The Highway Safety Information System (HSIS) is a multi-State safety database that contains crash, roadway inventory, and traffic volume data for a select group of States. The participating States—California, Illinois, Maine, Michigan, Minnesota, North Carolina, Ohio, Utah, and Washington—were selected based on the quality of their data, the range of data available, and their ability to merge the data from the various files. The HSIS is used by FHWA staff, contractors, university researchers, and others to study current highway safety issues, direct research efforts, and evaluate the effectiveness of accident countermeasures.
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SUMMARY REPORT
Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways
Publication No. FHWA-HRT-05-001
View PDF (149 KB)
As traffic volumes grow on urban freeways, highway agencies
face an ongoing challenge to maintain efficient traffic operations and acceptable
levels of service. Increasing the capacity of a freeway by adding a lane can be
difficult and expensive if it involves widening the existing roadbed, regrading
roadside areas, and/or acquiring additional right-of-way. A number of highway
agencies, however, have implemented projects in which a travel lane is added on
an urban freeway by restriping the traveled way with narrower lanes, converting
all or part of the shoulder to a travel lane, or a combination of both. The
traffic operational benefits of such conversions are immediate and obvious, but
the safety effects are uncertain. This study addresses these safety effects.
Literature Review
McCasland(1) evaluated two freeway segments in Houston, TX
on which narrower lanes and a narrower outside shoulder were used to create an
additional travel lane. Reductions in the accident rate per million
vehicle-kilometers (veh-km) were found using a Poisson comparison of means
test. Urbanik and Bonilla(2) evaluated similar projects on urban freeway segments
in California using a two-sample t-test. Statistically significant changes in
the accident rate were found for three of the 10 projects evaluated. Two
projects experienced statistically significant reductions in the accident rate,
but one project experienced a statistically significant increase. In
particular, the entire accident rate increase for this project occurred near
the downstream end of the segment. There are concerns that both evaluations
addressed accident rate rather than accident frequency and did not compensate
for regression to the mean, both of which could have distorted the safety
benefits.
The current study attempts to account for these possible biases.
Methodology
Databases Used
Because of the need for adequate periods both before and
after treatment, the study analyzed data on urban freeway sites in
California—78.7 kilometers (km) (48.9 miles(mi)) of treatment sites, 31.1 km
(19.3 mi) of untreated sites downstream from treatment sites, and 398.5 km
(247.6 mi) of untreated reference sites similar to the treatment sites.
Supplemental data was also collected on sites immediately upstream of the
treatment sites. Sites in all three groups had median barriers present. All
crash, traffic volume, and roadway inventory data were extracted from 1991–2000
California files in the Federal Highway Administration's (FHWA) Highway Safety
Information System (HSIS).
Research Design
All treatments involved converting either four lanes in one
direction to five lanes, or five lanes in one direction to six lanes. New
travel lanes were developed from existing pavement width by converting paved
shoulders to travel-lane width, narrowing existing lanes by restriping, or a
combination of the two. The treatments had various combinations of before-and-after
geometrics (i.e., different amounts of shoulder conversion and lane narrowing),
which were grouped into "bundles" for analysis. The primary bundles in both
classifications involved narrowing only the inside shoulder, and either leaving
the lane width at 3.7 meters (m)(12 feet (ft)) or narrowing the lanes to at
least 3.4 m (11.2 ft). In all but one bundle, the added lane was a high
occupancy vehicle (HOV) lane during at least some part of the day.
The goal of the study was to examine the effects of the
treatment on three measures of effectiveness:
Total accidents (fatal, injury, and property-damage-only (PDO) accidents, including both towaway and non-towaway accidents).
- Fatal, injury, and PDO towaway accidents (excluding PDO non-towaway accidents).
Fatal and injury accidents (excluding all PDO accidents).
In addition to examining changes in these three measures for
all four- to five-lane conversions combined and all five- to six-lane
conversions combined, the authors also attempted to examine the effects of
different "bundle" types, different crash types (e.g., sideswipe crashes), and
the number of interchange ramps in a section. They also studied possible
upstream effects of changes in flow and downstream effects of "accident migration" when the new lane was dropped.
All treatments were implemented in 1993, and crash, traffic,
and inventory data were available in HSIS from 1991–2000, so the data allowed
for a before-and-after study. To control for possible regression to the mean
and other biases, the empirical Bayes (EB) methodology described by Hauer(3,4)
was used. Here, a prediction of what would have happened at the treatment sites
in the after period without treatment is based on a weighted combination of two
factors: (1) the frequency of crashes on the treated sites in the before
period, and (2) crash-frequency predictions from regression models developed
with data from the untreated reference sites. The prediction of what would have
happened without treatment is then compared to what actually happened with
treatment to estimate the safety effect of the treatment. The methodology
corrects for the regression bias, changes in traffic volume at the treatment
sites, and other possible confounding factors. Details of the methodology are
in the paper referenced at the end of this summary.
Data Collection
As noted above, crash, inventory, and traffic data for
treatment, downstream, and reference sites were extracted from the 1991–2000 California HSIS files. Table 1 presents basic
descriptive statistics for the three types of sites, and table 2 presents the
crash data for the before-and-after periods at the treatment sites. For all
three types, the mileage was divided into "homogeneous sites" for analysis
purposes, with each site being homogeneous for conversion type (number of lanes
for the reference sites) and traffic volume. All of the treatment and
downstream sites were located in two southern California counties, and the
reference sites were located in these two counties plus four surrounding
counties.
Table 1. Descriptive statistics of evaluation sites.
|
|
|
|
AADTa (VEH/DAY) (1994) |
NUMBER OF RAMPS |
TYPE
OF SITE |
NUMBER
OF
LANES |
NUMBER
OF
SITES |
TOTAL
LENGTH
OF SITES
(MI) |
MINIMUM |
MEAN |
MAXIMUM |
ON-RAMPS |
OFF-RAMPS |
TOTAL |
Treatment |
|
4 to 5b |
79 |
36.4 |
79,000 |
104,081 |
128,000 |
60 |
51 |
111 |
|
5 to 6b |
45 |
12.5 |
77,000 |
107,497 |
126,500 |
14 |
15 |
29 |
|
Total |
124 |
48.9 |
77,000 |
104,951 |
128,000 |
74 |
66 |
140 |
Downstream |
|
4 to 5c |
45 |
11.4 |
62,5000 |
103,267 |
128,000 |
28 |
23 |
51 |
|
5 to 6c |
33 |
7.9 |
77,000 |
114,121 |
126,500 |
14 |
19 |
33 |
|
Total |
78 |
19.3 |
62,500 |
107,859 |
128,000 |
42 |
42 |
87 |
Reference |
|
3d |
92 |
45.7 |
5,600 |
63,958 |
142,500 |
205 |
222 |
427 |
|
4d |
270 |
138.6 |
14,250 |
79,965 |
164,000 |
559 |
534 |
1,093 |
|
5d |
128 |
63.4 |
48,500 |
109,245 |
164,000 |
154 |
149 |
303 |
|
Total |
490 |
247.7 |
5,600 |
81,227 |
164,000 |
918 |
905 |
1,823 |
a Annual average daily traffic volume (veh/day) for one direction of travel. 1 mile = 1.6 kilometers
b Number of lanes before and after the project (i.e., conversion type).
c Number of lanes before and after the project on the adjacent treated site.
d Number of lanes on the reference site. |
Analysis
As noted above, one component of the prediction of
after-period accident frequencies at the treatment sites without treatment is a
regression model (i.e., a safety performance function (SPF)) developed using
data from the untreated reference sites. In this study, SPFs using a
negative-binomial distribution were developed with the following primary
independent variables:
Examination of several model forms indicated that the most
appropriate and useful models had the following form:
Expected number of accidents per year = exp(1) x AADT2
(segment length)
The regression coefficients 1(intercept) and 2(exponent of
AADT), the overdispersion parameter of the negative binomial distribution, and
two goodness-of-fit measures (i.e., the ordinary multiple correlation
coefficient, R2, and the Freeman-Tukey coefficient, RFT2) were estimated by the
method of maximum likelihood using a commercially available SAS® statistical
analysis software named PROC GENMOD.(5)
While the EB approach compensates for regression to the mean
and adjusts for the effect on safety of changes in AADT over time, the effect
on safety of changes in other factors over time (e.g., accident reporting
practices, demography, weather) also needs to be addressed. This was
accomplished by developing a series of yearly calibration factors to ensure
that the SPF-predicted and observed accidents at each treated site during the
before period are the same(4) and using these calibration factors to adjust the
predicted accidents for each specific year. For the examination of off- and
on-ramp effects, modified SPFs including an independent variable for the number
of ramps were developed.
Results
Estimated Safety Effects of Four- to Five-Lane and Five- to
Six-Lane Conversions
The results of the primary analyses for different crash
injury levels within the two categories of treatment sites are shown in table
3. Note that the designation of the statistical significance of the change in
crash frequency is based on the ratio of the mean treatment effect to its
standard error. Hauer(3) recommends that a ratio of 2.0 or greater be used in
judging the results of the EB analysis. Although not a formal test of
significance, this could be equated to an approximate 98 percent (one-sided)
test.
The EB analysis results in table 3 indicate that the four-
to five-lane conversions, on the average, resulted in a statistically
significant increase in accident frequency of 10 to 11 percent. The five- to
six-lane conversion projects resulted in an increase in accident frequency of 3
to 7 percent, not statistically significant. The sample size for five- to
six-lane conversions was about half that for four- to five-lane conversions, so
the five- to six-lane analysis would be less likely to result in statistically
significant results as reflected by the larger standard errors.
Table 2. Accident frequencies at treatment sites.
|
BEFORE PERIOD (1991-1992) |
|
NUMBER OF ACCIDENTS |
AVERAGE
AADTa
(VEH/DAY) |
EXPOSURE
(106 VEH-MI) |
CONVERSION TYPE |
FATAL |
INJURY |
PDO
TOWAWAY |
PDO
NON-TOWAWAY |
TOTAL |
4 to 5 lanes |
8 |
629 |
201 |
947 |
1,785 |
105,461 |
2,804.5 |
5 to 6 lanes |
2 |
243 |
71 |
340 |
656
|
110,605 |
1,020.0 |
Total |
10 |
872 |
272 |
1,287 |
2,441 |
106,772 |
3,824.5 |
a AADT is for the treated direction of travel only. |
|
AFTER PERIOD (1994-2000) |
|
NUMBER OF ACCIDENTS |
AVERAGE
AADTa
(VEH/DAY) |
EXPOSURE
(106 VEH-MI) |
CONVERSION
TYPE |
FATAL |
INJURY |
PDO
TOWAWAY |
PDO
NON-TOWAWAY |
TOTAL |
4 to 5 lanes |
26 |
2,310 |
2,204 |
3,103 |
7,643 |
107,267 |
9,983.7 |
5 to 6 lanes |
13 |
809 |
731 |
1,048
|
2,601 |
111,874 |
3,613.3 |
Total |
39 |
3,119 |
2,935 |
4,151 |
10,244 |
108,441 |
13,597.0 |
a AADT is for the treated direction of travel only. |
Supplemental Results
The examination of crash types (both changes in single- and
multivehicle proportions and in individual crash types) indicated no
statistically significant change on the treatment sites. In general, the results
show that the frequency of rear-end collisions increased after project
implementation. The frequency of sideswipe accidents increased for the four- to
five-lane conversions, but decreased for the five- to six-lane conversions
(which may help explain the difference in effects in the primary analysis
above).
The analyses of the individual "bundle" types did not show
differences between bundles that could be used in deciding how best to apply
the treatment. The analysis of ramp locations did not show statistically
significant results, but it was interesting to note that crash frequency
increased both near and away from ramps on the four- to five-lane conversions
and near ramps in the five- to six-lane conversions. Crash frequency decreased
in the five-lane conversion sites away from ramps.
The examination of possible "accident migration" to adjacent
downstream sites indicated a nonsignificant increase for the four- to five-lane
conversions of 5 to 12 percent, and a statistically significant 17 to 21 percent
increase downstream from the five- to six-lane conversions. An effect that
potentially offsets the accident migration on the five- to six-lane conversions
was a nonsignificant decrease in crash frequencies for freeway segments
upstream of the conversion site.
Table 3. Empirical Bayes analysis results for primary
evaluation of specific conversion types.
CONVERSION
TYPE |
MEASURE OF EFFECTIVENESS/
DEPENDENT VARIABLE |
NUMBER OF
SITES |
PERCENT CHANGE IN ACCIDENT FREQUENCY |
RATIOb |
SIGNIFICANT?c |
MEANa |
STANDARD ERROR |
4 to 5 lanes |
|
Total accidents |
79 |
10.96 |
2.88 |
3.8 |
Yes |
|
Fatal, injury, and PDO
towaway accidents |
78 |
9.67 |
3.89 |
2.5 |
Yes |
|
Fatal and injury
accidents |
78 |
10.59 |
4.56 |
2.3 |
Yes |
5 to 6 lanes |
|
Total accidents |
43 |
3.02 |
4.56 |
0.7 |
No |
|
Fatal, injury, and PDO
towaway accidents |
45 |
3.71 |
6.08 |
0.6
|
No |
|
Fatal and injury
accidents |
45 |
7.08 |
7.22 |
1.0 |
No |
a A positive mean percent change indicates an increase in accident frequency, and a negative mean indicates a decrease.
b Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency.
c Significant if ratio 2, and not significant if ratio <2. |
Discussion
The analysis results indicate that narrow-lane or
shoulder-use-lane projects on urban freeways increase accident frequencies for
four- to five-lane conversion projects. Such conversions may increase accident
frequencies for five- to six-lane conversion projects as well, but the results
for those projects were not statistically significant. Because of the different
findings for these two types of conversions, the results obtained are difficult
to generalize to urban freeways as a whole.
One possible explanation for the increase in accident
frequency on conversion projects is that the added lanes in most of the
projects were HOV lanes. Speed differentials between the main lanes and HOV
lanes on freeways have the potential to increase sideswipe and lane-changing
accidents, although this effect has not been satisfactorily quantified in the
literature. The crash type results in this study indicated a nonsignificant
increase in sideswipe collisions on the four- to five-lane conversions, but a
decrease on the five- to six-lane conversions. If this is indeed true, it may
help explain why the results differ between the two classes.
The results also suggest that, at least for the five- to
six-lane conversions, the effect of the project may have been to dissipate
congestion upstream of the treatment site by removing the treatment site as a
bottleneck. It is possible that the effects of the four- to five-lane
conversions have been partially because of the displacement of a bottleneck as
well. The bottleneck may have been transferred to a location downstream of the
treatment site, with a corresponding increase in accident frequency at that
location and possibly within the treatment site itself.
In summary, the findings are more complex than expected.
Differences may exist in the crash-related effects of lane conversion
treatments at four-lane versus five-lane sites. The differences between road
classes observed may be explained by differences in traffic operations (e.g.,
speeds, lane-changing behavior) that could not be analyzed in this study. In
addition, the observed increases in accident frequency cannot necessarily be
attributed to the use of narrower lanes or the conversion of a shoulder to a
travel lane. The use of the added lanes as HOV lanes, which may introduce a
difference in speed between adjacent lanes, may be another explanation for the
increase in accidents. The analysis results also suggest that the conversion
projects may decrease accident frequencies upstream of the project and increase
accident frequencies within and downstream of the project because the projects
may result in the relocation of a traffic operational bottleneck. These various
effects on safety are confounded in the data and could not be separated in this
study.
-
McCasland, W. R.,
"Use of Freeway Shoulders to Increase Capacity," Transportation Research Record
666, Transportation Research Board, Washington, DC, 1978, pp. 46-51.
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Urbanik, T.,
Bonilla, C. R., "California Experience with Inside Shoulder Removals,"
Transportation Research Record 1122, Transportation Research Board, Washington,
DC, 1987, pp. 37-36.
-
Hauer, E.,
Observational Before-After Studies in Road Safety, Pergamon/Elsevier Science,
Inc., Tarrytown, NY, 1997.
-
Hauer, E.,
Council, F.M., and Mohammedshah, Y., "Safety Models for Urban Four-Lane
Undivided Road Segments," Presentation, 8th Annual Meeting of the
Transportation Research Board, Washington DC, January 2005. Accepted for
publication in Transportation Research Record (Draft in CD of Annual Meeting of
the Transportation Research Board, 2004).
-
SAS/STAT® User's
Guide, Version 8, SAS Institute Inc., Cary, NC, 1999, pp. 1,363-1,464.
For More Information
This research was conducted by Karin M. Bauer, Douglas W.
Harwood, and Karen R. Richard of the Midwest Research Institute and Warren E.
Hughes of BMI-SG. The final report, "Safety Effects of Using Narrow Lanes and
Shoulder-Use Lanes to Increase the Capacity of Urban Freeways," appears in the
Transportation Research Board's Transportation Research Record: Journal of the
Transportation Research Board No. 1897, 2004.
For more information about HSIS, contact Carol Tan, HSIS
Program Manager, HRDS, 202–493–3315, carol.tan@fhwa.dot.gov.
The Highway Safety Information System
(HSIS) is a multi-State safety database that
contains crash, roadway inventory, and traffic
volume data for a select group of States. The
participating States—California, Illinois, Maine,
Michigan, Minnesota, North Carolina, Ohio,
Utah, and Washington—were selected based on
the quality of their data, the range of data available,
and their ability to merge the data from the various
files. The HSIS is used by FHWA staff, contractors,
university researchers, and others to study current
highway safety issues, direct research efforts, and
evaluate the effectiveness of accident countermeasures.
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