. The following
graph (FIGURE 7) shows how the rate of serious crashes per vehicle mile of travel varies
throughout the week. The chart shows the rate of serious or fatal crashes per hour per 100 million
VMT. The following graphs will examine freeway and non-freeway travel separately. FIGURE
7 shows that the period after midnight has the highest rate of serious and fatal crashes per VMT
for each day of the week, for all types of roads, for both years in this analysis. The serious and
fatal crash rates per VMT are always significantly lower on the freeway system than on the non-
freeways.
In FIGURE 7, the non-freeway serious or fatal crash rate per VMT is graphed by the hour
of the week. For nearly all hours, the serious or fatal crash rate per VMT is lower in 2010 than
for 2005. The rates for both years of analysis are the highest in the post-midnight periods on
Saturday and Sunday mornings. During the weekdays, based on VMT, the daily travel becomes
slightly more dangerous from the morning through the evening peak. Compared to 2005 travel,
the post-midnight crash rate for 2010 is lower for each day with the most significant decrease
occurring on early Tuesday morning and early Thursday morning.
FIGURE 6 Fatal crashes on the freeways by hour of the week in the Chicago region (average of 2005 and 2006 compared
to the average of 2009 and 2010).

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The freeway crash rates per VMT are examined separately in the following graph
(FIGURE 8). Similar to the non-freeway roads, each of the days also have their most dangerous
travel periods in the first hours after midnight, and the early morning periods of Friday, Saturday
and Sunday get progressively more dangerous each day. A trend that is even more pronounced
than for the non-freeway roads, between 2005 and 2010 on the freeways, there has been little
change in the general rate of fatal or serious crashes per 100 million VMT for these dangerous
periods late in the week. For both years in this analysis, the highest single observation for
serious or fatal crashes per VMT is the post-midnight period for Sunday 2010. The first part of
the week is relatively safer in 2010 compared to 2005. The daytime periods on the freeway
system provide the safest travel options on the road network.
Fatal Crash Rates per Vehicle Mile of Travel
FIGURE 7 Hourly distribution of serious or fatal crashes per 100,000,000 VMT throughout the week for 2005 and 2010
on the freeways and non-freeways in the Chicago region.
FIGURE 8 Hourly distribution of serious or fatal crashes per 100,000,000 VMT throughout the week for 2005 and
2010 on the freeways in the Chicago region.

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This analysis has focused on the combined frequencies of serious and fatal crashes in the
Chicago region because many of the factors for these crashes are similar, and the difference
between a fatal crash and a serious injury crash may be a result of quick and expert medical
attention, the age of the occupants, or simple luck. Also, for this analysis the sample size for
only fatal crashes makes the trends less visible. In order to examine just the fatal crashes, two
additional years of data have been added to the analysis. The comparisons in this section will be
for the combination of 2005 and 2006 compared to the combined fatal crashes for 2009 and
2010. Additionally, the hourly values of the fatal crashes use a three-point moving average to
smooth out the observations. Given the limitations of additional years and a low number of
observations, the next two charts examine the relationships between fatal crashes and VMT on
the freeways and non-freeways in the Chicago region
The following graph (FIGURE 9) shows that the fatality rate per 100 million VMT is
always the highest during the post-midnight period for the non-freeways for both 2005/2006 and
2009/2010. The same was true for the total of serious and fatal crashes in the previous sections,
but it is more pronounced for the analysis of fatal crashes. Whereas the combined rates for
serious and fatal crashes, added together, might have a multiple of seven between the most
dangerous hour and the safest travel times on the non-freeways, the fatal crashes are closer to 30
times more dangerous in the worst period compared to the generally safe periods. For 2009/2010,
at the extreme the highest late-night hourly fatality rate per VMT is 68 times greater than the
hour with the lowest fatality rate. Travel has become safer between 2005 and 2010, but the early
morning hours are still significantly more dangerous than the daylight hours. The most
dangerous hour of each day also continues to increase in the rate of fatal crashes from the
beginning of the week through the end of the week on the non-freeway roads.
FIGURE 9 Annual hourly fatal crashes per 100 million VMT on the non-freeways in the Chicago region for 2005/2006
and 2009/2010.

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The trend on the freeway system is not exactly the same as for the non-freeways. The
most dangerous times are after midnight as shown in FIGURE 10. It should be noted that the
crash rates for the most dangerous periods on Friday, Saturday and Sunday, in the early morning,
are as dangerous per VMT in 2010 as they were in 2005. The early portion of the week has
experienced a significant reduction in late-night and early morning fatalities based on the hourly
VMT. The number of travelers in the late-night period on the freeways has been reduced, but the
risk per VMT is similar between 2005 and 2010 on the freeway. Even with these trends, the
freeways are still the safest roads for travel.
Severity of Late-Night Crashes
Not only are serious crashes much more likely to occur late at night, per vehicle mile of travel,
each serious crash is much more likely to result in a fatality. All of the 2010 crashes in this
analysis were grouped into day and evening crashes (6:00 AM to 11:00 PM) and night/late-night
crashes (11:00 PM to 6:00 AM) and the ratio of fatal crashes to serious crashes was calculated.
• On freeways in the day/evening, 7.5% of the serious crashes included a fatality
• On freeways in the late-night, 15.6% of the serious crashes included a fatality
• On non-freeways in the day/evening, 5.0% of the serious crashes included a fatality
• On non-freeways in the late-night, 12.6% of the serious crashes included a fatality
The types of serious crashes that take place late at night are generally more than twice as likely
to result in a fatality compared to crashes that occur in the daytime or early evening. Not only are
serious crashes much more likely to occur late at night, per vehicle mile of travel, each crash is
much more likely to result in a fatality.
FIGURE 10 Fatal crashes per 100 million VMT on the freeways in the Chicago region for 2005/2006 and 2009/2010.

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CONCLUSION
The State of Illinois, counties, and many municipalities have made improving traffic safety a
priority. Their safety programs and enhancements have saved many lives. In addition to these
efforts, the reduction in travel and changes in the time when people travel have also affected the
crash rates.
The traffic crashes which had a fatality or traffic crashes which had a serious injury have
declined 36.4% from 2005 to 2010 on the group of roads classified as arterials, collectors or local
roads (the non-freeways in this analysis). Fatal crashes on these roads decreased by 34.0%.
These roads represent the majority of serious or fatal crashes in the region and also the majority
of the VMT. On the freeways, the fatal crashes fell greater a relative amount, a 46.0% reduction,
while the serious crashes only dropped by 31.6%.
The regional VMT fell by 4.9% on the non-freeways, but increased by 1.3% on the
freeway system (which expanded by 2.7%) over the analysis period. The lowest VMT per hour
occurs in the period between midnight and 5:00 AM on all of the roads in this analysis.
The distribution of VMT per hour of the week changed between 2005 and 2010. For all
travel, the largest proportional changes in VMT occurred at night between 6:00 PM and 4:00 AM.
During this time frame the 2010 VMT per hour was much less than the rate for 2005. The single
largest relative decline in VMT occurred in the hours just past midnight, the time period with the
highest crash rates per VMT.
On the freeway system, the large reduction in late-night VMT was more pronounced in the
earlier part of the week with up to a ten percent reduction in travel. In contrast to this reduction of
VMT, from 6:00 AM Saturday through Sunday evening, there was more VMT on the freeway
system in 2010 than there was in 2005. The freeways also experienced a growth in VMT during
the daytime on weekdays.
For the non-freeways, VMT was lower for almost every hour in the week in 2010
compared to 2005. The late-night period had the greatest reductions for each day, with VMT
often reduced by up to 20% in a single hour, compared to 2005.
In general, between 2005 and 2010, for each hour of the week, fatal or serious crashes per
VMT fell. An exception to this trend occurs on the early portions of Friday, Saturday and Sunday
morning. During this time the rate of fatal crashes per VMT on the freeways is very similar for
both periods in the analysis.
Serious crashes on the non-freeway roads are more likely to occur during the morning
and evening peak periods. For these roads, the serious crash rate per VMT is the highest in the
post-midnight hours of the weekend. For fatal crashes, the highest number of hourly fatalities as
well has the highest rate per VMT take place during the post-midnight hours of the weekend.
For the freeway system, both, the highest number of serious crashes, and the highest
number of fatal crashes take place during the post-midnight hours on Saturday and Sunday. The
rate per VMT is also the highest at these times. The freeways are the class of road with the lowest
fatal or serious crash rates per VMT,
For all of the roads in this analysis, and for both time periods, the ratio between fatal
crashes and serious crashes is more than twice as great in the late-night period compared to
driving in the daytime or early evening. Excess speed, the condition and judgment of the driver
and the lighting conditions probably all contribute to the higher number of fatalities.