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Table 7. AMFs for median width in four-lane rural non-freeway roads (21).
Median width (ft)
10 20 30 40 50 60 70 80 90
1.00 0.91 0.85 0.80 0.76 0.73 0.70 0.67 0.65
TWLTLs is the access density since it has the potential to actual before-and-after crash data to provide sound support
significantly affect the opportunity for crashes. The impact for the conclusions.
of these treatments has not been extensively evaluated, and Agent and Pigman (40) compared the safety impacts of either
their safety gains still require additional verification (38). (1) converting two-lane rural roads to four-lane roads or
Hauer (33) estimated that the AMF for most urban and sub- (2) realigning two-lane roads. The study examined 49 con-
urban TWLTLs ranges from 0.70 to 0.90 based on a review version locations and 24 locations where the two-lane roadway
of several studies. These AMFs are for total number of crashes was upgraded with realignment and widening of lanes and
and not the types of crashes associated with the installation shoulders. The study concluded that both conversions to
of the TWLTL. four lanes and upgrades of two-lane roadways reduced crashes
In summary, the presence of a median has a positive effect after project completion. There was a 56% reduction for
on safety, and some AMFs have been developed based on pre- converted roadways and a 51% reduction for upgraded
vious studies. The median width has also an impact on road- two-lane roadways. A comparison to statewide crash rates for
way safety where wider medians tend to have a larger AMF. each roadway type revealed that converted four-lane roads
Finally, the placement of a barrier is a balancing act because exhibited crash rates similar to the statewide average, while
a barrier has the potential to increase median-related crashes crash rates of upgraded two-lane roads dropped to approxi-
but to reduce cross-median crashes. Even though this element mately one-half the statewide rate for two-lane rural roads.
has been examined more than the other two elements, several The influence of volume on both upgraded and converted roads
of the reports reviewed indicated that for multilane roadways, was also cited, and the authors acknowledge that additional
additional research is required either to develop new AMFs work is needed to evaluate volume impact and determine which
or to validate existing AMFs. approach--conversion or upgrade--is more appropriate.
The important finding of this study is that both approaches
improve safety and should be considered as design alternatives.
Rural Two-Lane Conversions to Multilane
A typical project for rural roadways is the conversion of a
Summary
two-lane road to a four-lane road with or without a median.
Using crash data from four Highway Safety Information System A significant body of research that attempts to quantify the
(HSIS) states, Council and Stewart (34) attempted to estimate relationships between safety and roadway design elements
the safety effects from such conversions on rural roads. The has been compiled. As previously noted, NCHRP Synthesis of
study indicated that safety gains ranging from 40% to 60% Highway Practice 299 has reviewed and discussed several of
were achieved for divided roadways, while smaller gains-- these issues at length, and the reader seeking more detailed
approximately 20%--were achieved for undivided roads. These information is encouraged to review that publication (8).
estimates were developed using typical cross sections for each Several studies have focused on two-lane rural roads and have
roadway type. The authors cautioned that these findings were addressed issues relative to lane widths, shoulder widths and
based on a predictive model and should be validated with types, clear zones, and horizontal and vertical alignments. Even
Table 8. AMFs for median width in rural multilane roadways (22).
Median width (ft)
Barrier 15 20 30 40 50 60 70 80 90
With 1.019 1.012 1.000 0.988 0.977 0.967 0.953 0.944 0.935
Without 1.010 1.006 1.000 0.994 0.988 0.983 0.978 0.973 0.968
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though these are the general areas of interest for this research, elements, without considering the effects when multiple
there is a lack of information regarding any association between elements are varied in combination. An additional issue that
typical and other than typical design values for several design has not been discussed extensively is the potentially opposite
elements. effects that selected values for design elements can impart.
To some degree, the design elements selected for further For example, wider shoulders have shown the potential to
examination in this research have the potential to affect safety. improve safety. On the other hand, they also have the potential
The degrees of influence vary by design element and application to encourage increased operating speeds that, in turn, can
and often are specific to a set of roadway conditions. There are lead to increased crash severity. A similar counterbalancing
current parallel efforts under way to address the quantification potential was noted for the presence and type of barrier in
of the safety and operational impacts from design element medians. Therefore, design decisions and countermeasure
trade-off. Specifically, such models exist for two-lane rural applications should consider the types of crashes associated
highways, and similar models will be developed in the near with the modification and then determine the appropriate
future for multilane highways. design element.
The most directly applicable lesson from the literature is A summary of the literature reviewed and pertinent findings
that values for design elements can be varied. Most research relative to the objectives of this research project are presented
has been directed to the task of evaluating specific design in Table 9.
Table 9. Summary of literature review.
Reference Element Results Comments
AMF for lane width
Lane width (ft)
9 10 11 12
Harwood Four-lane undivided 1.11 1.06 1.00 0.99 AMF for lane width is based on
et al. 2003 Lane rural two-lane roads and from
(26) width Four-lane divided 1.08 1.04 1.00 0.99 expert panel recommendation
AMF for lane width
Lane width (ft)
Roadway 9 10 11 12
Undivided 1.13 1.08 1.02 1.00 AMF for undivided is expert
Lord et al. Lane panel based in the HSM; divided
2008 (22) width Divided 1.09 1.05 1.01 1.00 is based on models
AMF for shoulder width
Paved shoulder width (ft; one side)
Harwood 3 4 5 6 7 8 AMF for shoulder width is based
et al. 2003 Shoulder on rural two-lane roads and from
(26) width 1.0 0.97 0.95 0.93 0.91 0.90 expert panel recommendation
AMF for shoulder conversion
Shoulder width (ft; one side)
Treatment 3 4 5 6 7 8
Turf to paved 0.99 0.98 0.97 0.97 0.97 0.96 AMF for shoulder conversion is
Harwood Gravel to paved 1.00 1.00 1.00 0.99 0.99 0.99 based on rural two-lane roads
et al. 2000 Shoulder and from expert panel
(32) type Turf to composite 1.00 0.99 0.98 0.97 0.98 0.98 recommendation
AMF for paved shoulder width
Paved shoulder width (ft)
Harkey et 0 2 4 6 8 AMF is developed from expert
al. 2008 Shoulder panel evaluating ITS
(27) width 1.18 1.11 1.05 1.00 0.95 improvements
AMF for paved shoulder width
Paved shoulder width (ft)
Roadway 0 2 4 6 8
Undivided 1.18 1.11 1.05 1.00 0.95 AMF is from expert panel for
Lord et al. Shoulder paved shoulders; recommended
2008 (22) width Divided 1.18 1.13 1.09 1.04 1.00 in the HSM.
(continued on next page)
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Table 9. (Continued).
Council &
Stewart Median Based on study of converting
1999 (34) presence Crashes for roads with medians 0.76xADT-0.05 2-to 4-lane roads
Strathman
et al. 2001 Median Larger than Council and Stewart
(35) presence AMF for roads with medians 0.46 but consistent trend; all crashes
Elvik and
Vaa 2004 Median AMF for all crashes for roads with medians 0.88 Based on meta-analysis of
(36) presence AMF for property damage crashes on roads with medians 0.82 several prior studies
General statement by review of
iTrans Median prior studies; difficult to be
2005 (21) presence AMF range 0.500.85 precise
AMF for median width
Median width (ft)
10 20 30 050 70 90 AMF for shoulder width is based
iTrans Median on rural two-lane roads and from
2005 (21) width 1.00 0.91 0.85 0.80 0.70 0.65 expert panel recommendation
AMF for median guardrails: 1.24 all crashes
Elvik and AMF for concrete barriers: 1.15 injury crashes
Vaa 2004 Median AMF for steel barriers: 0.65 injury crashes Based on meta-analysis of
(36) type AMF for cable barriers: 0.71 injury crashes several prior studies
AMF for median width
Median width (ft)
Barrier 15 30 50 70 90
With 1.019 1.000 0.877 0.953 0.935
Lord et al., Median Based on expert panel and
2008 (22) width Without 1.010 1.000 0.988 0.978 0.968 recommended in the HSM
Hauer
2000 (33) TWLTL AMF range for presence 0.70 to 0.90 Reviewing previous studies
Elvik Median Based on prior studies for roads
1995 (39) presence Estimated increase 30% for all crashes with barriers
Fitzpatrick Median
et al., 2008 and left Roads with median, increasing left shoulder by 1 ft will result in 12% AMF developed for roadways in
(38) shoulder reduction in crashes at 4- and 6-lane highways Texas
