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C-1
APPENDIX C
GUIDELINES FOR THE USE, LOCATION, AND
DESIGN OF UNSIGNALIZED MEDIAN OPENINGS
This appendix presents guidelines for the use, location, CLASSIFICATION AND ASSESSMENT OF
and design of unsignalized median openings. The guidelines TYPICAL MEDIAN OPENING DESIGNS
include a classification and assessment of typical median
opening designs, a discussion of the factors that influence the A classification system is presented to identify how par-
safety and operational performance of median openings, and ticular median openings function and where they are located
a methodology that can be used to compare the expected relative to other elements of the highway system. This sec-
safety performance of two or more median opening design tion presents the factors used in the classification process and
alternatives. provides an overview of typical median designs.
MEDIAN TREATMENTS Factors Used in Classification of Median
Opening Designs
The selection of median treatment has important bearing
on how well a roadway will operate, its safety performance, The four key factors used to classify or describe the design
and the access provided to adjacent developments. The basic of a median opening are:
options for median treatment include:
· Type of geometry
· No median (undivided roadway) · Degree of access served
· Continuous two-way left-turn lane (TWLTL) · Presence of left-turn lanes
· Nontraversable median · Presence of loons
NCHRP Report 395, Capacity and Operational Effects of The first factor, type of geometry, determines which move-
Mid-Block Left-Turn Lanes (15), compares the relative traf- ments are possible at a median opening. Conventional median
fic operational and safety performance of these three median openings (sometimes referred to as "full median openings")
options. Table C-1 presents a comparison of these three median typically permit all movements, while directional median
options, indicating which option is preferred with respect to openings may restrict certain movements. Jughandles are an
operational, safety, access, and other factors. indirect left-turn treatment that enable drivers to make U-turn
Many studies have analyzed the safety benefits of installing and left-turn maneuvers efficiently on divided highways with
TWLTLs or nontraversable medians on undivided highways, relatively narrow medians.
and replacing TWLTLs with nontraversable medians. NCHRP The second factor, degree of access served, not only deter-
Report 420, Impacts of Access Management Techniques (4), mines what movements need to be accommodated at a median
presents a summary of the individual studies and compares opening, but also the number of potential conflict points a
their results for each of the three median options. Figures C-1 median opening will have. For example, a median opening
and C-2 present accident rates by median type and total access that only serves U-turn maneuvers will have considerably
density (both directions) for urban-suburban and rural road- fewer conflicting maneuvers than a median opening at a
ways, respectively. Roadways with nontraversable medians three- or four-leg intersection, where U-turns use the same
consistently have a lower accident rate than undivided road- roadway as left-turn and crossing maneuvers. Median open-
ways or roadways with TWLTLs. ings can be classified by whether access points are present on
The preceding tables and figures show the reasons why high- neither side, one side, or both sides of the roadway. Access
way agencies are increasingly using nontraversable medians points at median openings may include either intersecting
on arterial highways. Provision of a nontraversable median is public roads or driveways.
likely to result in increased U-turn demand. However, it is The third factor used in the classification is whether or not
unlikely that the increased U-turn demand can be met solely a median opening has a left-turn lane. Median openings gen-
at signalized intersections. Therefore, unsignalized median erally operate better when left-turn lanes are present to pro-
openings are likely needed to accommodate both U-turn vide a deceleration and storage area for vehicles before they
demand and left-turn access, where appropriate. The follow- enter the median. In fact, the AASHTO Green Book specifi-
ing discussion addresses various types of unsignalized median cally encourages the use of left-turn lanes at median open-
openings and guidelines for their use and design. ings to reduce or eliminate stopping on the through lanes (3).
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C-2
TABLE C-1 Comparison of effects of three alternative cross-sections with differing midblock left-turn
treatment types (15)
Figure C-1. Estimated accident rates by type of median-urban and suburban areas.
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C-3
Figure C-2. Estimated accident rates by type of median-rural areas (4).
The final factor in classification of median openings is Overview of Typical Median Opening Designs
whether or not a median opening is accompanied by a loon.
A loon is an expanded paved apron on the shoulder opposite Using the first two classification factors (geometry type
a median crossover. The purpose of loons is to provide addi- and degree of access served), typical median openings can be
tional space for larger vehicles (particularly trucks) to nego- classified into the following six categories:
tiate turns, and thus, to allow the installation of conventional
or directional median openings along narrow medians. The 1. Conventional Midblock Median Opening
provision of loons to serve U-turns by large vehicles is a new 2. Directional Midblock Median Opening
technique that formalizes past use of paved shoulders for the 3. Conventional Median Opening at Three-Leg Intersection
same purpose. Initial results by highway agencies that have 4. Directional Median Opening at Three-Leg Intersection
used loons appear promising (55, 56). 5. Conventional Median Opening at Four-Leg Intersection
Based on the four factors discussed above, median open- 6. Directional Median Opening at Four-Leg Intersection
ings can be classified based on their design characteristics as
follows: These six categories of median openings can be subdivided
based on the presence of left-turn lanes or loons and the types
· Type of geometry (traffic movements permitted) of turning maneuvers permitted. With these subdivisions,
conventional (all movements permitted) there are a total of fifteen typical median opening designs. The
directional following discussion presents each of the six categories of
· Degree of access served median openings and the specific designs used for those open-
U-turn only (midblock median opening) ings. The discussion of each median opening design includes
access on one side (at three-leg intersection) a figure with a diagram of each median opening design and a
access on two sides (at four-leg intersection) list of the advantages and disadvantages associated with each
· Presence of left-turn lane design.
no left-turn lane present
left-turn lane present Conventional Midblock Median Opening
· Presence of loon
no loon present A conventional midblock median opening permits vehicles
loon present to make U-turns, but does not provide separate channelized
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C-4
roadways for vehicles making U-turns in opposing directions. Type 1c--Conventional Midblock Median Opening With
Median openings at midblock locations are appropriate on Left-Turn Lanes and Loons
arterials where providing for U-turn maneuvers between inter-
sections may improve operations at intersections by reducing Figures C-3 through C-5 illustrate these three median
the U-turn volumes at those intersections or reducing the opening designs and their advantages and disadvantages. The
amount of out-of-direction travel for vehicles trying to reach presence of left-turn lanes in Types 1b and 1c reduces the
a destination without direct left-turn access. Conventional potential for rear-end collisions between U-turn vehicles and
median openings are appropriate where U-turn volumes are following through vehicles. The presence of loons in Type 1c
relatively low, such that U-turn vehicles in opposing direc- provides a widening in the pavement to accommodate U-turn
tions of travel create minimal interference with one another. movements by larger vehicles, such as emergency vehicles
The conventional midblock median opening design is fur- and trucks.
ther classified into three subcategories based on the presence
of left-turn lanes and/or loons: Directional Midblock Median Opening
Type 1a--Conventional Midblock Median Opening With- A directional midblock median opening permits vehicles to
out Left-Turn Lanes make U-turns and provides separate channelized roadways for
Type 1b--Conventional Midblock Median Opening With vehicles making U-turns in opposite directions. Thus, oppos-
Left-Turn Lanes ing U-turn vehicles will not overlap. Median openings at mid-
Type 1a--Conventional Midblock Median Opening Without Left-Turn Lanes
Advantages
· Midblock access is provided for vehicles to (a) make a U-turn and (b) reach
driveways on the opposite side of the street.
· Since only major-road traffic is involved, delays to vehicles making U-turns will
be less than where an intersection is present.
· If U-turn traffic would otherwise proceed downstream to an intersection with a
median opening, then this treatment should reduce VMT.
· Since vehicles making a U-turn only need to enter, but not cross, the opposing
roadway, a minimum gap of only 4 to 6 sec will be needed.
· There are only four conflict points, which is less than at a three-leg or four-leg
intersection.
· Providing median openings for U-turns between intersections reduces the number
of turning maneuvers at the intersections.
· Accident rates at midblock median openings are lower than at three- or four-leg
median openings.
Disadvantages
· The absence of left-turn lanes increases potential for rear-end collisions between
U-turn vehicles and following through vehicles.
· U-turn vehicles forced to stop in the median opening may encroach on adjacent
lanes and interfere with through traffic.
· U-turn vehicles entering the through lanes may delay full-speed through traffic.
· Narrow medians may not provide enough space for larger vehicles to negotiate a
U-turn.
· With no directional island, opposing U-turn vehicles may have to overlap.
Figure C-3. Advantages and disadvantages of median opening type 1a--
conventional midblock median opening without left-turn lanes.
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C-5
Type 1b--Conventional Midblock Median Opening With Left-Turn Lanes
Advantages
· The presence of left-turn lanes reduces potential for rear-end collisions
between U-turn vehicles and following through vehicles.
· The presence of left-turn lanes mitigates the problem of U-turn vehicles
encroaching on adjacent lanes and interfering with through traffic while
waiting for a gap in the opposing traffic.
· Midblock access is provided for vehicles to (a) make a U-turn and (b)
reach driveways on the opposite side of the street.
· Since only major-road traffic is involved, delays to vehicles making U-
turns will be less than where an intersection is present.
· If U-turn traffic would otherwise proceed downstream to an intersection
with a median opening, then this treatment should reduce VMT.
· Since vehicles making a U-turn only need to enter, but not cross, the
opposing roadway, a minimum gap of only 4 to 6 sec will be needed.
· There are only four conflict points, which is less than at a three-leg or
four-leg intersection.
· Providing median openings for U-turns between intersections reduces
the number of turning maneuvers at the intersections.
· Accident rates at midblock median openings are lower than at three- or
four-leg median openings.
Disadvantages
· U-turn vehicles entering the through lanes may delay full-speed through
traffic.
· Narrow medians may not provide enough space for larger vehicles to
negotiate a U-turn.
· With no directional island, opposing U-turn vehicles may have to
overlap.
Figure C-4. Advantages and disadvantages of median opening type
1b--conventional midblock median opening with left-turn lanes.
block locations are appropriate on arterials where providing Type 2c--Directional Midblock Median Opening With
for U-turn maneuvers between intersections may improve Left-Turn Lanes and Loons
operations at intersections by reducing the U-turn volumes at
those intersections or reducing the amount of out-of-direction Figures C-6 through C-8 illustrate these three median open-
travel for vehicles trying to reach a destination without direct ing designs and their advantages and disadvantages. The pres-
left-turn access. Directional median openings are appropriate ence of left-turn lanes in Types 2b and 2c reduces the potential
where U-turn volumes are relatively high, such that U-turn for rear-end collisions between U-turn vehicles and following
vehicles in opposing directions of travel would otherwise through vehicles. The presence of loons in Type 2c provides a
interfere with one another. widening in the pavement to accommodate U-turn movements
The directional midblock median opening design is further by larger vehicles, such as emergency vehicles and trucks.
classified into three subcategories based on the presence of
left-turn lanes and/or loons: Conventional Median Opening at Three-Leg
Intersection
Type 2a--Directional Midblock Median Opening Without
Left-Turn Lanes A conventional median opening at a three-leg intersection
Type 2b--Directional Midblock Median Opening With permits vehicles on the major road to make U-turn move-
Left-Turn Lanes ments on the major road and left- or right-turning movements
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C-6
Type 1c--Conventional Midblock Median Opening With
Left-Turn Lanes and Loons
Advantages
· Widening on the far side of the turn makes it possible to make a U-turn
without stopping or backing and reduces interference between U-turn and
through traffic, particularly for large vehicles.
· Without performing major reconstruction, additional space can be provided
to facilitate the larger turning path of commercial vehicles along narrow
medians.
· The presence of left-turn lanes reduces potential for rear-end collisions
between U-turn vehicles and following through vehicles.
· The presence of left-turn lanes mitigates the problem of U-turn vehicles
encroaching on adjacent lanes and interfering with through traffic while
waiting for a gap in the opposing traffic.
· Midblock access is provided for vehicles to (a) make a U-turn and (b) reach
driveways on the opposite side of the street.
· Since only major-road traffic is involved, delays to vehicles making U-
turns will be less than where an intersection is present.
· If U-turn traffic would otherwise proceed downstream to an intersection
with a median opening, then this treatment should reduce VMT.
· Providing median openings for U-turns between intersections reduces the
number of turning maneuvers at the intersections.
· Accident rates at midblock median openings are lower than at three- or
four-leg median openings.
Disadvantages
· U-turn vehicles entering the through lanes may delay full-speed through
traffic.
· With no directional island, opposing U-turn vehicles may have to overlap.
· The presence of loons may make snow removal and other maintenance
work more difficult.
Figure C-5. Advantages and disadvantages of median opening type
1c--conventional midblock median opening with left-turn lanes and
loons.
onto the minor road. Vehicles on the minor road may make Type 3a--Conventional Median Opening Without Left-
left or right turns onto the major road. No separate channel- Turn Lanes at Three-Leg Intersection
ized roadways are provided for vehicles making U-turns in Type 3b--Conventional Median Opening With One Left-
opposing directions. Thus, U-turn vehicles may overlap with Turn Lane at Three-Leg Intersection
opposing U-turn or left-turn vehicles. Median openings at Type 3c--Conventional Median Opening With Two Left-
three-leg intersections are appropriate along arterial roadways Turn Lanes at Three-Leg Intersection
at street intersections or driveways to major developments Type 3d--Conventional Median Opening With Left-Turn
where providing access across the median will not create Lanes and Loons at Three-Leg Intersection
undesirable safety or traffic operational effects. Conventional
median openings are appropriate where it is desirable to Figures C-9 through C-12 illustrate these four median
allow left-turning movements from both the major road and opening designs and their advantages and disadvantages. The
the minor road (or driveway) and where U-turn volumes are presence of left-turn lanes in Types 3b, 3c, and 3d reduces
relatively low, such that U-turn vehicles in opposing direc- the potential for rear-end collisions between U-turn vehicles
tions of travel create minimal interference with one another. and following through vehicles. The presence of loons in
The conventional median opening design at a three-leg Type 3d provides a widening in the pavement to accommo-
intersection is further classified into four subcategories based date U-turn movements by larger vehicles, such as emer-
on the presence of a left-turn lane and/or loon: gency vehicles and trucks.
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C-7
Type 2a--Directional Midblock Median Opening Without Left-Turn
Lanes
Advantages
· This design prevents overlapping U-turns.
· Midblock access is provided for vehicles to (a) make a U-turn and (b)
reach driveways on the opposite side of the street.
· Since only major-road traffic is involved, delays to vehicles making U-
turns will be less than where an intersection is present.
· If U-turn traffic would otherwise proceed downstream to an intersection
with a median opening, then this treatment should reduce VMT.
· Since vehicles making a U-turn only need to enter, but not cross, the
opposing roadway, a minimum gap of only 4 to 6 sec will be needed.
· There are only four conflict points, which is less than at a three-leg or
four-leg intersection.
· Providing median openings for U-turns between intersections reduces
the number of turning maneuvers at the intersections.
· Accident rates at midblock median openings are lower than at three- or
four-leg median openings.
Disadvantages
· The absence of left-turn lanes increases potential for rear-end collisions
between U-turn vehicles and following through vehicles.
· U-turn vehicles forced to stop in the median opening may encroach on
adjacent lanes and interfere with through traffic.
· U-turn vehicles entering the through lanes may delay full-speed through
traffic.
· Narrow medians may not provide enough space for larger vehicles to
negotiate a U-turn.
Figure C-6. Advantages and disadvantages of median opening
type 2a--directional midblock median opening without left-turn
lanes.
Directional Median Opening at Three-Leg the minor road. Median openings at three-leg intersections
Intersection are appropriate along arterial roadways at street intersections
or driveways to major developments where providing access
There are two types of directional median openings at three- across the median will not create undesirable safety or traf-
leg intersections: fic operational effects.
Directional median openings are appropriate where U-turn
Type 4a--Directional Median Opening for Left Turns from or left-turn volumes are relatively high, such that a conven-
Major Road at Three-Leg Intersection tional median opening would experience considerable inter-
Type 4b--Direction Median Opening for Left Turns onto ference between vehicles entering the median opening. Direc-
Major Road at Three-Leg Intersection tional median openings are also appropriate where there is a
disproportionately high left-turn demand from either the
The first type, designated as Type 4a, permits vehicles to major road or the minor road and, therefore, either Type 4a
turn left off the major road onto the minor road and to make or Type 4b would accommodate the needs of the intersection.
U-turn maneuvers on the major road, but does not permit Directional median openings are desirable where an inter-
vehicles to turn left from the minor road onto the major road. section is going to be signalized, since it only impacts major-
The second type, designated as Type 4b, permits vehicles to road traffic in one direction and effective two-direction sig-
turn left or right from the minor road onto the major road and nal coordination can be maintained (5).
vehicles on the major road to make U-turn maneuvers, but Figures C-13 through C-14 illustrate these two median
does not permit vehicles to turn left off the major road onto opening designs and their advantages and disadvantages.
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C-8
Type 2b--Directional Midblock Median Opening With Left-Turn Lanes
Advantages
· The presence of left-turn lanes reduces potential for rear-end collisions
between U-turn vehicles and following through vehicles.
· The presence of left-turn lanes mitigates the problem of U-turn vehicles
encroaching on adjacent lanes and interfering with through traffic while
waiting for a gap in the opposing traffic.
· This design prevents overlapping U-turns.
· Midblock access is provided for vehicles to (a) make a U-turn and (b)
reach driveways on the opposite side of the street.
· Since only major-road traffic is involved, delays to vehicles making U-
turns will be less than where an intersection is present.
· If U-turn traffic would otherwise proceed downstream to an intersection
with a median opening, then this treatment should reduce VMT.
· Since vehicles making a U-turn only need to enter, but not cross, the
opposing roadway, a minimum gap of only 4 to 6 sec will be needed.
· There are only four conflict points, which is less than at a three-leg or
four-leg intersection.
· Providing median openings for U-turns between intersections reduces the
number of turning maneuvers at the intersections.
· Accident rates at midblock median openings are lower than at three- or
four-leg median openings.
Disadvantages
· U-turn vehicles entering the through lanes may delay full-speed through
traffic.
· Narrow medians may not provide enough space for larger vehicles to
negotiate a U-turn.
Figure C-7. Advantages and disadvantages of median opening type
2b--directional midblock median opening with left-turn lanes.
Conventional Median Opening at Four-Leg atively low, such that U-turn vehicles in opposing directions
Intersection of travel create minimal interference with one another.
The conventional median opening design at a four-leg inter-
A conventional median opening at a four-leg intersection
section is further classified into two subcategories based on
permits vehicles on the major road to make U-turn move-
the presence of left-turn lanes:
ments on the major road and left- or right-turning movements
onto the minor road. Vehicles on the minor road may make
left or right turns onto the major road. No separate channel- Type 5a--Conventional Median Opening Without Left-
ized roadways are provided for vehicles making U-turns in Turn Lanes at Four-Leg Intersection
opposing directions. Thus, U-turn vehicles may overlap with Type 5b--Conventional Median Opening With Left-Turn
opposing U-turn or left-turn vehicles. Median openings at Lanes at Four-Leg Intersection
four-leg intersections are appropriate along arterial roadways
at street intersections or driveways to major developments
where providing access across the median will not create Figures C-15 and C-16 illustrate these two median opening
undesirable safety or traffic operational effects. Conventional designs and their advantages and disadvantages. The presence
median openings are appropriate where it is desirable to allow of left-turn lanes in Type 5b reduces the potential for rear-
left-turning movements from both the major road and the end collisions between U-turn vehicles and following through
minor road (or driveway) and where U-turn volumes are rel- vehicles.
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C-9
Type 2c--Directional Midblock Median Opening With
Left-Turn Lanes and Loons
Advantages
· Widening on the far side of the turn makes it possible to make a U-turn
without stopping or backing and reduces interference between U-turn and
through traffic, particularly for large vehicles.
· Without performing major reconstruction, additional space can be provided
to facilitate the larger turning path of commercial vehicles along narrow
medians.
· The presence of left-turn lanes reduces potential for rear-end collisions
between U-turn vehicles and following through vehicles.
· The presence of left-turn lanes mitigates the problem of U-turn vehicles
encroaching on adjacent lanes and interfering with through traffic while
waiting for a gap in the opposing traffic.
· This design prevents overlapping U-turns.
· Midblock access is provided for vehicles to (a) make a U-turn and (b) reach
driveways on the opposite side of the street.
· Since only major-road traffic is involved, delays to vehicles making U-turns
will be less than where an intersection is present.
· If U-turn traffic would otherwise proceed downstream to an intersection with
a median opening, then this treatment should reduce VMT.
· Providing median openings for U-turns between intersections reduces the
number of turning maneuvers at the intersections.
· Accident rates at midblock median openings are lower than at three- or four-
leg median openings.
Disadvantages
· The presence of loons may make snow removal and other maintenance work
more difficult.
Figure C-8. Advantages and disadvantages of median opening Type
2c--directional midblock median opening with left-turn lanes and loons.
Directional Median Opening for Left Turns from and effective two-direction signal coordination can be main-
Major Road at Four-Leg Intersection tained (NCHRP Report 348). This median opening design is
represented in the classification as:
A directional median opening at a four-leg intersection per-
mits vehicles to turn left off the major road onto the minor Type 6a--Directional Median Opening for Left Turns from
road and to make U-turn maneuvers on the major road, but Major Road at Four-Leg Intersection
does not permit vehicles to turn left from the minor road onto
the major road. Median openings at four-leg intersections are Figure C-17 illustrates this median opening design and its
appropriate along arterial roadways at street intersections or advantages and disadvantages.
driveways to major developments where providing access
across the median will not create undesirable safety or traffic
operational effects. Directional median openings are appro- LOCATION AND DESIGN GUIDELINES
priate where U-turn or left-turn volumes are relatively high,
such that a conventional median opening would experience This section presents guidelines on the location and design
considerable interference between vehicles entering the median of median openings based on the following factors:
opening. This particular directional median opening design is
also appropriate where there is a disproportionately high left- · Major- and minor-road volumes
turn demand from the major road. Directional median open- · Left-turn and U-turn volumes
ings are desirable where an intersection is going to be signal- · Median width
ized, since it only impacts major-road traffic in one direction · Median opening length
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C-10
Type 3a--Conventional Median Opening Without Left-Turn
Lanes at Three-Leg Intersection
Advantages
· Vehicles on the minor road can make both left and right turns onto
the major road.
· Vehicles on the major road can make left and right turns onto the
minor road and make U-turns on the major road.
· Delays will be less than at a four-leg intersection.
· No additional VMT is incurred by vehicles making left-, right-, or
U-turning maneuvers.
· Since vehicles making a U-turn only need to enter, but not cross,
the opposing roadway, a minimum gap of only 4 to 6 sec will be
needed.
· There are only 11 conflict points, which is less than at a four-leg
intersection.
· Accident rates at three-leg intersections are lower than at four-leg
intersections.
Disadvantages
· The absence of left-turn lanes increases potential for rear-end
collisions between left-turn or U-turn vehicles and following
through vehicles.
· U-turn vehicles forced to stop in the median opening may encroach
on adjacent lanes and interfere with through traffic.
· U-turn vehicles entering the through lanes may delay full-speed
through traffic.
· Narrow medians may not provide enough space for larger vehicles
to negotiate a U-turn.
· With no directional island, opposing U-turn vehicles may have to
overlap.
· Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even
10 sec where six-lane or eight-lane arterials must be crossed.
These gaps will be limited during busy travel period on heavily
used arterials.
· Where peak-hour traffic exceeds critical volumes, vehicles entering
the major road from the minor road may experience long delays.
Figure C-9. Advantages and disadvantages of median opening
type 3a--conventional median opening without left-turn lanes at
three-leg intersection.
· Minimum spacing between median openings able. Safety prediction models for divided highway intersec-
· Sight distance tions that consider major- and minor-road traffic volumes are
· Loons to assist vehicles in completing U-turn maneuvers currently being developed for use in the Highway Safety Man-
· Median opening types ual. Once appropriate safety prediction models are developed,
they should be used in comparing alternative median opening
designs. While the full major- and minor-road traffic volumes
cannot currently be considered in the location and design of
Major- and Minor-Road Volumes median openings, consideration can be given to median open-
ing volumes (U-turns and left turns), as discussed in the next
Unsignalized median openings may be used for a broad
section.
range of major- and minor-road traffic volumes. However, if
the major- and minor-road volumes exceed the traffic vol-
umes given in the MUTCD signalization warrants, signal- U-turn and Left-Turn Volumes
ization of the median opening should be considered.
No safety prediction models relating median opening acci- Field studies at a variety of median opening types in urban
dent frequencies to major- and minor-road volumes are avail- arterial corridors have found estimated U-turn volumes rang-
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C-11
Type 3b--Conventional Median Opening With One Left-Turn Lane at
Three-Leg Intersection
Advantages
· The presence of a left-turn lane in one direction of travel reduces
potential for rear-end collisions between left-turn or U-turn vehicles and
following through vehicles for one direction of travel only on the major
road.
· The presence of a left-turn lane in the direction of travel mitigates the
problem of U-turn vehicles in that direction encroaching on the adjacent
lane and interfering with through traffic while waiting for a gap in the
opposing traffic.
· Vehicles on the minor road can make left and right turns onto the major
road.
· Vehicles on the major road can make left and right turns onto the minor
road and make U-turns on the major road.
· Delays will be less than at a four-leg intersection.
· No additional VMT is incurred by vehicles making left-, right-, or U-
turning maneuvers.
· Since vehicles making a U-turn only need to enter, but not cross, the
opposing roadway, a minimum gap of only 4 to 6 sec will be needed.
· There are only 11 conflict points, which is less than at a four-leg
intersection.
· Accident rates at three-leg intersections are lower than at four-leg
intersections.
Disadvantages
· U-turn vehicles in the direction of travel without a left-turn lane forced to
stop in the median opening may encroach on adjacent lanes and interfere
with through traffic.
· Narrow medians may not provide enough space for larger vehicles to
negotiate a U-turn.
· Vehicles on the major road in the direction of travel without a left-turn
lane must initiate U-turn maneuvers from the through lane, which may
delay through vehicles and create the potential for rear-end collisions.
· With no directional island, opposing U-turn vehicles may have to overlap.
· Minimum 6- to 8-second gaps are needed to cross traffic, possibly even
10 sec where six-lane or eight-lane arterials must be crossed. These gaps
will be limited during busy travel period on heavily used arterials.
· Where peak-hour traffic exceeds critical volumes, vehicles entering the
major road from the minor road may experience long delays.
Figure C-10. Advantages and disadvantages of median opening type
3b--conventional median opening with one left-turn lane at three-leg
intersection.
ing from 2 to 977 veh/day, representing from 0.01 to 3.2 per- frequency to median opening volumes. It is also not possible
cent of the major-road traffic volumes at those locations. At to separately account for the effects of U-turn and left-turn
median openings in rural areas, U-turn volumes were found volumes on safety at median openings. However, Table C-2
to range from 88 to 374 veh/day, representing 0.41 to 1.36 presents the accident rates per median opening traffic move-
percent of the major-road traffic volumes at those locations. ment for various area and median opening types. The median
The available data are not sufficient to develop satisfactory opening traffic volume is the sum of the U-turn and left-turn
regression relationships to relate median opening accident volumes through the median opening. Computational proce-
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Type 6a--Directional Median Opening for Left Turns From Major Road at
Four-Leg Intersection
Advantages
· The presence of left-turn lanes reduces potential for rear-end collisions between
left-turn or U-turn vehicles and following through vehicles for both directions
of travel on the major road.
· The presence of left-turn lanes mitigates the problem of U-turn vehicles
encroaching on adjacent lanes and interfering with through traffic while waiting
for a gap in the opposing traffic.
· Vehicles on the major road have direct left-turn access to the minor road.
· Vehicles on the major road in both directions of travel can make U-turn
maneuvers.
· Right-turn maneuvers are unaffected by this treatment.
· Delays will be less than at a conventional median opening at a four-leg
intersection.
· Since vehicles making a U-turn only need to enter, but not cross, the opposing
roadway, a minimum gap of only 4 to 6 sec will be needed.
· There are only 12 conflict points, which is less than at a conventional median
opening at a four-leg intersection.
· Accident rates at directional four-leg median openings are lower than at
conventional four-leg median openings.
Disadvantages
· Minor-road vehicles do not have direct left-turn access to the major road.
· U-turn vehicles entering the through lanes may delay full-speed through traffic.
· Accident rates at four-leg intersections are higher than at three-leg
intersections.
· No direct crossing maneuver on minor road.
· Additional VMT will be incurred by vehicles whose drivers desire to make
crossing maneuvers on the minor road and left-turn maneuvers from the minor
road to the major road.
Figure C-17. Advantages and disadvantages of median opening type
6a--directional median opening for left turns from major road at four-leg
intersection.
ance for median widths large enough to store the design · where left and/or right turns are not permitted from a par-
vehicle and for median widths not large enough to store the ticular approach and the crossing maneuver is the only
design vehicle. legal maneuver;
· where the crossing vehicle would cross the equivalent
width of more than six lanes; or
Crossing Maneuvers (Case B3) · where substantial volumes of heavy vehicles cross the
highway and steep grades that might slow the vehicles
Case B3 involves a situation in which a vehicle is stopped while its back portion is still in the intersection are pre-
on the minor road awaiting an opportunity to cross the major sent on the departure roadway on the far side of the
road by clearing traffic approaching from the left and from intersection.
the right. Green Book Exhibit 9-58 presents design ISD cri-
teria for Case B3. In most cases, the sight distance provided
in Case B1 will provide more than adequate sight distance for Left Turns from the Major Road (Case F)
minor-road vehicles to cross the major road. However, in the
following situations, it is advisable to check the availability Case F involves a situation in which a vehicle is stopped
of sight distance for crossing maneuvers: on the major road awaiting an opportunity to complete a left-
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TABLE C-2 Median opening accident rate by median opening type
Total median
opening Median opening
accident Median opening accident rate
Median Number frequencya movements (accidents per
opening of median (for entire study (106 turns during 106 turning
type openings period) entire study period) vehicles)
URBAN ARTERIAL CORRIDORS
Midblock
1a 7 1 b b
2b 20 4 17.20 0.23
2c 10 5 13.42 0.37
Three-leg
3a 11 9 2.23 4.04
3b 19 32 13.04 2.46
3c 2 10 1.20 8.35
4a 4 7 4.87 1.44
Four-leg
5a 8 26 11.16 2.33
5b 17 76 22.77 3.34
6a 5 42 16.36 2.57
RURAL ARTERIAL CORRIDORS
Midblock
1a 7 3 0.96 3.13
Three-leg
3a 4 4 4.65 0.86
Four-leg
5a 1 4 1.41 2.84
a
The duration of the study period was generally five years. However, only four years of accident and exposure data
were available for sites in New Jersey, and six years of accident and exposure data were available for sites in
New York.
b
Data too limited to be meaningful.
turn maneuver by waiting for an appropriate gap in opposing are defined as expanded paved aprons opposite a median
traffic to complete their turn. Green Book Exhibit 9-67 pre- crossover. Their purpose is to provide additional space to facil-
sents design ISD criteria for Case F. itate the larger turning path of commercial vehicles along nar-
On divided highways, while the geometry of the roadway row medians. Figure C-19 presents a typical loon design.
may provide sufficient sight distance for left turns from the Where a large truck is used as the design vehicle for a median
major road, some intersections may experience additional opening and a median width of 21 to 31 m (70 to 100 ft) can-
sight-distance concerns, such as: (1) sight obstructions in the not be provided, consideration should be given to provid-
median or (2) opposing left-turn vehicles obstructing the ing a loon.
view of a left-turning vehicle. The sight restrictions created Several unsignalized median openings with loons were
by opposing left-turn vehicles on divided highways can be evaluated as part of this research. No specific problems related
minimized by the use of parallel and tapered offset left-turn to loon operations were noted at these sites. Specifically,
lanes, as shown in Green Book Exhibit 9-98. while median opening Type 2c was found to have a higher
average median opening accident rate than median opening
Loons to Assist Vehicles in Completing U-turn
Type 2a, the individual accident patterns at these sites were
Maneuvers reviewed; it was confirmed that the accidents at median open-
ings of Type 2c did not involve trucks and were not related
A common problem associated with accommodating U-turn to loon usage. Although the sample size is very limited, there
maneuvers at unsignalized median openings is the difficulty of is no indication that provision of loons or their use by large
larger vehicles to negotiate U-turns along cross-sections with trucks leads to safety problems. At the same time, there are
narrow medians. This situation often affects the operation and not sufficient data to determine whether the provision of
safety of commercial vehicles that typically require more loons provides safety benefits.
space in order to perform a U-turn maneuver. One possible Based on a study by Sisiopiku and Aylsworth-Bonzelet
solution to this problem is the construction of a loon. Loons (55, 56), Table C-3 presents recommended loon widths for
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Figure C-18. AASHTO minimum median widths to accommodate U-turns (3).
Figure C-19. Typical loon design at a directional median opening (55, 56).
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TABLE C-3 Recommended loon widths for four-lane divided roadways (55, 56)
Type of design vehicle
P SU BUS WB-12 WB-15 WB-18
Median Length of design vehicle (m)
width 5.7 9.0 12.0 15.0 16.5 19.5
(m) Width of loon (m)
0 5 15 15 15 18 18
5 0 10 10 10 13 13
10 0 5 5 5 8 8
15 0 0 0 0 3 3
20 0 0 0 0 0 0
NOTE: Loon width equal to 0 indicates that the standard shoulder width is sufficient.
four-lane divided roadways. Usually, loons are not needed on presented in the preceding section of these guidelines (see
six-lane divided highways; usually a paved shoulder pro- Figures C-3 through C-17).
vides all the additional width needed for U-turns by large
trucks.
METHODOLOGY FOR COMPARING THE
EXPECTED SAFETY PERFORMANCE OF
MEDIAN OPENING DESIGN ALTERNATIVES
Median Opening Types
This section presents a methodology for comparing the
The four key factors used to classify or describe the design expected safety performance of median opening design alter-
of a median opening are: natives. An overview of the methodology is presented first,
followed by a detailed discussion of the methodology and
· Type of geometry--determines which movements are two examples with sample calculations.
possible at a median opening. Conventional median
openings typically permit all movements, while direc-
tional median openings may restrict certain movements. Overview of Methodology
· Degree of access served--determines what movements
need to be accommodated at a median opening and the The methodology for comparing the expected safety per-
number of potential conflict points a median opening will formance of median opening design alternatives consists of
have. Access points at median openings may include five logically sequenced steps as follows:
either intersecting public roads or driveways.
· Presence of left-turn lanes--indicates whether or not a Step 1: Select median opening design alternatives
median opening has a left-turn lane. Step 2: Enter all turning volumes
· Presence of loons--indicates whether or not a median Step 3: Enter the average non-intersection accident rate
opening is accompanied by a loon. for the major road
Step 4: Calculate the accident rate for each median open-
Based on these four factors, median openings can be clas- ing design alternative
sified based on their design characteristics as follows: Step 5: Compare accident rates for each median open-
ing design alternative
· Type of geometry (traffic movements permitted)
conventional (all movements permitted) Methodology
directional
· Degree of access served Step 1: Select median opening design alternatives
U-turn only (midblock median opening)
access on one side (at three-leg intersection) Typically, highway agencies are faced with selecting the
access on two sides (at four-leg intersection) most appropriate median opening design from a number of
· Presence of left-turn lane alternatives. In Step 1, each of the possible median opening
no left-turn lane present design alternatives are identified. Note that an alternative
left-turn lane present may be an individual median opening design or may include
· Presence of loon a combination of median opening designs. For example, at a
no loon present three-leg intersection, a highway agency may be considering
loon present the following alternatives:
Drawings illustrating each of the median opening types, · conventional median opening
along with their advantages and disadvantages of each, are · directional median opening
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C-22
· directional median opening in combination with a Volume E
directional midblock median opening located near the (Minor-road left turns in Direction 3) = ___ (veh/day)
intersection. Volume F
(Minor-road left turns in Direction 4) = ___ (veh/day)
Alternative 1: ____________________________________ Volume G
(Minor-road throughs in Direction 3) = ___ (veh/day)
Alternative 2: ____________________________________ Volume H
(Minor-road throughs in Direction 4) = ___ (veh/day)
Alternative 3: ____________________________________ Total turning volume = ___ (veh/day)
Step 2: Enter all turning volumes
Step 3: Enter the average non-intersection accident
Turning volumes--actual or estimated--for all possible rate for the major road
turning movements must be provided. Figure C-20 illustrates
all possible turning volumes at a four-leg intersection. If the If any of the alternatives identified in Step 1 represents a
median opening design alternative under consideration is a combination of median opening designs, the average non-
three-leg or midblock median opening, enter a zero for those intersection accident rate for the major road is needed. This
turning volumes that do not apply. may be obtained from actual accident data or may be esti-
Some median openings, such as a three-leg median open- mated based on historical accident experience or engineering
ing or a directional median opening, will include only a por- judgment. In the absence of any reliable site-specific esti-
tion of these turning volumes since not all turning move- mate, NCHRP Report 282 (22) suggests 2.90 accidents per
ments are accommodated. million vehicle-miles is an average non-intersection accident
Enter all turning volumes for the median opening design rate for all four-lane divided suburban highways.
alternatives considered:
Non-intersection accident
Volume A rate for the major road = _________ (acc/MVM)
(Major-road U-turns in Direction 1) = ___ (veh/day)
Volume B
(Major-road U-turns in Direction 2) = ___ (veh/day) Step 4: Calculate the accident rate for each median
Volume C opening design alternative
(Major-road left turns in Direction 1) = ___ (veh/day)
Volume D The accident rate for each median opening design alterna-
(Major-road left turns in Direction 2) = ___ (veh/day) tive consists of at least one of the following components:
Direction 4
Direction 2
H F
A
C
D
B
E
G
Direction 1
Direction 3
Figure C-20. Turning movements at a four-leg intersection.
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C-23
· Accident rate for each individual median opening--For [ ARmr ]acc × (2 × d )
median opening design alternatives that consist of a sin- Accident Rate( acc/yr ) =
10 6 veh - mi
gle median opening, this is the only accident rate that
needs to be computed. For median opening design alter- ×
[TV ]veh × 365 days
natives that consist of a combination of median open- day yr
ings, this accident rate must be computed for each indi-
vidual median opening that makes up the combination. where:
· Accident rate for exposure due to additional travel dis- ARmr = average non-intersection accident rate for major
tance--For combinations of median openings, vehicles road
often travel an additional distance along the major road to TV = sum of all turning volumes traveling additional
complete their turning maneuver. This additional travel distance
distance must be accounted for when computing the d = distance between midblock median opening and
estimated safety performance of the combination of intersection (mi)
median openings. Therefore, the accident rate for the
exposure due additional travel distance must be com- Step 4c: Calculate the total accident rate for each
puted as well. median opening design alternative
The equations for calculating each accident rate are pre- For alternatives that consist of a single median opening,
sented below. Each equation utilizes Table C-4, which pre- the total accident rate is equal to the accident rate of that indi-
sents median opening accident rates by geometry type (con- vidual median opening, as computed in Step 4a.
ventional vs. directional) and number of intersection legs For alternatives that consist of a combination of median
(midblock vs. three-leg vs. four-leg). openings, the total accident rate is equal to the accident rates
for each individual median opening, as computed in Step 4a,
Step 4a: Calculate the accident rate for each individual and the accident rate for exposure related to additional travel
median opening distance, as computed in Step 4b.
The accident rate for each median opening can be esti- Alternative 1: ____________________________________
mated by the following equation: Accident rate (individual median opening) = ___ (acc/yr)
Accident rate (individual median opening) = ___ (acc/yr)
Accident Rate( acc/yr ) =
[ AR]acc × [TV ]veh × 365 days Accident rate (individual median opening) = ___ (acc/yr)
10 6 veh day yr Accident rate (due to exposure) = ___ (acc/yr)
Total accident rate = ___ (acc/yr)
where:
AR = accident rate from Table C-2 Alternative 2: ____________________________________
TV = sum of all turning volumes through median opening
Accident rate (individual median opening) = ___ (acc/yr)
Accident rate (individual median opening) = ___ (acc/yr)
For combinations of median openings, repeat this step for
Accident rate (individual median opening) = ___ (acc/yr)
each individual median opening.
Accident rate (due to exposure) = ___ (acc/yr)
Total accident rate = ___ (acc/yr)
Step 4b: Calculate the accident rate for exposure due to
additional travel distance Alternative 3: ____________________________________
The accident rate for the exposure related to additional Accident rate (individual median opening) = ___ (acc/yr)
travel distance can be estimated by the following equation: Accident rate (individual median opening) = ___ (acc/yr)
TABLE C-4 Median opening accident rates by geometry type and number of
intersection legs at urban sites
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Accident rate (individual median opening) = ___ (acc/yr) vers from the major road onto the minor road and vehicles
Accident rate (due to exposure) = ___ (acc/yr) making U-turns in Direction 2 of the major road. Left-turn
Total accident rate = ___ (acc/yr) vehicles on the minor road and U-turn vehicles in Direc-
tion 1 of the major road must use the directional midblock
Step 5: Compare accident rates for each median opening median opening to complete their turning maneuvers.
design alternative
Step 2: Enter all turning volumes
The final step in the methodology is to compare the accident
rates for each median opening design alternative. If one alter- Enter all turning volumes for the median opening design
alternatives considered:
native has a substantially lower accident rate than the other
alternative(s), that alternative may be a preferable median
Volume A
opening design from a safety standpoint. However, the alter-
(Major-road U-turns in Direction 1) = 25 veh/day
natives should also be compared from an operational stand-
Volume B
point. For example, if one alternative accommodates fewer (Major-road U-turns in Direction 2) = 25 veh/day
turning movements than another alternative, selecting that Volume C
alternative may have a negative impact on the traffic opera- (Major-road left turns in Direction 1) = N/A
tional performance of the arterial corridor. Volume D
(Major-road left turns in Direction 2) = 225 veh/day
Example 1: Comparison of the safety performance of Volume E
conventional and directional median open- (Minor-road left turns in Direction 3) = 225 veh/day
ings at three-leg intersections Volume F
(Minor-road left turns in Direction 4) = N/A
Step 1: Select median opening design alternatives Volume G
(Minor-road throughs in Direction 3) = N/A
Alternative 1: Conventional median opening at a three- Volume H
leg intersection (Minor-road throughs in Direction 4) = N/A
Total turning volume = 500 veh/day
Alternative 2: Directional median opening at a three-leg
intersection in combination with a direc- Step 3: Enter the average non-intersection accident
tional midblock median opening rate for the major road
Figure C-21 illustrates Alternatives 1 and 2. In Alterna- Non-intersection accident
tive 2, the directional median opening at the three-leg inter- rate for the major road = 2.90 acc/MVM
section accommodates vehicles making left-turn maneu- (from NCHRP Report 282)
a) Conventional Median Opening at Three-Leg Intersection (Type 3b)
b) Directional Median Opening For Left Turns From Major Road at Three-Leg Intersection With Directional Midblock Median
Opening (Type 4a + Type 2b)
Figure C-21. Conventional and directional median openings at three-leg intersection.
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C-25
Step 4: Calculate the accident rate for each median 0.23 acc 250 veh
opening design alternative Accident Rate( acc/yr ) = ×
10 6 veh day
365 days
For Alternative 1, only the accident rate for a conventional × = 0.02 acc/ yr
yr
median opening at a three-leg intersection needs to be
computed.
From Table C-2, the median opening accident rate for a
directional midblock median opening is 0.23 accidents per
For Alternative 2, the accident rates for a directional median million turning vehicles. The total turning volume for this
opening at a three-leg intersection, a directional midblock median opening is 250 veh/day, since the midblock median
median opening, and the exposure due to additional travel opening accommodates major-road U-turns in Direction 1
distance need to be computed. and left turns from the minor road (Direction 4).
Step 4a: Calculate the accident rate for each individ- Step 4b: Calculate the accident rate for exposure due
ual median opening to additional travel distance
Conventional three-leg: In this example, the directional midblock median open-
ing is assumed to be located 0.2 mi from the three-leg inter-
The accident rate for a conventional median opening at section. Thus, the accident rate for the exposure related to
a three-leg intersection is computed as follows: additional travel distance is computed as follows:
2.90 acc
Accident Rate( acc/yr ) =
2.69 acc 500 veh
× Accident Rate( acc/yr ) = × (2 × 0.2 mi )
10 6 veh day 10 6 veh - mi
250 veh 365 days
×
365 days
= 0.49 acc/ yr × ×
yr day yr
= 0.11 acc/ yr
From Table C-2, the median opening accident rate for a con-
ventional three-leg median opening is 2.69 accidents per From NCHRP Report 282, the average non-intersection
million turning vehicles. The total turning volume for this accident rate for a four-lane divided highway is 2.90 acci-
median opening is 500 veh/day, since all turning move- dents per million-vehicle-miles of travel. The total num-
ments are permitted. ber of vehicles traveling the additional distance is equal to
the total turning volume through the directional median
opening, which is 250 veh/day.
Directional three-leg:
The accident rate for a directional median opening at a Step 4c: Calculate the total accident rate for each
three-leg intersection is computed as follows: median opening design alternative
Alternative 1: Conventional Median Opening at a Three-
1.40 acc 250 veh
Accident Rate( acc/yr ) = × Leg Intersection
10 6 veh day
365 days Accident rate
× = 0.13 acc/ yr
yr (conventional three-leg) = 0.49 acc/yr
Accident rate
From Table C-2, the median opening accident rate for a (individual median opening) = ___ (acc/yr)
directional three-leg median opening is 1.40 accidents per Accident rate
million turning vehicles. The total turning volume for this (individual median opening) = ___ (acc/yr)
median opening is 250 veh/day, since the only turning Accident rate
movements permitted include left turns from the major (due to exposure) = ___ (acc/yr)
road onto the minor road (Direction 2) and U-turns on the Total accident rate = 0.49 acc/yr
major road in Direction 2.
Alternative 2: Combination of Directional Median Open-
ing at a Three-Leg Intersection and a Direc-
Directional Midblock:
tional Midblock Median Opening
The accident rate for a directional midblock median Accident rate
opening is computed as follows: (directional three-leg) = 0.13 acc/yr
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Accident rate from the major road onto the minor road and vehicles mak-
(directional midblock) = 0.02 acc/yr ing U-turns on the major road. Left-turn vehicles from the
Accident rate minor road approaches must use the directional midblock
(individual median opening) = ___ (acc/yr) median openings to complete their turning maneuvers.
Accident rate (due to exposure) = 0.11 acc/yr
Total accident rate = 0.26 acc/yr Step 2: Enter all turning volumes
Step 5: Compare accident rates for each median open- Enter all turning volumes for the median opening design
ing design alternative alternatives considered:
Alternative 1 = 0.49 acc/yr Volume A
Alternative 2 = 0.26 acc/yr (Major-road U-turns in Direction 1) = 40 veh/day
Volume B
In this example, the combination of directional median (Major-road U-turns in Direction 2) = 40 veh/day
openings (Alternative 2) represents a 47% reduction in Volume C
accident rate over the conventional median opening at a (Major-road left turns in Direction 1) = 180 veh/day
three-leg intersection (Alternative 1). Volume D
(Major-road left turns in Direction 2) = 180 veh/day
Example 2: Comparison of the safety performance of Volume E
conventional and directional median open- (Minor-road left turns in Direction 3) = 180 veh/day
ings at four-leg intersections Volume F
(Minor-road left turns in Direction 4) = 180 veh/day
Step 1: Select median opening design alternatives Volume G
(Minor-road throughs in Direction 3) = 100 veh/day
Alternative 1: Conventional median opening at a four-leg Volume H
intersection (Minor-road throughs in Direction 4) = 100 veh/day
Total turning volume = 1,000 veh/day
Alternative 2: Directional median opening at a four-leg
intersection in combination with two direc- Step 3: Enter the average non-intersection accident
tional midblock median openings rate for the major road
Figure C-22 illustrates Alternatives 1 and 2. In Alternative Non-intersection accident
2, the directional median opening at the four-leg intersec- rate for the major road = 2.90 acc/MVM
tion accommodates vehicles making left-turn maneuvers (from NCHRP Report 282)
a) Conventional Median Opening at Four-Leg Intersection (Type 5b)
b) Directional Median Opening Four-Leg Intersection With Two Directional Midblock Median Openings (Type 2b + Type 6a +
Type 2b)
Figure C-22. Conventional and directional median openings at four-leg intersection.
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Step 4: Calculate the accident rate for each median 0.23 acc 280 veh
opening design alternative Accident Rate( acc/yr ) = ×
10 6 veh day
365 days
For Alternative 1, only the accident rate for a conven- × = 0.02 acc/ yr
tional median opening at a four-leg intersection needs to yr
be computed.
From Table C-2, the median opening accident rate for a
For Alternative 2, the accident rates for a directional directional midblock median opening is 0.23 accidents per
median opening at a four-leg intersection, two directional million turning vehicles. The total turning volume for each
midblock median openings, and the exposure due to addi- median opening is 280 veh/day, since each median open-
tional travel distance need to be computed. ing accommodates left-turn maneuvers from one of the
minor-road approaches.
Step 4a: Calculate the accident rate for each individ-
ual median opening Step 4b: Calculate the accident rate for exposure due
to additional travel distance
Conventional four-leg:
In this example, each directional midblock median open-
The accident rate for a conventional median opening at ing is assumed to be located 0.2 mi from the four-leg inter-
a four-leg intersection is computed as follows: section. Thus, the accident rate for the exposure related to
additional travel distance is computed as follows:
3.01 acc 1000 veh
Accident Rate( acc/yr ) = × 2.90 acc
10 6 veh day Accident Rate( acc/yr ) = × (2 × 0.2 mi )
10 6 veh - mi
365 days
× = 1.10 acc/ yr 560 veh 365 days
yr × ×
day yr
From Table C-2, the median opening accident rate for a = 0.24 acc/ yr
conventional four-leg median opening is 3.01 accidents
per million turning vehicles. The total turning volume for From NCHRP Report 282, the average non-intersection
this median opening is 1,000 veh/day, since all turning accident rate for a four-lane divided highway is 2.90 acci-
movements as well as through movement on the minor dents per million-vehicle-miles of travel. The total num-
road are permitted. ber of vehicles traveling the additional distance is equal to
the total turning volumes through both directional median
Directional four-leg: openings, which is 560 veh/day.
The accident rate for a directional median opening at a Step 4c: Calculate the total accident rate for each
four-leg intersection is computed as follows: median opening design alternative
2.57 acc 440 veh
Accident Rate( acc/yr ) = × Alternative 1: Conventional Median Opening at a Four-
10 6 veh day Leg Intersection
365 days
× = 0.41 acc/ yr
yr Accident rate
(conventional four-leg) = 1.10 acc/yr
From Table C-2, the median opening accident rate for a Accident rate
directional four-leg median opening is 2.57 accidents per (individual median opening) = ___ (acc/yr)
million turning vehicles. The total turning volume for this Accident rate
median opening is 440 veh/day, since the only turning (individual median opening) = ___ (acc/yr)
movements permitted include left turns from the major Accident rate
road onto the minor road and U-turns on the major road. (due to exposure) = ___ (acc/yr)
Total accident rate = 1.10 acc/yr
Directional Midblocks:
Alternative 2: Combination of Directional Median Open-
The accident rate for each of the directional midblock ing at a Four-Leg Intersection and Two
median openings is computed as follows: Directional Midblock Median Openings
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Accident rate Step 5: Compare accident rates for each median open-
(directional four-leg) = 0.41 acc/yr ing design alternative
Accident rate
Alternative 1 = 1.10 acc/yr
(directional midblock) = 0.02 acc/yr
Alternative 2 = 0.69 acc/yr
Accident rate
(directional midblock) = 0.02 acc/yr In this example, the combination of directional median
Accident rate openings (Alternative 2) represents a 32% reduction in acci-
(due to exposure) = 0.24 acc/yr dent rate over the conventional median opening at a four-
Total accident rate = 0.69 acc/yr leg intersection (Alternative 1).
