This page is an extraction by Pierre Joubert

from the US FHWA web page entitled Signalized Intersections Informational Guide (FHWA-HRT-04-091) obtainable from http://www.tfhrc.gov/safety/pubs/04091/10.htm


This page is included on this web site because SANRAL's Geometric Design Guidelines does not include it as an option, and because it can be widely applied to speed up traffic flow in Port Elizabeth and elsewhere in RSA, as shown in examples on this web site




See also

the michigan left



CAUTION when viewing these pages, they are written for the right hand rule of the road and tends to be confusing for persons familiar with the left hand rule



Skip to contentUnited States Department of Transportation - Federal Highway Administration Go to TFHRC homeGo to FHWA website


further note by Pierre Joubert

This extraction is from a dissertation by the FHWA on signalised (robot controlled) intersections. The authors of the material that follows are neutral to the concept of the median left turn (right turn in our case), and make a number of references to the use thereof in conjunction with traffic signals, which negates to some extent my purpose in demonstrating this concept. In my case I am presenting the concept as a means of eliminating signals, and wish to emphasise that any attempt to apply this concept should be approached with the first intention being to eliminating traffic signals. If traffic signals were to be retained in any specific case, it should be on the basis of selected functioning using peak / off-peak considerations, or alternatively to substantially lengthen green time on the major route without undue delay to the entering minor route.

I also mention that the concept is not universal in the USA and subject to some controversy, as is alluded to in the web page the michigan left, where it is stated that the concept was first developed in Michigan in 1960. However, examples can be found in London east of Hyde Park, a very busy area worth looking at closely, where some dedicated bus lanes can also be seen. For that matter, examples can also be found in Bogota , Colombia, from where NMBM has obtained guidance on dedicated bus lanes.

Also, the median turn as described here is generally associated with divided highways that have wide medians, where the concept works more easily, in our case Uitenhage Road. However, the concept can be applied where narrow medians exist as will be seen in the proposals I have made for Stanford Road, William Moffat and Buffelsfontein, where "right turn pads" can be employed. Also see reference to fig 89 below.

Re narrow medians, the reference in Fig 89 below is intended to address that question. However, the drawing as presented in Fig 89 is specifically to allow large trucks to turn on narrow medians. It does not show that normal traffic would have to stop for trucks to make the turn. Also the concept as shown will not work correctly for normal traffic because the mandatory weaves cannot function. My sketches for Buffelsfontein, William Moffat and other locations show the concept as working correctly with narrow medians.

Re dedicated bus lanes , the weaving element in indirect turning is a problem and could preclude the advantages of indirect turns from being applied where dedicated bus lanes are proposed

End note by Pierre Joubert




10.2.2 Median U-Turn Crossover

Median U-turn crossovers eliminate left turns at intersections and move them to median crossovers beyond the intersection. For median U-turn crossovers located on the major road, drivers turn left off the major road by passing through the intersection, making a U-turn at the crossover, and turning right at the cross road. Drivers wishing to turn left onto the major road from the cross street turn right onto the major road and make a U-turn at the crossover.

Figure 85 illustrates a median U-turn configuration, and figure 86 illustrates some of the vehicle movements at such an intersection.

Figure 85. Diagram of a median U-turn crossover from the main line. Diagram. The intersection diagram shows a four-lane divided east/west major street with a wide median and a four-lane north/south minor street with right-turn lanes and a narrow median. Two median U-turn crossovers on the major street eliminate left turns at intersections and move them to median crossovers beyond the intersection.

Figure 85. Diagram of a median U-turn crossover from the main line.(adapted from 145)



Figure 86. Vehicular movements at a median U-turn intersection. Diagram. Major street traffic that desires to turn left at the intersection instead travels through the intersection, makes a median U-turn beyond the intersection, and turns right onto the minor street.
(a) Major street movements.
Diagram: Minor street traffic that desires to turn left at the intersection instead turns right, makes a median U-turn, and travels through the intersection on the major street.
(b) Minor street movements.

Figure 86. Vehicular movements at a median U-turn intersection.

The median crossover may also be located on the minor road. In this case, drivers wishing to turn left from the major road turn right on the minor road, and left through the median crossover. Minor road vehicles turn left onto the major road by proceeding through the intersection, making a U-turn, and turning right at the major road. Median U-turn crossovers also may be provided on both the major and minor roads at an intersection.

Median U-turn crossovers are very common in Michigan, and drivers are very familiar with them. They have been in use for more than 30 years, and the signing has evolved to become more user friendly.(141) Figure 87 shows an example of median U-turn signing used in Michigan.

Figure 87. Example of median U-turn signing in Michigan. Photo. The picture shows a direction sign in advance of a U-turn location. The through arrow points north to Business 96 (Larch Street). The U-turn arrow points south to Cedar Street.

Photograph Credit: Lee Rodegerdts, 2002

Figure 87. Example of median U-turn signing in Michigan.

Applicability

Due to the design, median U-turn crossovers require a wide median to enable the U-turn movement. Median U-turns may be appropriate at intersections with high major-street through movements, low-to-medium left turns from the major street, low-to-medium left turns from the minor street, and any amount of minor street through volumes.(141) Locations with high left-turning volumes may not be good candidates because the out-of-direction travel incurred and the potential for queue spill back at the median U-turn location could outweigh the benefits associated with removing left-turns from the main intersection.(141) Median U-turns can be applied on a single approach.

Design Features

Key design features of median U-turns identified in the literature are summarized below:

  • Median U-turn lanes should be designed to accommodate the design vehicle.
  • Appropriate deceleration lengths and storage lengths should be provided based on the design volume and anticipated traffic control at the median
    U-turn.
  • The Michigan Department of Transportation advises that the optimum location for the crossover is 170 to 230 m (560 to 760 ft) from the main intersection as shown in figure 88.(147)
  • To accommodate a semi-trailer combination design vehicle, AASHTO policy recommends that the median on a four-lane arterial should be 18 m (60 ft) wide.(3) If design vehicles do not have enough space to turn, additional pavement should be added outside the travel lane to allow these vehicles to complete the maneuver, as shown in figure 89.
Figure 88. Diagram of general placement of median U-turn crossover.   Diagram. The diagram shows a median U-turn on one side of a major intersection. The note reads that the nose of the crossover must align with the center lane  of the side street. Another arrow points to the intersection and mentions that the optimum directional crossover spacing for signal progression is 200 meters (660 feet) (plus or minus 30 meters (100 feet)) from a major intersection. Another note reads, "The number of crossovers per mile is determined by need." Generally, 200 meters (660 feet) spacing is used in urban areas, and 400 meters (1320 feet) spacing is used in rural areas.

Figure 88. Diagram of general placement of a median U-turn crossover.(adapted from 147)


Figure 89. Diagram of a median U-turn crossover from the main line with a narrow median. Diagram. The intersection shows four legs with four lanes of traffic and a turn lane. The medians are narrow, and the pavement is widened using jughandles to accommodate the turning radius of large vehicles making the U-turn.

Figure 89. Diagram of a median U-turn crossover from the main line with a narrow median.(adapted from 148)

Operational Features

Key items regarding the operational features of median U-turns are summarized below:

  • Median U-turn crossovers allow for two-phase signal operation. This can reduce signal cycle length and delays for through vehicles. Left-turning vehicles have to travel further to complete the turn, which may offset some operational benefits achieved for through vehicles.
  • Signing is needed to alert motorists of the presence of median U-turns and the restriction of left-turn movements at the signalized intersection.
  • Installing traffic signals at median U-turn locations requires additional storage for the U-turn movement and requires coordination with adjacent signalized intersections.
  • The reduction in phases at the signalized intersection improves the ability to coordinate traffic signals along a corridor.

Safety Performance

According to NCHRP 420, the collision rate along road sections having directional median openings (facilitating U-turn and left turns) versus road sections having full median openings (facilitating all movements) was 49 to 52 percent less for signalized corridors having more than one traffic signal per mile.(85)

Table 74 shows the number of conflict points at a four-leg signalized intersection as compared to a four-leg signalized intersection with a median U-turn crossover. A median U-turn crossover configuration eliminates all crossing (left turn) conflict points. It also reduces the number of merge/diverge conflict points as compared to a four-leg signalized intersection. Figure 90 shows the conflict point diagram for a four-leg signalized intersection with a median U-turn crossover configuration.

Table 74. Number of conflict points at a four-leg signalized intersection compared to a four-leg signalized intersection with a median U-turn crossover configuration.

Conflict type

Four-Leg Signalized Intersection

Median U-Turn Crossover Configuration

Merging/diverging

16

12

Crossing (left turn)

12

0

Crossing (angle)

4

4

Total

32

16

Figure 90. Conflict diagram for a four-leg signalized   intersection with median U-turns. Diagram. A median U-turn configured intersection has four crossing angle conflicts and 12 merging/diverging conflicts.

Figure 90. Conflict diagram for a four-leg signalized intersection with median U-turns.

Table 75 summarizes the expert opinion of the authors with regard to the safety benefits of a conversion of a four-leg signalized intersection to a median U-turn crossover configuration.

Table 75. Safety benefits of converting a four-leg signalized intersection to median U-turn crossover configuration: Expert opinion.

Treatment

Surrogate

Implication

Convert signalized four-leg intersection to a median U-turn crossover configuration

Conflict points

Offers the potential for a minor decrease in merging/diverging collisions

Offers the potential for a major decrease in left-turn collisions

Operational Performance

Key elements regarding the operational performance of median U-turns are summarized below:

  • Median U-turns reduce the number of stops for mainline through movements.(141)
  • The median crossovers can be signalized or unsignalized. Signalized crossovers can be synchronized with the other signals in a corridor to provide progression. If a traffic signal is installed at a median U-turn, the median should be designed to accommodate the maximum design queue to avoid spillover to the main line.
  • The operations of a median U-turn should be evaluated using a microsimulation model to determine the effect of progression and queue interaction from the signalized intersection.
  • A study on a Michigan corridor used simulation to compare median U-turn crossovers with two-way left-turn lanes (TWLTL). The study showed that during peak hours, the corridor with median U-turn crossovers had a lower travel time by 17 percent and a 25 percent higher average speed than the same corridor with a TWLTL. However, vehicles made more stops on the arterial with median U-turn crossovers. In nonpeak hours, the median U-turn crossovers had the same efficiency as the TWLTL, even though a higher delay for left-turning vehicles had been expected due to the higher travel distance a vehicle must cover to turn left using a median crossover.(149)
  • Simulation studies using a range of intersection configurations (number of through lanes on the major and minor street) and volumes from intersections in Virginia and North Carolina suggest a reduction in overall travel time for all movements through the intersection when compared to a conventional intersection: -21 to -2 percent during off-peak conditions, and -21 to +6 percent during peak conditions. The studies also show a general increase in the overall percent of stops when compared to a conventional intersection: -20 to +76 percent during off-peak conditions, and -2 to +30 percent during peak conditions.(145)
  • Results from a simulation analysis using TRANSYT-7F and CORSIM found that the percentage of stops was reduced for the median U-turn configuration compared with a conventional intersection.(148)

Multimodal Impacts

Roadways with median U-turns generally have a greater cross section width resulting in an increased crossing distance for pedestrians. The number of movements that conflict with pedestrians at intersections with upstream/downstream median U-turns is reduced.

Turning paths of the median U-turn should be evaluated to ensure that vehicle paths do not encroach on bike lanes.

Socioeconomic Impacts

Access should be restricted on facilities within the influence of median U-turn locations. Local property owners may oppose such restrictions, particularly if the access already exists.

Education, Enforcement, and maintenance

Education and enforcement are needed to ensure that vehicles are not making illegal left turns at the main intersection.

Summary

Table 76 summarizes the issues associated with median U-turn crossovers.

Table 76. Summary of issues for median U-turn crossovers.

Characteristic

Potential benefits

Potential Liabilities

Safety

Potential major reduction in left-turn collisions.

Potential minor reduction in merging/diverging collisions.

None identified.

Operations

Potential reduction in overall travel time.

Reduction in stops for mainline through movements.

Mixed findings with respect to overall stops.

Mixed findings with respect to overall stops.

Multimodal

Number of conflicting movements at intersections is reduced.

Increased crossing distance for pedestrians.

Turning paths of the median U-turn may encroach in bike lanes.

Physical

None identified.

May be additional right-of-way needs depending on width of existing median.

Socioeconomic

None identified.

Access may need to be restricted within the influence of the median U-turn locations.

Enforcement, Education, and maintenance

None identified.

Enforcement and education may be necessary to prevent illegal left turns at the main intersection.



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