Selection of Appropriate Control
For at-grade intersections, the right-of-way for all users is determined by state law and the type of control in place, which can be uncontrolled (i.e., no regulatory signs or traffic signals), YIELD sign control (including roundabouts), STOP sign control, or traffic signal control. The objective in selecting the appropriate type of intersection control is to provide for the safety of all users while allowing the intersection to operate as efficiently as possible.
According to the Manual on Uniform Traffic Control Devices (MUTCD) Section 2B.04 (Right-of-Way at Intersections), the selection of the appropriate control for a given intersection is to be an engineering judgment dependent upon the following factors:
- Vehicular, bicycle, and pedestrian traffic volumes on all approaches.
- Number and angle of approaches.
- Approach speeds.
- Sight distance available on each approach.
- Crash history.
At the higher end of the intersection control types, the MUTCD provides nine warrants that—if one or more are met—could be used to justify, but not necessarily require, installation of a traffic control signal (see Chapter 4C [Traffic Control Signal Needs Studies]). The MUTCD provides general guidance on the selection of control type at unsignalized intersections as summarized below.
At an uncontrolled intersection, state or local laws consistent with the Uniform Vehicle Code (Section 11-401) specify the following rules for determining right-of-way at uncontrolled intersections:
- The driver of an approaching vehicle must yield the right-of-way to any vehicle or pedestrian already in the intersection; and
- When two vehicles draw near an intersection from different approaches at approximately the same time, the driver of the vehicle on the left is required to yield the right-of-way to the vehicle on the right.
This latter right-of-way rule with respect to vehicles is modified at an uncontrolled T-intersection where the through street has the right-of-way.
The MUTCD does not provide any guidance as to when there is not a need for either a YIELD or STOP sign. It could be assumed that if the conditions suggesting a YIELD or STOP sign are not present, then the intersection could operate without either of these regulatory signs. While not stated in the MUTCD, general conditions for allowing an intersection to remain uncontrolled would be:
- Adequate sight distance along the approach and at the intersection to all other legs of the intersection.
- Very low traffic volume—less than 400 vehicles per day for both connecting roads.
- Residential street network meeting both of the above conditions.
Be advised, however, that the existence of these conditions does not imply that the intersection should be changed to or remain uncontrolled.
YIELD Sign Control
According to the MUTCD (see Section 2B.09 [YIELD Sign Applications]), a YIELD sign may be installed in the following conditions:
- On the approaches to a through street or highway where conditions are such that a full stop is not always required.
- At the second crossroad of a divided highway, where the median width at the intersection is 30 feet or greater.
- For a channelized turn lane that is separated from the adjacent travel lanes by an island, even if the adjacent lanes at the intersection are controlled by a highway traffic control signal or by a STOP sign.
- At an intersection where a special problem exists and where engineering judgment indicates the problem to be susceptible to correction by the use of a YIELD sign.
- Facing the entering roadway for a merge-type movement if engineering judgment indicates the control is needed because acceleration geometry and/or sight distance is not adequate for merging traffic operation.
The first and fourth conditions are broad statements open to interpretation and, therefore, allow variation in their application. Further guidance would be that if the conditions for a STOP sign (below) are not present, and if it is deemed that the intersection should not be uncontrolled, then the YIELD sign may be the most appropriate control.
STOP Sign Control
The MUTCD (see Section 2B.06 [STOP Sign Applications]) states that a STOP sign would be appropriate on the minor-street approaches if it is determined that a stop is always required because of one or more of the following conditions:
- The vehicular traffic volumes on the through street or highway exceed 6,000 vehicles per day;
- A restricted view exists that requires road users to stop in order to adequately observe conflicting traffic on the through street or highway; and/or
- Crash records indicate that three or more crashes that are susceptible to correction by the installation of a STOP sign have been reported within a 12-month period, or that five or more such crashes have been reported within a 2-year period. Such crashes include right-angle collisions involving road users on the minor-street approach failing to yield the right-of-way to traffic on the through street or highway.
Multi-Way STOP Sign Control
Multi-way STOP sign control may be appropriate for locations where the volumes of traffic on the intersecting roads are approximately equal, and it is not readily apparent which is the major through street. In addition, the following criteria are listed in the MUTCD (see Section 2B.07 [Multi-Way Stop Applications]):
- Five or more reported crashes in a 12-month period that are susceptible to correction by a multi-way stop installation. Such crashes include right-turn and left-turn collisions, as well as right-angle collisions.
- Minimum Vehicular volumes:
- An average of at least 300 vehicles per hour for any 8 hours of an average day entering the intersection from the major street approaches (total of both approaches); and
- The combined vehicular, pedestrian, and bicycle volume entering the intersection from the minor street approaches (total of both approaches) averages at least 200 units per hour for the same 8 hours, with an average delay to minor-street vehicular traffic at least 30 seconds per vehicle during the highest volume hour; but
- If the 85th percentile approach speed of the major-street traffic exceeds 40 mph, the minimum vehicular volume warrants are 70 percent of the values provided in Items 1 and 2.
Other criteria that may be considered in an engineering study for multi-way STOP control include the following:
- There is a need to control left-turn conflicts.
- There is a need to control vehicle/pedestrian conflicts near locations that generate high pedestrian volumes.
- Locations where a road user, after stopping, cannot see conflicting traffic and is not able to negotiate the intersection unless conflicting cross traffic is also required to stop; and
- An intersection of two residential neighborhood collector (through) streets of similar design and operating characteristics where multi-way stop control would improve traffic operational characteristics of the intersection.
While essentially a yield-controlled intersection, modern roundabouts are a distinct form of unsignalized intersection. The MUTCD does not provide guidance as to when this type of intersection is appropriate. Comprehensive information on this type of control is found in NCHRP Report 672—Roundabouts: An Informational Guide, Second Edition. This document does not provide guidance on when this type of control would be appropriate but does provide information about its advantages, especially with regard to its safety performance. In many situations, roundabouts are the technically preferred alternative to signal control. FHWA has identified the roundabout as one of nine proven safety countermeasures as documented here.
Mini-roundabouts are a variation of modern roundabouts characterized by a smaller inscribed diameter and traversable center island. While mini-roundabouts are not discussed in the MUTCD, they are described in NCHRP Report 672—Roundabouts: An Informational Guide, Second Edition, as well as FHWA’s Technical Summary: Mini-Roundabouts. The latter document states that this type of roundabout is “best suited to environments where speeds are already low and environmental constraints would preclude the use of a larger roundabout with a raised central island.” They are becoming more common in residential neighborhoods as a form of traffic calming and an alternative to all-way STOP sign control or for traffic calming and speed control purposes. At times mini-roundabouts are installed to break up a long stretch of a straight street and/or to add a landscape element to the street environment.
Residential Traffic Circle
As with mini-roundabouts, residential traffic circles are typically built at the intersections of neighborhood local streets for reasons of traffic calming and/or aesthetics. They typically are operated as two-way or all-way stop-controlled intersections and frequently do not include raised channelization to guide approaching traffic into the circulatory roadway. At some neighborhood traffic circles, left-turning vehicles must turn in front of the central island, potentially conflicting with other circulating traffic.
Over the last 30 years, the City of Seattle, Washington has installed over 1,000 traffic circles on city streets. While they have no stated guidelines for when this type of control is appropriate, they do have a process for evaluating an application from its citizens; see the Seattle Department of Transportation website for more information.