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Geometric Design

Scope

The scope of work is to design a new interchange between Main Street and Franklin Street in the City of Lake Elsinore. Deliverables for the Franklin Street Geometrics team are to design an interchange that will meet the purpose and need of this project. All existing roads within the project boundary are subject to redesign. New roads and bridges may be designed to appropriately handle traffic between the new interchange and existing communities. The City of Lake Elsinore General Plan is to be accounted for in all designs. Geometrics Approval Drawings will be submitted and include the following: horizontal alignments of the interchange and all new or existing roads in the project design area, vertical alignments, superelevation tables, and typical sections.

Existing Conditions and Constraints

Franklin Street lies just Northwest of Railroad Canyon Road, one of the busiest interchanges in Lake Elsinore. While there is currently no congestion at Franklin Street, the future development of the area combined with the growing population of the city has brought about a need for a brand new interchange. The City General Plan will widen Franklin Street and calls for the design of new roads to handle local traffic.

The new interchange will be approximately one-third of a mile West of the existing Franklin Bridge. Along with the local roads, it will be designed to handle all traffic flow in the area , provide a safe and efficient means for residents to travel within the city by car, bus, or foot, as well as provide safe and easy access to the Interstate 15.

Alternative Consideration

Initial interchange designs were created during a preliminary study of the site. These designs were considered, at the time, as being able to achieve the purpose and need of the interchange design project, and were altered according to specifications shown in the Caltrans Highway Design Manual. The purposes of this project include offering safe transportation options for vehicles, pedestrians, and bicyclists, as well as providing access to the Interstate 15 which accommodates the future growth of the study area. The needs of the project involved evaluation of the City's general plan for the study area. Franklin Street is surrounded mainly by residential facilities. As such, providing access to the highway will help to draw some of the traffic from Railroad Canyon Road to the new interchange. As the Franklin Street site is generally open, there are minimal constraints for geometric designs. Although there were not many constraints, the ones that are applicable include interchange spacing and the presence of a landfill near the design area. In terms of interchange spacing, Franklin Street is located between Main Street and Railroad Canyon Road interchanges. As a result, interchange spacing will be less than the minimum required by Caltrans Highway Design Manual of one mile in the direction of one of the aforementioned interchanges. For the purposes of this project, we designed a new bridge between Franklin Street and Main Street. This gives us ample spacing between Franklin Street and Railroad Canyon Road, but requires a design exception for the spacing between Franklin Street and Main Street interchanges. The reasoning behind the new bridge is to eliminate the design exception for intersection spacing between the designed interchanges and existing intersections. Aside from interchange spacing, there is a nearby landfill that we needed to design around. After the preliminary study of the site was completed, the interchanges that we decided upon are as follows:

  • Tight Diamond Interchange
  • Split Diamond Interchange
  • Partial Clover Interchange
  • Single Point Urban Interchange
  • Roundabout Variations


  • Alternative Development

    After the preliminary study was conducted, more detailed analyses were completed. These include traffic studies and AutoTURN analyses. Traffic studies were completed to determine whether or not the given geometric design would be able to accommodate the projected traffic volumes for the future build of the study boundary. According to the traffic study, geometric designs were altered as needed, and if possible, to allow for the projected traffic volumes to be accommodated. The AutoTURN analysis was done because the site is located on a truck route. The study is done to determine whether or not the alternative is capable of allowing for turning movements for STAA-Long trucks. While these studies were being done, the geometric designs of each alternative were constantly being changed in accordance with the information that was determined during the studies.

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    Tight Diamond Interchange

    The tight diamond interchange is an interchange design that is commonly used. It is characterized by being a very simple design that meets driver expectation quite easily. In the case of this interchange design project, this interchange causes very few problems. For this design project, the ramps connect to a bridge. Since the ramps for a tight diamond interchange are closely spaced, it eliminates the need for a design exception for intersection spacing on the bridge. Unfortunately, this design does not allow for the elimination of the interchange spacing design exception between the Franklin Street and Main Street interchanges.

    Please click the following link to view/download the Design Exceptions Fact Sheet.


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    Split Diamond Interchange

    The Split Diamond Interchange connects two major crossroads into one large interchange. It does this by sharing the on and off ramps located between the crossroads. This is appropriate when the crossroads are closely spaced. In this case, the new proposed bridge is located less than one mile (approximately 0.78 miles) away from the Main Street interchange. This may causing weaving issues between the interchanges as Interstate 15 becomes congested. The Split Diamond eliminates these weaving issues while moving a large amount of traffic volume. It also has the advantage of being expandable with room to add two loop ramps in the future.

    Please click the following link to view/download the Design Exceptions Fact Sheet.


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    Partial Clover Interchange

    The Partial Clover Interchange is another interchange that is rather simple to understand. One of the more attractive attributes of this alternative is that the ramps are located on only one side of the bridge. What this does is increase the distance between the Franklin Street interchange and the Main Street interchange. Despite having ramps that are more spaced out than the tight diamond alternative, there are still no problems with intersection spacing on the newly designed bridge. The design is characterized by two loop ramps on the inner side of the outer ramps. As the design does not incorporate any irregular movements, it easily meets driver expectations.

    Please click the following link to view/download the Design Exceptions Fact Sheet.


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    Single Point Urban Interchange (SPUI)

    The Single Point Urban Interchange or SPUI design is a somewhat different design than the other three. The SPUI acts almost like a tight diamond interchange, but there is one major difference. All four of the ramps converge onto one point. This is where the interchange gets its name from. Since all the ramps converge into one point, that one intersection is controlled by a single traffic light. The notable thing about this type of interchange is that it actually performs better than more conventional interchange designs. The way it does this is that it allows for two movements that would normally not move together to happen concurrently.

    Please click the following link to view/download the Design Exceptions Fact Sheet.


    The fact sheets for each alternative can be found with the descriptions for each alternative shown above. Provided below is a link to the Geometric Approval Drawings (GADs) for each alternative. GADs show information for both horizontal and vertical designs for each intersection. Aside from those designs, the roadway cross section designs are shown as well.