233.5 Intersection Alternatives
Alternative intersection configurations may provide advantages over traditional at-grade intersections, or in some cases, even grade-separated interchanges. A given alternative intersection may provide safety, operational, or cost benefits at a specific project location. This article provides policy guidance and planning-level information on three types of potential alternative intersections, as well as traditional at-grade intersections and roundabouts.
This article does not attempt to provide geometric or traffic control guidance for any given intersection type. Appropriate articles within EPG 200 and EPG 900, respectively, shall be used for design guidance on geometric or traffic control elements for specific projects or proposed improvements.
Further analysis of an alternative intersection type at a given location may be required as part of a project scope (EPG 104), guidelines for which are described in EPG 905.
Glossary of Terms for Intersection Alternatives
| AWSC: | All-Way Stop Control |
| TWSC: | Two-Way Stop Control |
| MUT: | Median U-Turn |
| RCI: | Reduced Conflict Intersection |
| RCUT: | Restricted Crossing U-Turn |
| DLT: | Displaced Left-Turn |
| CFI: | Continuous Flow Intersection |
| PDLT: | Partial Displaced Left-Turn |
| XDLT: | Crossover Displaced Left-Turn |
233.5.1 Alternative Intersection Comparisons
233.5.1.1 Alternative Intersection Summary
This article provides guidance, typically in the form of tables, figures, or nomographs, for further evaluation of given alternative intersection types. A range of criteria may be utilized for alternative intersection selection, such as intersection vehicular volumes (on an hourly or daily basis) or vulnerable road user safety considerations.
233.5.1.2 Intersection Selection by Traffic Volumes
The following figures provide potential guidance for alternative intersection selection either by the total entering daily volume for the intersection or by the peak two-way hourly volumes on the major and minor streets at the intersection.
Potential Intersection Control by Total Entering Daily Volume (ADT) provides guidance for generic ADT-based scenarios under which a specific intersection control type could be used. The term “Non-traditional Intersection” refers to intersection types with unique geometrics or flow patterns. Non-traditional intersections considered by MoDOT include: Roundabouts, J-Turns (Restricted Crossing U-Turn Intersection (RCUT)/Reduced Conflict Intersection (RCI)), Median U-Turns (MUT), and Displaced Left Turns (DLT/Continuous Flow Intersection (CFI)).
Intersection Control Type by Peak Hour Volume Thresholds utilizes the peak hour directional distribution splits and two-way traffic volumes for both the major and minor roadways to determine the most appropriate intersection treatment, with a primary focus on traditional intersection control types. All lines shown are defined by the MUTCD 8-hour traffic signal warrant, MUTCD all-way stop warrant, and HCM methods for calculating roundabout capacity and stop-controlled intersection delay. Dashed lines represent an extrapolation of a capacity threshold for a given intersection type and geometric configuration, such as comparing a single-lane roundabout to a multi-lane roundabout.
233.5.1.3 Intersection Selection by Pedestrian and Bicyclist Safety Criteria
The following figures present the optimum feasible intersection configuration choice (per original source research study, not through MoDOT determination) for pedestrians and bicyclists given the number of through lanes and daily traffic volumes on the minor and major streets.
Pedestrian Optimum Feasible Intersection Design
Bicyclist Optimum Feasible Intersection Design
233.5.1.4 Intersection Selection by Vehicular Safety Criteria
The following figures show the intersection type that maximizes safety (per original source research study, not through MoDOT determination) based on various approach configurations and daily roadway volumes. Areas without a specific intersection type listed are either infeasible regions in which the minor street demand exceeds the major street demand, or where the alternative intersection types being considered are not feasible.
233.5.2 Traditional Intersections
233.5.2.1 Traditional Intersection Summary
Traditional intersections, which may also be referred to as conventional or standard intersections, allow direct movements (left, through, and right) on all approaches. These intersections may operate under traffic signal, all-way stop, or two-way stop control.
233.5.2.2 Traditional Intersection Applicability
Traditional intersections may be considered across a wide variety of contexts. Several articles of EPG 900 (Traffic Control) provide specific directional guidelines for when a given traffic control device may be used. Specifically, EPG 903.5.4 provides guidance on stop sign applications, and EPG 902.3 provides guidance for traffic signal installations.
233.5.2.3 Traditional Operational Based Criteria
There are several daily volumetric thresholds at which a given traffic control method at a traditional intersection may become infeasible and require further improvements.
| Maximum Major Street Volume (vpd) | Maximum Total Entering Volume (vpd) | |
|---|---|---|
| Two-Way Stop Control (One Through Lane) | ||
| All-Way Stop Control (One Through Lane) | ||
| Signalized Intersection with Left-Turn Lanes | ||
| Signalized Intersection with Left- and Right-Turn Lanes |
233.5.2.4 Traditional Intersection Advantages and Disadvantages
Advantages
- This commonly used intersection layout leads to familiarity and intuitiveness for all users.
Disadvantages
- Safety issues may arise under certain geometric conditions, such as when slip lanes are included or left -turn visibility is obstructed by opposing movements.
- Two-way stop control is ineffective at serving high minor roadway volumes.
- All-way stop control has the lowest capacity of any intersection type.
233.5.2.5 Traditional Intersection Pedestrian and Other Nonmotorized User Considerations
Under a two-way stop controlled condition, there are no protected movements across the major roadway without supplemental traffic control devices. At signalized intersections, long traffic signal cycle lengths may limit crossing opportunities and lead to poor pedestrian levels of service. At both signalized and unsignalized intersections, the presence of channelizing and median islands may simplify movements for pedestrians by minimizing the number of conflicting movements for the pedestrians.
233.5.2.6 Traditional Intersection Costs and Maintenance
Due to the widespread use of traditional intersections, the costs associated with construction and maintenance are often predictable. Stop controlled intersections are typically low cost, and signal controlled intersections often have reasonably estimated costs. Signalized intersections require additional equipment and timing maintenance.
233.5.2.7 Traditional Intersection Conflict Points
Conflict points for a 4-leg traditional intersection may be viewed in the table below and in the Traditional Intersection Conflict Point Diagram.
| Conflict Point Type | Conflict Points |
|---|---|
| Vehicle-Vehicle - Total | 32 |
| Vehicle-Vehicle - Crossing | 16 |
| Vehicle-Vehicle - Merging | 8 |
| Vehicle-Vehicle - Diverging | 8 |
| Nonmotorized-Vehicle | 24 |
233.5.2.8 Traditional Intersection Additional Resources
EPG 233.2 At-Grade Intersections with Stop and Yield Control
EPG 233.4 At-Grade Intersections with Signal Control
233.5.3 Roundabouts
233.5.3.1 Roundabout Summary
Roundabouts remove direct left-turn movements on all intersection approaches, which typically operate under yield control. All vehicular movements circulate counter-clockwise around a center island. Where appropriate, right-turn bypass lanes may be added for potential capacity or operational benefits. Roundabouts may either be placed at a single intersection or be placed in series along a corridor.
Information included as part of this article does not replace information provided in EPG 233.3, which provides specific guidance and information on roundabouts.
233.5.3.2 Roundabout Applicability
Roundabouts may be applicable where high frequencies of left-turn or right-angle crashes are being experienced, or in some situations where there are heavy traffic delays. Furthermore, roundabouts may work well in situations with non-conventional approach geometry (i.e., skewed intersections, more than four legs, etc.). At some intersections, roundabouts may also be used as an alternative to a traffic signal installation.
233.5.3.3 Roundabout Operational Based Criteria
There are several volumetric thresholds for which a given roundabout type may be viable. One available threshold estimates the hourly sum of the entering flow on a given approach with the conflicting circulating flow, while another threshold approximates the maximum daily capacity of a given roundabout configuration. These thresholds are outlined in both the table below and in Planning Level Practical Estimates for Roundabouts Using Peak Hour Volumes.
| Roundabout Type | Sum of Entering and Conflicting Flows (vph) | Maximum Daily Capacity (vpd) |
|---|---|---|
| Mini | 10,000 | |
| Urban Compact | 15,000 | |
| Urban Single-Lane | 1,000-1,300 | 20,000-25,000 |
| Urban Double-Lane | 1,300-2,300 | 40,000-50,000 |
| Rural Single-Lane | 1,000-1,300 | 20,000-25,000 |
| Rural Double-Lane | 1,300-2,300 | 40,000-50,000 |
233.5.3.4 Roundabout Advantages and Disadvantages
Advantages
- Reduces overall conflict points and eliminates left-turn conflicts.
- Geometry and yield control leads to reduced vehicle speeds and crash severity, especially for fatal/injury crashes as compared to signalized control.
- Provides an opportunity for a transitional zone along a corridor, facilitates access management, and provides traffic calming.
Disadvantages
- Cannot provide explicit priority for specific users without supplemental traffic control devices.
- Increase in single-vehicle and fixed-object crashes as compared to other intersection treatments.
- Implementation of multi-lane roundabouts may create unique challenges, such as path overlap and higher crash rates.
- Multi-lane roundabouts may require supplemental lane markings and wayfinding signage for correct utilization.
- Roundabouts operating near volume/capacity thresholds lose efficiency or may even gridlock.
233.5.3.5 Roundabout Pedestrian and Other Nonmotorized User Considerations
At roundabouts, pedestrians typically only cross one direction of conflicting traffic at a time, and splitter islands provide refuge for two-stage crossings. Multilane approaches to roundabouts may require additional pedestrian protective measures, such as activated signals, beacons, or raised crosswalks. Cyclists may be provided multiple options to navigate through roundabouts based on skill and comfort level. Pedestrian users with visual impairments may have navigational difficulties at roundabouts due to the non-traditional vehicular movements and multi-stage crossings required.
233.5.3.6 Roundabout Costs and Maintenance
Roundabouts have comparable, or sometimes higher, initial geometric costs when compared to a new signalized intersection with auxiliary turn lanes. Some roundabouts may require more right-of-way than a traditional intersection, leading to increased acquisition costs. Additionally, some roundabouts may have costs associated with aesthetics and landscaping maintenance. Roundabouts eliminate the need for ongoing traffic signal equipment, maintenance, and power supply costs.
233.5.3.7 Roundabout Conflict Points
Conflict points for a 4-leg roundabout may be viewed in the table below and in the Roundabout Conflict Point Diagram.
| Conflict Point Type | Conflict Points Roundabout (Traditional Intersection) |
|---|---|
| Vehicle-Vehicle - Total | 20 (32) |
| Vehicle-Vehicle - Crossing | 4 (16) |
| Vehicle-Vehicle - Merging | 8 (8) |
| Vehicle-Vehicle - Diverging | 8 (8) |
| Nonmotorized-Vehicle 8 (24)
233.5.3.8 High Speed Roundabout Design Considerations Roundabouts located on high-speed roadways may require special geometric and safety design considerations. Appropriate design related measures may include, but are not limited to: • Provide a minimum of stopping sight distance to the entry point based on approach operating speed. • Align approach roadways and vertical profiles to make the central island conspicuous with landscaping and sight-blocking amenities. • Extend splitter islands at least 200’ upstream to a point at which entering drivers are expected to begin decelerating. • Use landscaping on extended splitter islands and roadside to create a tunneling effect for approaching vehicles. • Provide roadway illumination in transition to the roundabout. • Use proper signage and pavement markings to advise the appropriate speed and path for approaching vehicles. 233.5.3.9 Roundabout Additional Resources EPG 620.3 Roundabout Markings EPG 903.6.37 Intersection Warning Signs MoDOT Roundabouts NCHRP 1043 Guide for Roundabouts FHWA Roundabouts Level 2 - Issue 1 (Page 17 of |