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As a practitioner, you may implement multiple countermeasures at a single location,
such as adding pavement markings, widening shoulders, or adding rumble strips.
But what is the combined safety effect of applying multiple countermeasures?
It makes sense that additional improvements would provide an added safety benefit beyond
that of the first countermeasure.
But how much more?
Would you expect the same safety benefit from the second countermeasure as if it was installed
by itself?
What about the third?
Ideally, a crash modification factor (or CMF) would be available that represents the effect
of combined countermeasures.
If so, it is relatively easy to use a single CMF to estimate the change in crashes.
More than likely, however, you will need to use multiple CMFs and select an appropriate
method to estimate the combined effect.
Welcome to Part 1 of FHWA's instructional video on analyzing the combined effect of
multiple countermeasures.
This video describes the process of selecting an appropriate method to estimate the combined
effect when you need to use multiple CMFs.
Part 2 focuses on the application of these methods, assuming you are familiar with the
general concepts of selecting and applying standalone CMFs.
You can refer to FHWA's CMF Clearinghouse and other resources on the web page at the
end of this video for further guidance on selecting and applying CMFs.
While these videos focus on scenarios with two countermeasures, there is the potential
to apply these concepts to three or more measures.
This video uses the term countermeasure to describe a general change in the geometric
or operational conditions implemented to achieve a specific performance outcome of the roadway.
The first edition of the Highway Safety Manual (or HSM) suggests the Multiplicative Method
for estimating the combined effect of multiple countermeasures.
While this method is relatively straightforward in that you simply multiply two CMFs to obtain
the combined effect, it may not be the most reliable method.
Other methods, such as the Additive, Dominant Effect, and Dominant Common Residuals methods,
may provide a more reliable estimate depending on the scenario.
So, when would you select one method over another?
In this video, you will learn how to select an appropriate method by completing the following steps.
Define the scenario of interest. Understand the limits of the combined countermeasure effect.
Determine the potential for overlapping effects among countermeasures.
And categorize the magnitude of individual countermeasure effects.
The first step toward selecting an appropriate method is to define the scenario of interest.
Specifically, you need to answer questions such as: what are the target crash types and
what are the countermeasures of interest?
Defining the scenario of interest will help you select appropriate CMFs and determine
the potential for overlapping effects later.
First, identify the general conditions for the location of interest.
This can include the roadway characteristics and traffic volume.
Next, for each countermeasure, define the target crash type and severity,
applicable time of day (such as day or night), and specific portion of the road, such as a curve, to which
the countermeasure will be applied.
For example, if you plan to construct a roundabout at a stop-controlled intersection,
this applies to the entire intersection at all times of the day.
Or, if you install lighting along sections of a corridor, then it would only apply to
those sections during the night.
It is important to understand the limits of the combined effect.
Keep in mind the maximum effect of any countermeasure or combination of countermeasures
is a crash reduction of 100 percent (or a CMF of 0).
The next step is to determine the potential overlap among individual countermeasure effects.
Potential overlap is defined with respect to target crashes and represents the likelihood
that the individual countermeasure would address the same crash types.
This is different than the applicability of the CMF, which is based on the crash types
used to develop the CMF.
For example, an agency may construct a roundabout to target right-angle crashes while a researcher
develops a CMF to evaluate the impact on fatal and injury crashes.
In this case, the target crashes associated with the countermeasure are right-angle crashes.
The applicable crashes associated with the CMF are fatal and injury crashes.
This is an important distinction to remember as we move forward.
Consider the potential overlap for installing lighting on the major road and installing
advance intersection warning signs on the minor road of a two-way, stop-controlled intersection?
Do you think the two individual countermeasures target the same crash types?
Would there be no overlap, some overlap, or complete overlap?
Engineering judgment is critical in this process.
To determine the potential for overlap, consider the crashes and locations targeted by the
countermeasures as defined in step 1.
There is more potential for countermeasure effects to overlap if the individual countermeasures
target the same crash types and are applied to the same location.
For further information on target crashes, refer to the resources at the end of this video.
Two or more countermeasures may complement, replace, or counteract each other.
Building on the potential for overlap, select the case that best matches your scenario.
Zero overlap represents two independent effects where you would expect the complete benefit
of both countermeasures.
For example, you might select this scenario if the first countermeasure targets motorcycle crashes,
the second countermeasure targets pedestrian crashes, and both are applied to
different portions of the site of interest.
Complete overlap represents two non-independent effects where the second countermeasure targets
some or all the same crash types and severities as the first.
For example, widening both the lane width and the shoulder width may target the same
run-off-road and opposite direction crashes.
In many cases, there is likely some overlap, where both countermeasures provide an added benefit,
but the second countermeasure has some overlap with the first.
For example, cable median barrier and inside shoulder rumble strips on a four-lane, median-divided
facility may have some overlapping effects.
The cable barrier and inside shoulder rumble strips both target cross-median crashes.
There may be an added benefit as the inside shoulder rumble strips also target run-off-road-left
crashes prior to the vehicle leaving the road.
If the combined effect is likely to be greater than the sum of the individual effects, then
this is considered an enhancing effect.
For example, installing shoulder rumble strips in conjunction with shoulder widening may
be more effective than the product of the individual CMFs because the added shoulder
width provides recovery room for drivers alerted by the rumble strips.
On the contrary, when one countermeasure negates the effect of another countermeasure or roadway feature,
this is considered a counteracting effect.
For example, a highway designer may consider installing advance curve warning signs to
offset the potential impacts of reducing the radius of a curve due to topographical constraints.
Depending on the magnitude of the individual countermeasure effects, the method selected
to estimate the combined effect may have a nominal or significant impact on the result.
For example, if both countermeasures are expected to reduce crashes by 10 percent or less,
then the method you use to estimate the combined effect probably won't make much difference.
As the magnitude of the effect increases, the methods produce much different estimates,
and it becomes more important to select an appropriate method.
This table defines the magnitude of effect as small, medium, or large.
Use it to categorize the magnitude of effect for each individual countermeasure for your scenario.
For example, a CMF of 0.95 represents a 5 percent reduction in crashes and you would
classify the magnitude as small.
A CMF of 0.5 represents a 50 percent reduction and you would classify the magnitude as large.
How would you classify the magnitude of individual countermeasure effects if the CMF for the
first countermeasure is 0.92 and the CMF for the second countermeasure is 0.85?
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The classification would be small and medium, respectively.
At this point, you've considered the potential overlap and magnitude of effects.
The last step before selecting a method is to determine the applicability of the individual CMFs.
Specifically, identify the crash type and severity to which each CMF is applicable.
Again, this may be different than the crash type and severity targeted by the countermeasure.
If the CMFs apply to the same crash type and severity, use this table to select an appropriate method.
If the CMFs apply to different crash types or severities, refer to the examples later in this video.
If one or more CMFs are greater than 1.0, the multiplicative method is appropriate.
This is the method presented in the first edition of the Highway Safety Manual where
you simply multiply the two CMFs.
If both CMFs are less than 1.0, consider the potential for overlap.
If you expect zero overlap or enhancing effects, then the additive method is appropriate,
which assumes the full effect of both countermeasures with a maximum reduction of 100 percent.
If you expect complete overlap, the dominant effect method is appropriate because it only
considers the countermeasure with the smallest CMF.
If you expect some overlap, the dominant effect or dominant common residuals method
may be appropriate.
Try both and select the method that produces the largest combined reduction (or smallest CMF).
The dominant effect method tends to work well when one or both individual effects are large.
The dominant common residuals method tends to work well when the individual effects are not large.
If the CMFs apply to different crash types or severities, use this five-step process
to apply the CMFs individually to the applicable crashes, and then aggregate the results.
Apply the smallest CMF to applicable crashes, excluding those crashes associated with other
CMFs in the analysis.
Apply the next smallest CMF to applicable crashes, excluding those crashes associated
with other CMFs in the analysis.
If necessary, estimate the CMF for the combined countermeasure effect
and apply it to the applicable crashes.
The applicable crashes would be those common to both CMFs.
Note, this step is not necessary if there are no applicable crashes common to both CMFs
as illustrated in the next example.
Sum the estimated change in crashes to calculate the combined effect.
Check that the estimated change does not exceed 100 percent.
If it does, then assume the maximum reduction of 100 percent.
The key is to first apply each CMF individually to any crashes that are applicable to that
CMF but not to other CMFs.
Then estimate the CMF and apply it to the applicable crashes common to both CMFs.
As an example, consider one direction of a rural, four-lane, divided highway segment
where there is currently no median barrier.
The agency identifies cross-median crashes, as well as run-off-road-right crashes,
which include fixed object and rollover crashes, as concerns along this section.
There are shoulder rumble strips along the right shoulder but no rumble strips along
the median shoulder.
The two countermeasures of interest are installing median shoulder rumble strips and installing
new guardrail along the right side of the road.
The total crashes on this section comprise 3 cross-median crashes, 4 run-off-road-right crashes,
and 2 sideswipe same direction crashes.
There are no other crash types reported at this location.
The CMF for installing median shoulder rumble strips is 0.87, which applies to all cross-median crashes.
The CMF for installing new guardrail along the right side of the road is 0.93,
which applies to all run-off-road-right crashes.
In this example, the two CMFs apply to different crash types, meaning there are no applicable
crashes common between the individual CMFs.
As such, the CMFs can be applied directly to the applicable crashes.
CMF1 represents the countermeasure with the smallest CMF, median shoulder rumble strips,
and CMF2 represents the countermeasure with the next smallest CMF, shoulder barrier.
Using the five-step process, you would first apply CMF1 to cross-median crashes.
Then, apply CMF2 to run-off-road-right crashes.
You can assume no change in sideswipe same direction crashes because these CMFs are not
applicable to those crashes and there are no CMFs for sideswipe crashes related to median
shoulder rumble strips or right shoulder guardrail.
Next, sum the change in crashes across the two crash categories to estimate the combined effect.
Finally, check to make sure the estimated crash reduction does not exceed 100 percent.
If there are no applicable crashes common to both CMFs, as was the case in this example,
then you can assume the full effect of both countermeasures and skip step 3.
If the CMFs are applicable to any common crashes, then you need to apply all five steps as described.
Again, the key is to apply each individual CMF to the applicable crashes exclusive to
that CMF and not applicable to another CMF in the analysis.
You would then use an appropriate method from this table to estimate the change in crashes
that are applicable to two or more CMFs.
When two or more CMFs are applicable to the same crash types or severities,
follow these rules and apply the results to those crash types or severities.
If one or more CMFs are greater than 1.0, combine the CMFs using the multiplicative method.
If there is complete overlap in countermeasure effects, use the Dominant Effect method.
If there is some overlap in countermeasure effects, use the Dominant Effect or
Dominant Common Residuals method, whichever produces the greatest effect.
This video focused on scenarios with two countermeasures.
So, what happens when there are three or more countermeasures?
In general, this increases the potential for overlapping effects.
More research is needed to verify the accuracy of applying the methods presented in this
video to more than two countermeasures.
However, if you wanted to adapt the general processes outlined in this video to analyze
more than two countermeasures, this is how you would proceed.
First, you would select an appropriate method to estimate the combined effect of the two
countermeasures with the smallest CMFs, producing a single combined effect.
You would then continue in a pairwise process, combining this estimate with the next smallest CMF,
again producing a single combined effect.
This video demonstrated how to select an appropriate method to estimate the combined effect of
multiple countermeasures.
Refer to the companion video for guidance on how to apply these methods.
All the resources mentioned in this video can be found here, including the video for
Part 2: Applying Methods to Analyze Multiple Countermeasures.
At the Federal Highway Administration, safety is our top priority.
For more information, visit us online at safety.fhwa.dot.gov.