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Innovative Intersection Safety Improvement Strategies and Management Practices: A Domestic Scan

Chapter 6. Geometric Design Treatments

This chapter presents findings with respect to intersection geometric design treatments that were identified and investigated during the scanning study. Since very few of these treatments have been evaluated by the host agencies, it is not possible to reach a conclusion about their safety effectiveness. Many of these geometric treatments could produce a positive effect on intersection safety, but that will not be known until evaluations are done.

Michigan Indirect Left Turns Junction

After arriving in Michigan for the first set of meetings, several members of the scan team indicated that they did not have much personal experience in negotiating the Michigan Indirect Left Turn Junction, which is a fairly typical design in southeast Michigan. Their initial reaction was that these junctions were markedly different and therefore may be less safe compared to conventional intersections with left turn lanes. Before proceeding, it is necessary to describe a Michigan Indirect Left turn junction. Figure 63 is an aerial photograph of several intersections using the indirect left turns, where no left turns are allowed at the major intersection. As depicted in figure 63, drivers desiring to turn left from the major road (i.e., the road that is laid out horizontally) need to go through the intersection and then make a U-turn at a point downstream. Figure 64 (page 45) illustrates some vehicle movements at such an intersection. For many cases, signal control has been added at the intersection of the U-turn roadway and the directional roadway configuration. Several variations of this junction were encountered in Michigan. In fact, this type of intersection treatment has been in Michigan for well over 30 years. Since this type of intersection was not identified as a key treatment of interest on the original agenda for the scan, there were no prepared questions on this design. However, information was provided subsequent to the visit indicating that this treatment is safe. A Michigan DOT study, The Comparative Accident Experience of Directional and Bidirectional Signalized Intersections,(5) concluded that the operation of an intersection with a pair of stop-controlled directional crossovers where left turns are prohibited at the crossroad carry higher volumes at a lower accident rate than the standard signalized intersection type where all turns are permitted. The study concluded that the use of the Michigan Indirect Left Turn provides the following safety benefits to traditional intersection design and operation: 80 percent reduction in rear-end left-turn crashes and head-on left-turn crashes and 60 percent reduction in right-angle crashes.

Arial photograph of a bi-directional arterial with lanes separated by a median strip. The lanes are connected by a series of cut-throughs allowing drivers to make u-turns or to cross opposing traffic to the left.
Figure 63. Michigan Indirect Left Turn Junction.
Source: AAA Michigan

Diagram showing major and minor street movements at Michigan indirect left turn junctions.
Figure 64. Vehicular movements at a Michigan Indirect Left Turn Junction.
Source: FHWA's Signalized Intersections: Information Guide, Report No. FHWA-HRT-04-091, August 2004, Chapter 10, Alternative Intersection Treatments.(6)

Intersection Bulb-out

In terms of intersection safety, there is still much debate about the safety value of bulb-out intersections. Some can clearly see safety benefits that they offer to pedestrians by reducing the width needed to cross. Others see safety benefits more in terms of traffic calming benefits. There is intrinsic safety value if motorists drive slower. Since bulb-outs are perceived to induce drivers to drive slower, some perceive that safety benefits are accrued from their implementation. However, some question the safety benefits attributable to reduced speed if they produce more potential for rear-end crashes or if the design of the island is inadequate for certain design vehicles or if the design does not explicitly consider the turning radii of larger vehicles. It should be recognized that the safety benefits attributable to intersection bulb-outs are likely to range widely due to site conditions, traffic flows, vehicle mixes, and speeds, among other factors.

Figure 65 depicts a bulb-out that effectively created a landing area for a light rail/trolley car stop in Portland. Note how the parking lane has been created between an edgeline that effectively extends from the face of the curb for the raised bulb-out island. The photograph was taken looking south, approximately 50 feet south of the intersection.

Photograph of a bumped out area of sidewalk, called a bulb-out, that extends to the rails of a trolley line, creating a natural area for a trolley stop.
Figure 65. Intersection bulb-out for a light rail/trolley transit stop in Portland.

Figure 66 presents a photograph of an intersection bulb-out in a residential area in West Palm Beach. Note the evident tire marks on the face of the curb. One observation made by the city's engineering department was to note the telltale items that would indicate potential safety problems. It is conceivable that the turn was too tight or that a large vehicle off-tracked while making a turn.

Photograph of a bumped out (bulb-out) curb with sidewalks that shows signs of rubber marks on the curb.
Figure 66. An example of an intersection “bulb-out” in West Palm Beach, Florida.

Figure 67 presents a residential street where the throat was reduced by bulbing out the intersection corners. The effective street width is wider upstream of the intersection.

Photograph of two bulb-outs at an intersection that serve to narrow the width of the street, creating the shape of a bottlneck at the intersection.
Figure 67. Intersection bulb-outs to reduce the street width opening in a residential area in West Palm Beach.

Figure 68 illustrates an intersection bulb-out island created at an intersection for a commercial area. Note how the edgeline wraps around the bulb-out to define the curvature of the bulb-out and then delineate the parking lane. The transverse line at the bottom left of the photograph is a crosswalk.

Photograph of a bulb-out at an intersection that creates a parking lane on the street that is protected from turning vehicles.
Figure 68. Intersection bulb-outs to define parking lane in a commercial area of West Palm Beach.

An example of a slightly more ornate intersection geometric treatment is illustrated in Figure 69. Again, there is a bulb-out in the corners of the intersection, although it is not as discernible in figure 69. This is a fairly unique treatment since the white “zebra-style” markings at the top of the photo are the crosswalk markings. The brick pavement is not the crosswalk, as one might have originally guessed. The white lines that border the brick pavement are concrete headers on which white markings have been applied. The brick “border” around the intersection is a technique to “define” the intersection space. Figure 70 is a photograph of the same intersection showing how a school crossing warning sign is at the signal-controlled intersection. Barely visible on the right side of figure 71 is a pedestrian crosswalk across the major street. There is no crosswalk across the left leg in figure 70. There is a time and place for traffic calming practices, and clearly they have their greatest application in residential areas. However, the need for the brick “border” should be questioned if it was not meant to be used as the crosswalk. The tire marks on the curbs, which can be seen on figures 67, 70, and 71, raise concerns about the adequacy of the radii. Since there were no before and after speed data nor before and after crash data made available, conclusions could not be made on whether the treatment depicted in figures 69 and 70 resulted in enhanced safety.

Photograph showing a unique bulb-out in the corners of the intersection. At the top of the photo is painted crosswalk striping indicating; a brick paved strip is placed before the crosswalk, but is not intended for pedestrian use. Rather, the brick border around the intersection is a technique used to create visual definition to the intersection.
Figure 69. Illustrative example of an ornamental intersection bulb-out and pavement design.

Wider photograph of the previous photo that includes traffic signal heads, the intersection's in-pavement detector loops, and a school crossing sign in addition to the brick paving placed before the crosswalk markings.
Figure 70. Photograph of same intersection in West Palm Beach showing the traffic signal heads, the loop, the brick pavement, the transverse crosswalk markings and the school crossing sign.

Photograph of an island placed in the middle of a two-lane road at an intersection to separate traffic traveling in opposite directions.
Figure 71. Island implemented in median of a two-lane road at an intersection that serves as a gateway to a corridor in West Palm Beach.

Road Diets (Conversions of Four-lane Undivided to Three-Lane Cross-Sections)

As part of the AAA Michigan Road Improvement Project, road diets, which are a commonly accepted term to define projects where a four-lane undivided cross section was changed into a three-lane cross section, were implemented at several intersections in Detroit and Grand Rapids. Statements made by the host agencies suggest that this treatment is effective in terms of safety, citing results of a study of eight Michigan corridors where the following crash reductions were observed: 25.4 percent in total crashes, 30.1 percent in injury crashes, 36.5 percent in crashes involving 65 years of age or older, and 37 percent in pedestrian and bicycle crashes. (7) However, these considerable observed reductions lacked statistical testing and control group methodology.

Median Treatments

There were several median treatments that were identified by host agencies. In West Palm Beach, Florida, median islands were introduced on several streets just beyond intersections. Figure 71 presents an illustrative example. They were apparently installed for traffic calming purposes, but they also have serves as gateways into corridors. They were not implemented to achieve an improvement in intersection safety but more for traffic calming and aesthetics. Figure 71 is therefore presented to show the median islands that were encountered in West Palm Beach.

Another streetscaping project involved the replacement of a center Two-Way Left Turn Lane (TWLTL) with a raised median. Figure 72 presents a view of the treatment at a three-legged intersection, with the intersecting third leg to the right in the photograph. This treatment intuitively has a greater traffic safety benefit compared to an alternative cross section treatment in which the sidewalk, curb and gutter area are extended to reduce the overall street width. Such an alternative treatment would remove left turn lanes at intersections. Left turn lane pockets have intrinsic safety value.

Photograph of a median separating a four lane road.
Figure 72. Median treatment at intersections in West Palm Beach.

Charlotte, North Carolina, identified a very promising treatment that they thought was effective in terms of safety. The treatment consists of installing a raised channelizing barrier in the median on an undivided roadway as presented in figure 73. The intersection had experienced a relatively high number of crashes. Subsequent investigations revealed that many left turning drivers were turning at various locations to get through gaps in the opposing traffic stream. The device shown below was very effective in forcing drivers to turn left at the preferred location. It is easily applied to the pavement, although there were maintenance concerns after it was first applied. The device itself does not appear to be a hazard.

Narrow raised strip bordering a left turn lane. Strip is about the width of the double yellow line.
Figure 73. Channelizing median device used in Charlotte.

Offsetting left turn lanes enhances intersection sight distance and therefore improves intersection safety. Depending on the width of the median, there are many methods to achieve this. Figure 74 was taken in Wyoming, Michigan. The painted island allows the particular left turn lane in the photo to be pushed farther into the median. By doing the same on the opposite approach, the probability of having a vehicle in one left turn lane pocket block the view of a driver in a vehicle in the other left turn pocket is greatly reduced.

Photograph of a painted island that allows the left turn lane to be pushed farther into the median, reducing the probability of having a vehicle in the left turn lane pocket blocking the view of a driver in a vehicle in the other left turn pocket.
Figure 74. Illustrative example of one method to offset a left turn lane.

Michigan “Loons”

In addition to the Michigan Indirect Left Turn Junction that was described earlier, there is also a variation of that design in Grand Rapids. The treatment is frequently called a Michigan Loon, getting its name from the shape of the pavement. Figure 75a. provides a photo of one and figure 75b. provides an aerial sketch. When the median is too narrow for a Michigan Indirect Left Turn, larger vehicles will have difficultly making the U-turn. The Michigan Loon attempts to solve this problem by widening the outside pavement edge of the opposite direction so that the swept path of a larger vehicle can be accommodated. Grand Rapids indicated that this treatment was effective in improving intersection safety, but documented evaluations could not be found to confirm this. The scan team did believe that this was a notable practice that does positively affect intersection safety.

Photograph of a snowy intersection in the Michigan Loon layout.
Figure 75a. A Michigan “Loon.”

Sketch of a Michigan Loon shows an extended left turn lane that allows vehicles to cross the opposing traffic. This turning traffic enters a broadly curved loop that leads back to the previous intersection, where vehicles may then make a right turn.
Figure 75b. Aerial sketch of a Michigan Loon.

Roundabouts

In the Insurance Institute for Highway Safety May 13, 200 Status Report on Roundabouts, it was found that overall roundabouts produced highly significant reductions on the order of 39 percent for all crash severities combined and 76 percent for all injury crashes. Reductions in the numbers of fatal and incapacitating injuries were estimated to be about 90 percent, based on a study of 24 intersections that had been converted to roundabouts. The Michigan Intersection Safety Action Plan, developed jointly by the Michigan Department of Transportation and DLZ Michigan, which is a consulting firm, includes an action item to promote the design and construction of roundabouts on a trunk highway and to evaluate the effectiveness in both crash reduction and crash characteristics. Figures 76 and 77 present two roundabouts that have been implemented in Michigan. More about roundabouts can be found in Roundabouts: An Informational Guide. (8)

Aerial photograph of a roundabout.
Figure 76. Aerial view of roundabout constructed In Michigan.
(Photo courtesy of Wes Butch, DLZ Michigan, Inc.)

Photo showing a closer view of a roundabout.
Figure 77. View of another roundabout constructed in Michigan.
(Photo courtesy of Wes Butch, DLZ Michigan, Inc.).

Mini-roundabouts

At the end of our meeting with the agencies involved with the Michigan Intersection Safety Action Plan, the traffic engineer from AAA Michigan and the program officer for the Office of Highway Safety Planning identified a junction where a mini-roundabout had been successfully implemented in close proximity to Lansing. Both felt that this was an innovative treatment. The scan team was able to drive to the junction and concurs with the assessment. Figure 78 depicts the central portion of the mini-roundabout.

Photograph of small roundabout in which the round central area is painted on the pavement and painted arrows direct traffic around the circle.
Figure 78. Mini-roundabout in Michigan.

The diameter of the domed middle of the roundabout was approximately 16 feet, and drivers seemed to understand the traffic control devices. Figure 79 depicts one approach to the mini-roundabout. Despite the snow, the word “YIELD” is clearly visible on the pavement, adjacent to the YIELD sign.

Photograph of an approach to a mini-roundabout featuring a channel island, painted road markings and signage on both the left and right side of the approach to direct approaching vehicles to yield.
Figure 79. Approach to mini-roundabout in Michigan.

Figure 80 presents a close-up view of the splitter island, which features a small sign. The island itself is about 3 feet wide at its narrowest and about 7 feet wide at its widest. It is about 17 feet in length. Figure 81 presents an upstream view of the other approach. The yellow warning sign is clearly visible in the foreground. In the background is a yield-ahead warning sign.

Close up photograph of a channel island on the approach to a mini-roundabout with signage on the median directing approaching traffic to keep right.
Figure 80. Detailed view of channelizing island on approach to mini-roundabout.

Photograph of a sign on the approach to a mini-roundabout featuring left curved arrows indicating a counterclockwise direction.
Figure 81. View of warning sign on approach to mini-roundabout.

The scan team felt that this treatment was a safe intersection treatment for a junction that features a significant amount of turning traffic.

Speed Humps and Speed Tables

Both Charlotte and West Palm Beach have speed humps, speed tables, and raised intersections. Raised intersections are a special kind of intersection where the elevation of the entire intersection is raised and the transition between the elevation of the upstream pavement and the elevation at the intersection is discernable by the motorst. This has a traffic calming effect in a manner that drivers slow down as they negotiate the elevated intersections. Figures 82 and 83 provide views of this application at two intersections—note the pedestrian warning sign in figure 83.

Photograph of an intersection with raised pavement.
Figure 82. View of raised intersection/speed table in West Palm Beach.

Alternate photograph of an intersection with raised pavement and a pedestrian crossing sign.
Figure 83. Closer view of raised intersection/speed table in West Palm Beach.

Non-Traditional Intersection Geometric Treatments

Non-traditional intersections have been covered in great detail in Signalized Intersections: Information Guide, published by FHWA. Several non-traditional intersection treatments with potential application in other locations were identified in the study. Figure 84 presents a jughandle intersection design implemented in Bend, Oregon. Left turns are not allowed from the side road onto the major road at the signalized-intersection in the middle of the photo. Drivers can make a left turn maneuver at the junction where the ramps intersect the side road, which is shown vertically in the figure 84. This intersection configuration is a promising geometric treatment, given that it is reducing the number of conflicts in the intersection. It is key to note that similar designs are used extensively in New Jersey.

Photograph of a jughandle intersection design. Left turns are not allowed from the side road onto the major road at the signalized-intersection in the middle of the photo, but  the photo shows that drivers can make a left turn maneuver at the junction where the ramps intersect the side road, which is shown vertically. The shape of the design is similar to a figure 8.
Figure 84. At-grade intersection with jug-handle ramps in two quadrants located in Bend, Oregon.

Figure 85 presents a fairly unique intersection treatment that has been implemented at other intersections in the downtown area of West Palm Beach. The treatment consists of flush gutter, brick pavements, and crosswalks defined in different brick colors and styles. Photographs of another intersection with similar geometrics are presented in figure 86.

Photograph of an intersection with treatments consisting of a flush gutter, brick pavements, and crosswalks all defined in different brick colors and styles to delineate each elements visually.
Figure 85. Intersection in West Palm Beach.

Photograph of an intersection similar to the previous one, with treatments consisting of a flush gutter, brick pavements, and crosswalks all defined in different brick colors and styles.
Figure 86. Illustration of alternative intersection.

A close examination of figures 85 and 86 reveal that the sidewalk blends into the curb and that the intersection has the appearance of a plaza. This treatment has application only in highly developed areas at the intersections of roads with lower speed and lower volumes. The primary goal of these geometric treatments is improved aesthetics. It must be noted there is a lack of knowledge of the safety effects of these treatments, and there are potential safety challenges attributable to the lack of retroreflective pavement markings for the crosswalks and stop lines. Jurisdictions should exercise engineering judgement on the use of this treatment when compared to the need to enhance safety.

In terms of intersection design, one of the first streets retrofitted in West Palm Beach for traffic calming was Clematis Street, which is cited as an illustrative landmark example of new urbanism. Figures 87 and 88 provide photos of Clematis Street in the midblock area. It was here that traffic calming was tailored-made in applicability. Figure 89 presents the intersection of Clematis and Narcissus Streets, which essentially is a continuation of a plaza. Figure 90 depicts the intersection of Narcissus and Datura Streets. In all of these intersections, the design is very similar—an ornate brick pattern, flush corners with no curbs. In the last two figures, small bollards are visible. Although clearly these are different intersection designs, the applicability based on safety concerns are limited.

Photograph of an intersection that features bulb-outs to bracket parking areas, crosswalks connecting the bulb-outs to the opposite side of the street, brick patterns in the sidewalks, and flush corners with minimal curbs.
Figure 87. Photograph of Clematis Street in West Palm Beach, looking west.

Photograph of the intersection shown in the previous photo from the opposite direction; again, features include bulb-outs to bracket parking areas, crosswalks connecting the bulb-outs to the opposite side of the street, brick patterns in the sidewalks, and flush corners with minimal curbs.
Figure 88. Photograph of Clematis Street in West Palm Beach looking east.

Photograph of a T-intersection featuring ornate brick pavement and crosswalks of different patterns and colors of brick, but with no curbs to separate pedestrian sidewalks from the roadway.
Figure 89. Plaza at east end of Clematis Street.

Photograph of an intersection at a downtown plaza area featuring ornate brick pavement and crosswalks of different patterns and colors of brick. Flush curb corners feature low cement bollards to protect pedestrians at the edge, but no curbs to separate pedestrian sidewalks from the roadway at the intersection.
Figure 90. Intersection of Narcissus Street and Datura Street in West Palm Beach.

Charlotte, North Carolina, also identified several sites where innovative geometric treatments were installed. Figure 91 presents an aerial photograph view of an intersection before it was reconstructed. The geometry is somewhat complicated in that the main road splits into a one-way pair. A large medical center is located in the lower right corner. Consequently, traffic turning left from the top to the right is heavy.

Aerial photograph of an intersection where the main road splits into a one-way pair.
Figure 91. Aerial view of intersection in Charlotte prior to implementation of geometric treatment. (Courtesy of Charlotte DOT).

Charlotte developed a fairly comprehensive design for the intersection. At the time of the scanning study, construction was about completed with the exception of an accessible curb cut ramp and the construction of relocated sidewalk. The finished design is illustrated in figures 92, 93 and 94. The project included many notable items, such as brick sidewalks, raised concrete islands, LED signals, pedestrian signal heads, and street lighting.

Photograph of a new intersection featuring brick sidewalks, raised concrete islands, LED signals, pedestrian signal heads, and street lighting.
Figure 92. Photograph of Charlotte intersection after improvement.
(Courtesy of Charlotte DOT)

Two photographs showing alternate views of a brick crosswalk with a refuge island.
Figure 93. Two photographs of pedestrian refuge at an intersection in Charlotte after improvement.
(Courtesy of Charlotte DOT).

Diagram of modifications to a complex intersection. Treatments include bulbing out one corner and extending the median to create a median refuge for pedestrians. The design also notes plans to install a pedestrian signal head and set of push buttons in this median island.
Figure 94. Design plan showing geometric modifications and changes to pedestrian crossing at a somewhat complex intersection in Charlotte.
(Courtesy of Charlotte DOT).

One final example of innovative intersection geometric design is illustrated in the design plan presented as figure 94. The building on the far side of the photograph is part of a college campus. Hence, there were significant pedestrian volumes crossing at this location. The challenge was how to better accommodate the movement of pedestrians given the intersection site constraints. As seen in figure 94, the project consisted of bulbing out one corner and extending the median to create a median refuge for pedestrians. A decision was made to install a pedestrian signal head and set of push buttons in this median island and to reroute the pedestrian crosswalk to reduce the exposure of pedestrians to traffic.

Photographs of the intersection after construction was completed are presented in figures 95, 96, and 97.

Photograph of crosswalk and refuge island with pedestrian push button leading to a college lecture hall.
Figure 95. Depiction of completed intersection geometric treatment implemented in Charlotte.
(Courtesy of Charlotte DOT).

Close-up of previous photo featuring crosswalk and refuge island.
Figure 96. Close-up of crossing and median refuge island.
(Courtesy of Charlotte DOT).

Photograph of new bulb-out that serves as a pedestrian landing.
Figure 97. Close up of larger concrete landing in one corner.
(Courtesy of Charlotte DOT).

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