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Publication Number: FHWA-HRT-04-091
Date: August 2004
Signalized Intersections: Informational Guide
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CHAPTER 12 — INDIVIDUAL MOVEMENT TREATMENTS
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
12. Individual Movement Treatments
This section identifies treatments for vehicle movements at signalized intersections: left- and U-turn movements, through movements, and right-turn movements. In addition, this section addresses the use of variable lane use. The treatments in this section primarily address the following safety and operational deficiencies:
12.1 Left-Turn Treatments
This section discusses the key safety, operational, and design characteristics associated with left-turn treatments, including the addition of a single left-turn lane, multiple left-turn lanes, and left-turn prohibition.
Left-turning vehicles encounter safety problems from several sources of conflict: pedestrians; bicyclists; opposing through traffic; through traffic in the same direction; and crossing traffic. These conflict types often lead to angle, sideswipe same direction, and rear-end crashes. Left-turn-related crashes typically account for a high percentage of total crashes at an intersection.
The demand for a left-turn movement also affects the amount of green time that can be allocated to additional movements. Operational treatments may be justified to minimize the amount of green time that is allocated to left-turn movements to serve additional critical movements at an intersection.
12.1.1 Add Single Left-Turn Lane
Adding a single left-turn lane at an approach that currently has shared through and left-turn movements is applicable when the delay caused to through vehicles adversely affects the operations and/or safety of an approach. An example is shown in figure 116. A disproportionately high amount of rear-end crashes involving left-turning vehicles followed by through vehicles is an indication that a left-turn lane may be appropriate. Physically separating turning vehicles from the through stream removes slow or decelerating vehicles from through traffic, thus reducing the potential for rear-end collisions. Left-turn lanes also increase the capacity of the approach by adding an additional approach lane; they allow for a wider variety of phasing options. On the other hand, depending on how the left-turn lane is added, the left-turn lane may add distance, time, and exposure for pedestrians and may increase the overall intersection cycle length, adding delay to all users.
The adopted guidelines and practices of local agencies should be reviewed to determine whether left-turn lane warrants are in place for a particular roadway. Key elements that should be considered when determining whether a left-turn lane is warranted include:
In the absence of site-specific data, the HCM 2000 indicates the probable need for a left-turn lane if the left-turn volume is greater than 100 vehicles in a peak hour, and the probable need for dual left-turn lanes if the volume exceeds 300 vehicles per hour.(2) The HCM also indicates a left-turn lane should be provided if a left-turn phase is warranted.
Table 117 highlights several rule-of-thumb intersection capacities for various scenarios where exclusive left-turn treatments may be required on one or both approaches to an intersection. In general, exclusive left-turn lanes are needed when a left-turn volume is greater than 20 percent of total approach volume or when a left-turn volume is greater than 100 vehicles per hour in peak periods.(41)
Table 117. Rule-of-thumb intersection capacities assuming various exclusive left-turn treatments.
Key Design Features
Key design elements of an exclusive left-turn lane include: entering taper, storage length, lane width, and offset. Design criteria for left-turn lanes are presented in the AASHTO a policy on Geometric Design for Highways and Streets as well as in the policies of individual highway agencies.(3)
Entering taper. Entering tapers should be designed to: (1) allow vehicles to depart the through travel lane with minimum braking; and (2) provide adequate length to decelerate and join the back of queue. An appropriate combination of deceleration and taper length will vary according to the situation at individual intersections. A relatively short taper and a longer deceleration length may be applicable at busier intersections where speeds are slower during peak hours. This allows more storage space during peak hours and reduces the potential for spillover into the adjacent through lane. However, off-peak conditions should be considered when vehicle speeds may be higher, thus requiring a longer deceleration length.
AASHTO indicates a taper rate of 8:1 to 15:1 is common for high-speed roadways. Using a taper that is too short may require a vehicle to stop suddenly, thus increasing the potential for rear-end collisions. Using a taper that is too long may result in drivers inadvertently drifting into the left-turn lane, especially if located within a horizontal curve. AASHTO indicates that municipalities and urban counties are increasingly adopting the use of taper lengths such as 30 m (100 ft) for a single-turn lane.(3)
Storage length. The length of the left-turn bay should be sufficiently long to store the number of vehicles likely to accumulate during a critical period so the lane may operate independent of the through lanes. The storage length should be sufficient to prevent vehicles spilling back from the auxiliary lane into the adjacent through lane. Storage length is a function of the cycle length, signal phasing, rate of arrivals and departures, and vehicle mix. As a rule-of-thumb, the left-turn lane should be designed to accommodate one and one-half to two times the average number of vehicle queues per cycle, although methods vary by jurisdiction. The Highway Capacity Manual can also be used to estimate queues, as noted in chapter 7.(2)
Lane width. Lane width requirements for left-turn lanes are largely based on operational considerations. Generally, lane widths of 3.6 m (12 ft) are desirable to maximize traffic flow; however, right-of-way or pedestrian needs may dictate use of a narrower lane width. For situations where it is not possible to achieve the standard width for a left-turn lane, providing a less-than-ideal lane is likely an improvement over providing no left-turn lane. Lane widths less than 2.7 m (9 ft) are not recommended for new design, but in some very constrained retrofit situations on lower speed roadways, lane widths as low as 2.4 m (8 ft) for some left-turning movements may be a better choice than not providing any left-turn lane or having too few left-turn lanes. Achieving more lanes through restriping from 3.6-m (12-ft) lanes to narrower lanes should be considered where appropriate.(50,183) Figure 117 shows an example from Montgomery County, MD, where a narrow left-turn lane has been used effectively.
Offset. A left-turning driver's view of opposing through traffic may be blocked by left-turning vehicles on the opposite approach. When left-turning traffic has a permissive green signal phase, this can lead to collisions between vehicles turning left from the major road and through vehicles on the opposing major-road approach. Offset left-turn lanes position vehicles on approaches further to the left, which removes the vehicles from the sight lines of the opposing left-turners. This helps improve safety and operations of the left-turn movement by improving driver acceptance of gaps in opposing through traffic and eliminating the potential for vehicle path overlap. This is especially true for older drivers who have difficulty judging gaps in front of oncoming vehicles. AASHTO policy recommends that medians wider than 5.4 m (18 ft) should have offset left-turn lanes. One method for laterally shifting left-turning vehicles is to narrow the turn-lane width using pavement markings. This is accomplished by painting a wider stripe or buffer area at the right side of the left-turn lane, which causes left-turning vehicles to position closer to the median. Wider lane lines were implemented at six intersections in Nebraska with positive results.(184) The width of these lines ranged from 150 mm to 900 mm (0.5 ft to 3 ft). The wider the left-turn lane line used to offset vehicles, the greater the effect on improving sight distance.
Offset left-turn lanes should remain parallel to the through travel route. A tapered left-turn design positions a turning vehicle at an angle. If struck from behind, the left-turn vehicle could be pushed into the oncoming through lane. Figure 118 illustrates a positive offset of left-turn lanes at an intersection.(12)
Miscellaneous. In constrained areas, through lanes are sometimes converted to left-turn lanes. In this situation, it is important that through lanes converted to turn lanes do not appear to be through lanes. Making the driver aware of this situation using lane markings and/or signs is important. One study indicates that the incidence of rear-end crashes increases in these situations.(185) The design of the left-turn lane is critical to its effectiveness as a safety or operational improvement strategy.
Figure 118. Example of positive offset.(adapted from 12)
Physical channelization of left turns emphasizes separation of left-turning vehicles from the through traffic stream. It guides drivers through an intersection approach, increasing capacity and driver comfort.
A left-turn channelization design should incorporate consideration of the design vehicle, roadway cross section, traffic volumes, vehicle speeds, type and location of traffic control, pedestrians, and bus stops. In addition to these design criteria, consideration should be given to the travel path; drivers should not have to sharply change direction in order to follow the channelization. Channelizing devices should not cause drivers to make turns with angles that vary greatly from 90 degrees. If median treatments are used to channelized the left turn, pedestrian needs identified in chapter 8 should be considered. Additional guidance is provided in the AASHTO policy.(3)
Channelization can be provided using curbed concrete or painted islands, or delineators. The appropriateness of raised or flush medians depends on conditions at a given intersection. Painted channelization provides guidance to drivers without presenting an obstruction in the roadway, and would be more appropriate where vehicles may be proceeding through the intersection at high speeds. However, paint is more difficult to see at night, especially at intersections that are not lighted.
Raised curbed islands should provide guidance in the intersection area but should not present a significant obstruction to vehicles. Safety advantages of left-turn lanes with raised channelization include:
Raised pavement markings and "flex-post" delineators should be considered when use of raised channelization is not possible.
The type of signal phasing used for a left-turn movement directly affects safety and operational performance of the turn. In general, less-restrictive phasing schemes are preferable where appropriate because they result in lower delay to all users of the intersection.
Table 118 presents suggested guidelines for determining whether left-turn phasing is appropriate, and table 119 presents suggested guidelines for determining the type of left-turn phasing. In addition, table 120 presents the minimum recommended sight distance for permissive left turns. Note that many agencies have adopted guidelines such as these with localized variations to reflect State policy. Examples of deviations include the following:
Table 118. Guidelines for use of left-turn phasing.(186,187)
Table 119. Guidelines for selection of type of left-turn phasing.(186,187)
Table 120. Minimum recommended sight distance for allowing permissive left turns
Installation of a left-turn lane can be expected to decrease rear-end crashes and red light running crashes. NCHRP 279 reports a California study that found a 15 percent reduction in all crashes when left-turn lanes were constructed at signalized intersections without a protected left-turn signal phase, and a 35 percent reduction of crashes when a left-turn phase is provided.(41) A separate study found that the installation of a left-turn lane on one major-road approach at signalized intersections reduces total crashes by 18 percent, and by 33 percent when left-turn lanes are installed on both major-road approaches.(188)
The presence of a left-turn lane could create situations where vehicles are more likely to off-track. Large trucks and buses are more likely to off-track than passenger cars. Off-tracking increases the likelihood of sideswipe and head on crashes between left-turning and adjacent through vehicles and between opposing left-turning vehicles.
In providing left-turn lanes, vehicles in opposing left-turn lanes may block their respective drivers' view of approaching vehicles in the through lanes. This potential problem can be resolved by offsetting the left-turn lanes.
Table 121 shows safety benefits of left-turn geometric improvements. All collision modification factors suggest safety improvements associated with providing a left-turn lane at a signalized intersection. Collision types that would particularly benefit from a left-turn lane are rear-end and left-turn collisions. Provision of a left-turn lane in conjunction with protected left-turn phasing would appear to provide the most benefit.
Table 121. Safety benefits associated with left-turn lane design improvements: Selected findings.
The addition of a left-turn lane increases capacity for the approach by removing left-turn movements from the through traffic stream. The addition of a left-turn lane may allow for the use of a shorter cycle length or allocation of green time to other critical movements.
The additional pavement width associated with the left-turn lane increases the crossing width for pedestrians and may increase the minimum time required for pedestrians to cross. In addition, the wider roadway section likely will increase the amount of clearance time required for the minor street approach. Restriping the roadway with narrower lanes can minimize this problem.
If a left-turn lane is excessively long, through drivers may enter the lane by mistake without realizing it is a left-turn lane. Effective signing and marking of the upstream end of the left-turn lane should remedy this problem.
For cases where widening is required to add a left-turn lane, the crossing distance and conflict area for pedestrians will increase. For wide roadway sections, pedestrian refuges (along with push buttons) should be considered.
The design of a left-turn lane should consider the volumes of truck and bus traffic that would be using the lane.
Addition of a left-turn lane will increase the footprint of the intersection if no median is currently present, except when the approach is restriped with narrower lanes. The approach to the intersection will be wider to accommodate the auxiliary lane.
Designers should also use caution when considering restriping a shoulder to provide or lengthen a left-turn lane. Part of the safety benefits of installing the turn lane may be lost due to a loss of shoulder, less proximity to roadside objects, and a reduction in intersection sight distance. In addition, the shoulder may not have been designed and constructed to a depth that will support considerable traffic volumes and may require costly reconstruction.
The potential reduction in travel time and in vehicle emissions is a benefit of left-turn lanes. A certain degree of comfort is provided to drivers when they are able to wait to turn outside of the through traffic stream, since they are not delaying other vehicles and can wait for a comfortable gap.
The cost of construction and the accompanying signing and striping are one of the main economic disadvantages to installing a left-turn lane. Also, access to properties adjacent to the intersection approach may need to be restricted when a left-turn lane is installed.
Enforcement, Education, and maintenance
Periodic enforcement may be needed to prevent red light running.
Given that left-turn lanes are common at signalized intersections, no education should be needed to prepare drivers for installation of a lane at an intersection.
Maintenance issues for left-turn lanes will be the same as for other areas of the intersection. Pavement markings and signs should be kept visible and legible. Pavement skid resistance should be maintained.
Table 122 provides a summary of the issues associated with left-turn lanes.
Table 122. Summary of issues for left-turn lanes.
* Applies to situations where the left-turn lane is added by physical widening rather than restriping.
12.1.2 Multiple Left-Turn Lanes
Multiple left-turn lanes are becoming more widely used at signalized intersections where traffic volumes have increased beyond the design volume of the original single left-turn lane.
Multiple left-turn lanes can be used to address left-turn volumes that exceed or are expected to exceed a single turn lane. Multiple left-turn lanes allow for the allocation of green time to other critical movements or use of a shorter cycle length.
Double and triple left-turn lanes are appropriate at intersections with significantly high left-turn volumes that cannot be adequately served in a single lane. As a rule of thumb, dual left-turn lanes are generally considered when left-turn volumes exceed 300 vehicles per hour (assuming moderate levels of opposing through traffic and adjacent street traffic). A left-turn demand exceeding 600 vehicles per hour indicates a triple left-turn may be appropriate.
While effective in improving intersection capacity, double or triple lefts are not appropriate where:
The design of multiple left-turn lanes is similar to that of single turn lanes. In addition, the interaction between vehicles in adjacent lanes and also width of the receiving lanes should be considered. The following are design considerations for triple left-turn lanes provided by Ackeret.(191) These same considerations apply for double left-turn lanes:
The previous section provided criteria for selecting the type of signal phasing to be used. In general, protected-only left-turn phasing is used for most double-lane and triple-lane left-turn movements, although some agencies have used protected-permissive phasing for double left turns.
Drivers may be confused when attempting to determine their proper turn path on an approach with multiple left-turn lanes. Providing positive guidance for the driver in the form of pavement markings can help eliminate driver confusion and eliminate vehicle conflict by channeling vehicles in their proper turn path.
Delineation of turn paths is especially useful to drivers making simultaneous opposing left turns, as well as in some cases where drivers turn right when a clear path is not readily apparent. This strategy is also appropriate when the roadway alignment may be confusing or unexpected.
Delineation of turn paths is expected to improve intersection safety, though the effectiveness has not been well evaluated. The additional guidance in the intersection will help separate vehicles making opposing left turns, as well as vehicles turning in adjacent turn lanes.
Additional operational features of dual and triple left-turn lanes are identified below.
A literature review shows that dual left-turn lanes with protected-only phasing generally operate with minimal negative safety impacts. Common crash types in multiple turn lanes are sideswipes between vehicles in the turn lanes. Turn path delineation guides drivers through their lane and can help reduce sideswipes at left-turn maneuvers.
A study of double and triple left-turn lanes in Las Vegas, NV, showed that about 8 percent of intersection-related sideswipes occur at double lefts, and 50 percent at triple lefts.(192) These sideswipes are 1.4 and 9.2 percent of all crashes at the intersections with double and triple lefts, respectively. Turn path geometry and elimination of downstream bottlenecks are important considerations for reducing sideswipes.
One study indicates that triple left-turn lanes have been shown to operate well, and drivers do not have trouble understanding the triple left turns.(193) In addition, construction of triple left-turn lanes has not resulted in unexpected or unacceptable crash experiences. Another study showed that 10 percent of the crashes at intersections with triple lefts occurred in the approach for the triple left. These are angle crashes that occur when left-turning vehicles collide with through traffic on the cross street. These crashes are attributed to short clearance intervals and limited sight distance, not operation of the triple left. Public education of the proper use of triple left turns will be necessary where these are being considered at an intersection.
Table 123 presents selected findings of the safety benefits of multiple left-turn lanes.
Table 123. Safety benefits associated with multiple left-turn lanes: Selected findings.
Multiple left-turn lanes can improve intersection operations by reducing the time allocated to the signal phase for the left-turn movement. Triple left-turn lanes have been constructed to meet the left-turn capacity demand without having to construct an interchange. This configuration can accommodate left-turn volumes of more than 600 vehicles per hour. Vehicle delays, intersection queues, and green time for the left-turn movement are all reduced, improving operation of the entire intersection.
While dual left-turn lanes are largely operated with protected-only phasing, some agencies use protected-permissive signal phasing. This signal phasing improves capacity for the left-turn movements, particularly during nonpeak times when opposing traffic volumes are lower. Many agencies have safety concerns regarding permissive left-turns in a double turn lane. In fact, many agencies only allow dual left-turn lanes to be run as protected-only phasing. However, some agencies overcome this concern by offsetting the dual left turn lanes.
Tucson, AZ, uses protected-permissive offset dual left-turns at approximately 30 intersections. The city has been using this treatment for about 30 years with limited reported problems, and continues to install them where needed. The protected-permissive "offset" dual lefts are used on very high volume city streets (with ADTs exceeding 80,000). The capacity of the left-turn movement increases 75 to 80 percent and left-turn crashes increase only insignificantly with the protected-permissive phasing is implemented. One potential issue is sight distance for the left-turning vehicles. The City of Tucson addresses this concern by offsetting the far lane by 1.2 to 1.5 m (4 to 5 ft) so that it has the same sight distance as a single left-turn lane, enabling drivers to see beyond the opposing left-turn vehicles, as shown in figure 119.(194)
For protected-permissive dual lefts, Tucson, AZ, also uses a lagging left-turn phase operation. The Arizona Insurance information association studied this operation in 2002.(195) The study found that tucson, AZ, had lower crash rates than the leading left-turn operations in the Phoenix, AZ, area, and this benefit was attributed in part to the use of lagging left phases.
On the other hand, in a study of four non-offset intersections with dual left-turn lanes in atlanta, GA, operating with protected-permissive signal phasing, it was shown that this signal phasing needs to be carefully considered.(196) The advantage of increased capacity compared to the disadvantage of increased vehicle conflicts illustrated that this type of phasing may not be appropriate. This study was based on a limited data set, and more sites should be studied to verify these results.
Adding turn lanes increases the crossing distance for pedestrians, as well as their exposure to potential conflicts if roadway widening is required.
Installation of a second or third turn lane will increase the footprint of the intersection, except when additional lanes can be accommodated through restriping. As with single left-turn lanes, right-of-way costs and access to adjacent properties are significant issues to consider.
A shorter green time for left-turning vehicles, made possible by multiple turn lanes, can provide more green time to other movements. As this reduces delay, it will also reduce vehicle emissions.
Enforcement, Education, and maintenance
Little or no education should be needed for multiple left-turn lanes that operate with protected-only or split phasing other than lane assignment signing and markings. Some public information may be needed to educate drivers regarding a permissive movement at a double left-turn lane.
Table 124 summarizes the issues associated with multiple left-turn lanes.
Table 124. Summary of issues for multiple left-turn lanes.
12.1.3 Turn prohibition
Safety and operations at some signalized intersections can be enhanced by restricting turning maneuvers, particularly left turns, during certain periods of the day (such as peak traffic periods) or by prohibiting particular turning movements altogether. Signing or channelization can be implemented to restrict or prohibit turns at intersections.
Prohibiting or restricting left turns should practically eliminate crashes related to the affected turning maneuver. Alternative routes should be analyzed to ensure that crash rates and operational problems do not increase due to diversion of traffic to these alternatives. Also, the benefit of restricting turns may be reduced by an increase in accidents related to formation of queues (rear-end collisions).
U-turning vehicles proceed through an intersection at a slower speed than left-turning vehicles and can have an adverse effect on both operations and safety at the intersection. Prohibition of U-turns may be appropriate at intersections with high volumes for movement with which U-turns interfere. Slower moving U-turning traffic will reduce the capacity of a left-turn movement. Drivers attempting to make a U-turn during a permitted left-turn phase may interfere with opposing through traffic. Rear-end crashes involving U-turning vehicles followed by left-turning or through vehicles may be a sign of operational problems with the U-turn maneuver.
Sight distance limitations should be considered. If opposing left-turning vehicles waiting in a turn lane block a U-turning driver's view of oncoming through traffic, prohibition of
The turning radius of the design vehicle should be accommodated by the combination of the median and receiving lane width. A shorter turn radius will cause slower speeds for U-turning vehicles, and will result in more delay to following vehicles.
Due to the adverse effect U-turns have on intersection capacity and safety, it is sometimes preferable to prohibit U-turns, especially at busy intersections. U-turning vehicles have a greater operational effect on succeeding vehicles than do left-turning vehicles. One study suggests adjusting for U-turns differently from left-turns when determining saturation flow rates of left-turn lanes, to account for their larger effect on operations.(197)
Prohibition of a U-turn is typically implemented with signing. Enforcement may be necessary to ensure the prohibition is obeyed.
Selected findings of the safety benefits for various left-turn operational treatments are presented in table 125.
Table 125. Safety benefits associated with left-turn operational treatments: Selected findings.
Table 126 summarizes the issues associated with turn prohibitions.
Table 126. Summary of issues for turn prohibitions.
12.2 Through Lane treatments
12.2.1 Provide Auxiliary Through Lanes
Auxiliary through lanes (i.e. additional through lanes with limited length) can be added at signalized intersections to provide added capacity for through movements. The amount of added capacity achieved depends on the extent to which through vehicles use the auxiliary lane. Various factors (such as the length of the auxiliary lane, turn volumes, and overall operation of the intersection) contribute to how many vehicles will use an auxiliary lane.
Auxiliary lanes are generally provided on the approaches of a signalized intersection in advance of the intersection and dropped downstream of the intersection. Right-turn traffic may share the outside lane with a portion of the through vehicles, or there may be a separate exclusive right-turn lane. The auxiliary lane also provides an acceleration lane for vehicles turning right from the adjacent approach. Figure 120 illustrates an auxiliary through lane.
Figure 120. Diagram of an auxiliary through lane.(adapted from 199)
Auxiliary lanes are applicable for arterials that have adequate capacity along midblock segments but require additional capacity at signalized intersection locations. The full benefit of an auxiliary lane will not be realized if a bottleneck or constraint exists on the arterial upstream or downstream of the intersection.
The length of the auxiliary lane on both sides of an intersection is a determining factor in whether the lane will be used; longer lanes get more use by through vehicles than do shorter ones.(200) Ideally, the lane should be long enough to allow vehicles turning right off the road to decelerate, and vehicles turning right onto the road to accelerate and merge.
Unless a separate right-turn lane is provided, both through and right-turning vehicles may use the additional lane. More vehicles are likely to use the auxiliary lane if there is not adequate green time to clear the signal from the inside through lane. Using relatively short green times for the approach will clear vehicle queues and likely result in a higher utilization of the outside auxiliary lane.
Based on the subjective assessment of the authors, the safety experience of an intersection with auxiliary through lanes should not be significantly different from conventional intersections without the additional lane. The downstream merge maneuver that this design requires may lead to an increase in merge-related collisions (sideswipes), but studies have not evaluated this.
Tarawneh summarized research performed on auxiliary through lanes and concludes:(200)
Hurley introduces the concept of captive and choice users of an auxiliary through lane.(199) These concepts are described above. Sites were studied in Tennessee to identify the factors that affect the choice of using an auxiliary lane. These factors were:
Wider intersections result in longer crossing times for pedestrians and bicyclists, as well as increased exposure to vehicle conflicts.
Adding an auxiliary through lane will increase the footprint of the intersection if no median is currently present. The approach to the intersection will be wider to accommodate the auxiliary lane.
Driver perception of the benefits of the auxiliary through lane will determine how often the lane is used by through vehicles. If right-turn volumes are high enough that drivers do not benefit from using the lane, capacity of the through movement will not improve significantly.
The cost of construction and the accompanying signing and striping are among the main ecocomic disadvantages to installation of an auxiliary lane. Also, access to properties adjacent to the intersection approach may need to be restricted when another lane is constructed. Property owners affected by the restrictions, especially business owners, may be opposed to the auxiliary lanes.
Enforcement, Education, and maintenance
Auxiliary through lanes do not present any special enforcement issues.
No public education should be needed to inform drivers how to proceed through the intersection. Signs and pavement markings describing the lane arrangements should be sufficient.
Maintenance issues for through auxiliary lanes will be the same as for other areas of the intersection. Pavement markings and signs should be kept visible and legible.
Table 127 summarizes the issues associated with auxiliary through lanes.
Table 127. Summary of issues for auxiliary through lanes.
12.2.2 Delineate through Path
At complex intersections where the correct path through the intersection may not be immediately evident to drivers, pavement markings may be needed to provide additional guidance. The same markings are used to delineate turning paths through intersections for multiple turn lanes. These markings are a continuation of the longitudinal lane stripes, but have a different stripe and skip pattern. An example of these markings is given in figure 121.
Figure 121. Example of delineated paths.
Intersections where through vehicles cannot proceed through the intersection in a straight line may benefit from pavement markings that guide drivers along the appropriate path. Skewed intersections, intersections where opposing approaches are offset, and multileg intersections may all present situations where additional guidance can improve safety and operations.
Delineation of the through path should help reduce driver confusion in the intersection, which will reduce erratic movements as drivers steer into or out of the appropriate path. This would reduce the potential for sideswipe, rear-end, and head-on crashes.
Pavement markings through the intersection should account for off-tracking of large (design) vehicles. The markings should be spaced far enough apart to allow off-tracking without crossing over the markings.
The cost of installing and maintaining the pavement markings should be the only costs of this treatment, and should be similar to that of other pavement markings on the approaches.
Table 128 summarizes the issues associated with path delineation.
Table 128. Summary of issues for path delineation.
12.3 Right-Turn Treatments
The treatments in this section are: addition of a right-turn lane, double right-turn lanes, and a channelized right-turn lane.
12.3.1 Add Single Right-Turn Lane
Significant volumes of right-turning traffic can have an adverse effect on both intersection operations and safety. The deceleration of the turning vehicles creates a speed differential between them and the through vehicles. This can lead to delay for the through vehicles, as well as rear-end crashes involving both movements.
In addition to providing safety benefits for approaching vehicles, right-turn lanes at signalized intersections can be used to reduce vehicular delay and increase intersection capacity.
Figure 122 illustrates the design features of a right-turn lane.
Right-Turn Lane Warrants
Similar to left-turn lane warrants, adopted guidelines and practices from local agencies should be reviewed when determining if a right-turn lane is warranted. Factors that should be considered include vehicle speeds, turning and through volumes, percentage of trucks, approach capacity, desire to provide right-turn-on-red operation, type of highway, arrangement/frequency of intersections, crash history involving right turns, pedestrian conflicts, and available right-of-way.
NCHRP 279 identifies warrants for right-turn lanes on four-lane, high-speed roadways, shown in table 129.(41) These warrants are based on the percentage of vehicles turning right (as a percentage of through vehicles) during the peak period.
Table 129. Right-turn lane volume warrants.(41)
The key design criteria for right-turn lanes are: entering taper; deceleration length; storage length; lane width; corner radius; and sight distance. Design criteria for selecting an appropriate right-turn lane length are presented in a policy on Geometric Design for Highways and Streets as well as in the policies of individual highway agencies.(3)
Entering taper and deceleration length. The entering taper and deceleration length should be determined based on vehicle speed. The length of storage should be designed to accommodate the maximum vehicle queue expected for the movement under design year conditions. From a functional perspective, the entering taper should allow for a right-turning vehicle to decelerate and brake outside of the through traffic lanes. This is particularly important at higher vehicle speeds. In urban areas, this is often difficult to achieve and some deceleration of a turning vehicle is expected to occur in the through travel lane.
Storage length. A right-turn lane should be sufficiently long to store the number of vehicles likely to accumulate during a critical period. The storage length should be sufficient to prevent vehicles from spilling back from the auxiliary lane into the adjacent through lane. At signalized intersections, the storage length required is a function of the cycle length, signal phasing arrangement, and rate of arrivals and departures. As a rule of thumb, the auxiliary lane should be designed to accommodate one and one-half to two times the average number of vehicle queues per cycle, although methods vary by jurisdiction. See chapter 7 for additional discussion regarding methodologies for estimating queue lengths/storage requirements.
In some cases, a right-turn lane may already be provided, but an increase in traffic volumes may necessitate lengthening it, which can help improve operations and safety by providing additional storage for right-turning vehicles. If the length of a right-turn lane is inadequate, right-turning vehicles will spill back into the through traffic stream, thus increasing the potential for rear-end collisions. Longer entering tapers and deceleration lengths can reduce this potential.
Lane width. Lane width requirements for right-turn lanes are largely based on operational considerations. Generally, lane widths of 3.6 m (12 ft) are desirable to maximize traffic flow; however, right-of-way or pedestrian needs may dictate use of a narrower lane width. Achieving more lanes through restriping from 3.6 m (12 ft) lanes to narrower lanes should be considered where appropriate.(50) Figure 123 shows an example from Montgomery County, MD, where a narrow right-turn lane has been used effectively.
Corner Radius. The corner radius influences the turning speed of vehicles. Large corner radii allow vehicles to turn at higher speeds. If low-speed, right-turn movements are desired, particularly in locations where pedestrian crossings occur, the curb radius should be minimized, yet still accommodate the turning path of the design vehicle. Pedestrian crossing distances will be minimized if curb radius is minimized. In addition, lower vehicle speeds can reduce the probability of a crash.
A larger curb radius is appropriate for situations where it is desirable for right-turning vehicles to exit the through traffic stream quickly. The right turn may operate as a free-flow movement if an acceleration lane is provided on the cross street, or the movement may be controlled by a yield sign where the turning roadway enters the cross street.
Increasing the turning radius can reduce the potential for sideswipe or rear-end collisions by reducing lane encroachments as a vehicle approaches a turn and as it enters the cross street. Also, some older drivers and drivers of large vehicles may have difficulty maneuvering; the rear wheels of their vehicles may ride up over the curb or swing out into other lanes where traffic may be present. For situations where a large turning radius is desired, the use of a channelization island may be appropriate to reduce unused pavement area. Unused pavement area contributes to driver confusion regarding the appropriate path through the intersection.
Sight distance. Adequate sight distance should be provided for vehicles in the right-turn lane or channelized right-turn movement. If right turns on red are permitted, drivers turning right should be able to view oncoming traffic from the left on the crossroad.
Right-turn lanes are often used to preclude the undesirable effects resulting from the deceleration of turning vehicles. ITE s Transportation and Land Development indicates that a vehicle traveling on an at-grade arterial at a speed 16 km/h (10 mph) slower than the speed of the normal traffic stream is 180 times more likely to be involved in a crash than a vehicle traveling at the normal traffic speed.(86) Right-turn channelization has been shown to reduce right-turn angle crashes. However, the addition of a right-turn lane may result in an increase in sideswipe crashes. From a vehicular operations standpoint, larger curb radii generally result in vehicle turning paths that are in line with the pavement edge. In addition, larger curb radii produce higher vehicle speeds that can negatively impact the safety of pedestrians and bicyclists.
The provision of right-turn lanes minimizes collisions between vehicles turning right and following vehicles, particularly on high-volume and high-speed major roads. A right-turn lane may be appropriate in situations where there is an unusually high number of rear-end collisions on a particular approach. Installation of a right-turn lane on one major road approach at a signalized intersection is expected to reduce total crashes by 2.5 percent, and crashes are expected to decrease by 5 percent when right-turn lanes are constructed on both major-road approach.(188)
Selected findings of safety benefits associated with various right-turn lane improvements are given in table 130.
Table 130. Safety benefits associated with right-turn improvements: Selected findings.
Right-turn lanes will remove decelerating and slower-moving vehicles from the through traffic stream, which will reduce delay for following through vehicles. Lin concluded that a right-turn lane may reduce vehicle delays substantially, even with the percentage of right-turns as low as 10 percent.(202)
It is possible that installation of a right-turn lane could create other safety or operational problems at the intersection. For example, vehicles in the right-turn lane may block the cross street drivers' view of through traffic; this would be a significant issue where right turns on red are permitted on the cross street. If a right shoulder is restriped to provide a turn lane, there may be adverse impacts on safety due to the decrease in distance to roadside objects. Delineation of the turn lane should be carefully considered to provide adequate guidance through the intersection.
If a right-turn lane is excessively long, through drivers may enter the lane by mistake without realizing it is a right-turn lane. Effective signing and marking the upstream end of the right-turn lane may remedy this.
Also, if access to a right-turn lane is blocked by a queue of through vehicles at a signal, drivers turning right may block the movement of through traffic if the two movements operate on separate phases. This could lead to unsafe lane changes and added delay.
The speed of turning vehicles is a risk to pedestrian safety.
The addition of a turn lane increases the crossing distance for pedestrians and may require additional time for the flashing don't WALK phase. Other issues to consider when designing a right-turn lane include potential conflicts between turning vehicles and cyclists proceeding through the intersection.
Transit stops may have to be relocated from the near side of an intersection, due to possible conflicts between through buses and right-turning vehicles.
Addition of a right-turn lane will increase the footprint of the intersection, unless the shoulder is restriped to create a turn lane. The approach to the intersection will be wider to accommodate the auxiliary lane.
Designers should use caution when considering restriping a shoulder to provide or lengthen a right-turn lane. Part of the safety benefits of installing the turn lane may be lost due to loss of shoulder, the greater proximity of traffic to roadside objects, and a possible reduction in intersection sight distance.
Installing or lengthening a right-turn lane on an intersection approach may involve restricting right turns in and out of driveways on that approach. Techniques include signing or construction of a raised median.
The cost of construction (including relocation of signal equipment) and right-of-way acquisition is the main disadvantage to installation of a turn lane. Also, access to properties adjacent to the intersection approach may need to be restricted when a turn lane is installed.
Enforcement, Education, and maintenance
Periodic enforcement may be needed to prevent red-light violations, especially if right turns on red are prohibited.
Right-turn lanes are common, and minimal education should be needed to prepare drivers for their installation. drivers may need a reminder that they should be watching for pedestrians crossing the departure lanes.
Maintenance issues for right-turn lanes will be the same as for other areas of the intersection. Pavement markings and signs should be kept visible and legible. Pavement skid resistance should be maintained.
Table 131 summarizes the issues associated with right-turn lanes.
Table 131. Summary of issues for right-turn lanes.
12.3.2 Provide Double Right-Turn Lanes
High volumes of right-turning vehicles may support double right-turn lanes to increase capacity for the turns and reduce delay for other movements at the intersection. Double right-turn lanes can reduce both the length needed for turn lanes and the green time needed for that movement.
Approaches with right-turn volumes that cannot be accommodated in a single turn lane without excessively long green times (and delays for other approaches) may be appropriate locations for double turn lanes. Also, locations where right-of-way is not available to provide a long turn lane but there is space for two shorter turn lanes may be ideal for double turn lanes. Clearly, multiple turn lanes are not appropriate where only one receiving lane is available; however, consideration may be given to providing a departing auxiliary lane to allow for double right turns with a downstream merge.
As with single right-turn lanes, the design vehicle should be considered when determining length, width, and taper of the turn lane. The receiving lane should accommodate the turning radius of a large vehicle. Delineation of the turn path will guide drivers through the maneuver and help reduce crossing over into adjacent lanes while turning.
Typically, right turns on red are only permitted on the outside right-turn lane, if at all. NO TURN ON RED signing with appropriate lane-specific legends should be placed in a location visible to drivers (such as overhead), especially those in the inside turn lane.
Based on the subjective assessment of the authors, the safety experience of double right-turn lanes should be similar to that of single right-turn lanes. Rear-end collisions of decelerating right-turn vehicles and following through vehicles may be reduced after construction of the additional turn lane, because the turn lanes have a higher capacity for the slower vehicles. Even though the double turn lanes increase capacity, some deceleration may occur in the through lanes, depending on the length of the turn lanes. This could lead to rear-end crashes.
Sideswipes between turning vehicles are a possibility at double turn lanes. This is especially an issue if the turn radius is tight and large vehicles are likely to be using the turn lanes. Delineation of turn paths should help address this.
Construction of an additional right-turn lane can be reasonably expected to improve the operation of the intersection, provided that the affected right-turn movement is a critical movement. The additional deceleration and storage space should help prevent spillover into adjacent through lanes. Less green time should be needed for right-turn traffic, and this time thus can be allocated to other movements. However, a double turn lane will result in a wider footprint for the intersection and increase the distance pedestrians must cross, which increases their exposure to potential conflicts with vehicular traffic.
Acquisition of right-of-way to provide an additional turn lane may be expensive. If a departure auxiliary lane is to be constructed to allow for a downstream merge, this may also increase right-of-way costs. Access to adjacent properties may need to be restricted to provide a merge area. Owners of adjacent property should be involved in early discussions regarding the plans.
Lane use signing and signs prohibiting right turns on red from the inside turn lane should convey all the information that drivers would need. Periodic enforcement may be needed to ensure drivers obey any right turn on red prohibitions.
Table 132 summarizes the issues associated with double right-turn lanes.
Table 132. Summary of issues for double right-turn lanes.
12.3.3 Provide Channelized Right-Turn Lane
Channelization of the right turn with a raised or painted island can provide larger turning radii and allow for higher turning speeds, and also can provide an area for pedestrian refuge. Figure 124 illustrates a channelized right-turn lane.
Figure 124. Example illustration of a channelized right-turn lane.
Channelized right-turn lanes are applicable for intersections with a high volume of right-turning vehicles that experience excessive delay due to the traffic signal. The larger the turn radius, the higher vehicle speeds can be. An important consideration is the desired speed of the turning vehicles as they enter the crossroad. The turn radius can be used to control speed, especially if the speed varies greatly from the road the vehicle is turning from. Larger turn radii and higher speeds are a safety issue for pedestrians.
A channelized right-turn lane will have a larger footprint than an intersection with a conventional right-turn lane. Additional right-of-way may be needed to accommodate the larger corner radius. Constructing a departure auxiliary lane to allow for a downstream merge may also increase right-of-way costs.
Key Design Features
Channelizing islands can be raised or flush with the pavement. A Georgia study evaluated the effects of right-turn channelization in the form of painted islands, small raised islands, and large raised islands.(203) Results show that traffic islands appear to reduce the number of right-turn angle crashes, and the addition of an exclusive turn lane appears to correspond to an increased number of sideswipe crashes given the introduction of a lane change.
The right turn may operate as a free flow movement if an acceleration lane is provided on the cross street, or the movement may be controlled by a YIELD sign where the turning roadway enters the cross street. Periodic enforcement may be needed to ensure drivers obey any traffic control devices used for the right-turn roadway (such as a YIELD sign).
Visibility of channelizing islands is very important. Islands can be difficult for drivers to see, especially at night and in inclement weather. This is particularly true for older drivers. Raised islands have been found to be more effective than flush painted islands at reducing nighttime collisions, because they are easier to see.
Older drivers, in particular, benefit from channelization as it provides a better indication of the proper use of travel lanes at intersections. However, older drivers often find making a right turn without the benefit of an acceleration lane on the crossing street to be particularly difficult.
A reduction in rear-end collisions involving right-turning vehicles and following through vehicles could be expected after construction of a right-turn roadway. Turning vehicles will not need to decelerate as much as they would for a standard right-turn lane, and therefore the speed differentials between turning and through vehicles would not be as great.
The potential for rear-end and sideswipe crashes on the departure lanes may increase as the vehicles turning onto the crossroad merge with the vehicles already on the road.
Higher speeds and a possibly longer crossing distance and exposure could lead to an increase in crashes involving pedestrians, and the resulting crashes will likely have more serious consequences.
Safety benefits of right-turn channelization are shown in table 133.
Table 133. Safety benefits associated with right-turn channelization: Selected findings.
Through vehicles will experience less delay if right-turning vehicles do not have to decelerate in a through lane. If the volume of right turns is significant enough that the right turn is the critical movement on an approach, provision of a right-turn roadway may increase capacity enough that more green time can be provided for other movements.
Curbed islands offer a refuge for pedestrians. Crossing paths should be clearly delineated, and the island itself should be made as visible as possible to passing motorists.
Right-turn roadways can reduce the safety of pedestrian crossings if an area is not provided for pedestrian refuge. Crossing distances are increased, as is pedestrian exposure to traffic. Elderly and mobility-impaired pedestrians may have difficulty crossing intersections with large corner radii. Right-turn channelization also makes it more difficult for pedestrians to cross the intersection safely, adequately see oncoming traffic that is turning right, and know where to cross. Proper delineation of the turning roadway may help, particularly at night.
Larger turn radii result in higher vehicle speeds. In areas with significant pedestrian traffic, consideration should be given to minimizing the curb radii while still accommodating the turning path of the design vehicle. Minimizing the curb radii will reduce vehicular turning speeds, minimize pedestrian crossing distances, and reduce the potential severity of vehicle-pedestrian collisions.
Access to adjacent properties may need to be restricted to provide a merge area. Owners of adjacent property should be involved in early discussions regarding the plans.
Table 134 summarizes the issues associated with channelized right-turn lanes.
Table 134. Summary of issues for channelized right-turn lanes.
12.4 Variable Lane Use treatments
12.4.1 Provide Reversible Lanes
Reversible lanes are used along a section of roadway to increase capacity without additional widening when flows during peak periods are highly directional. These peak periods could be regular occurrences, as with normal weekday morning and evening peak traffic, or with special events, as with roadways near major sporting venues. Reversible lanes often extend for a considerable length of an arterial through multiple signalized intersections.
According to the MUTCD, reversible lanes are governed by signs (section 2B.25) and/or the following lane use control signals (section 4J.02):(1)
At least three sources provide good information on the implementation of reversible lanes. First, the MUTCD provides guidance on the allowable applications of these lane use control signs and signals, as well as when lane use signals should be used instead of signs. Second, the Traffic Control Devices Handbook provides additional information on signal control transition logic that can be used when reversing the directional flow of a lane or changing a lane to or from two-way left-turn operation.(61) Third, the Traffic Safety Toolbox provides further discussion on planning and implementation considerations, in addition to a discussion of the effects on capacity and safety.(95)
Reversible lanes help reduce congestion and thus are likely to reduce rear-end collisions. As reported in the Traffic Safety Toolbox, "Studies of a variety of locations where reversible lanes have been implemented have found no unusual problem with head-on collisions compared to other urban facilities. Typically, the reversible lanes will have either no effect on safety conditions or will achieve small but statistically significant reductions in accident rates on the facility." (95, p. 130)
Reversible lanes may preclude the use of median treatments as an access- management technique along an arterial street.
Reversible lanes directly benefit operational performance by allowing better matching of the available right-of-way to peak direction demands.
The operation of a reversible lane precludes the use of a fixed median to physically separate opposing travel directions. Therefore, reversible lane operation precludes the use of medians as a refuge area for pedestrians, thus requiring pedestrians to cross the arterial in one stage.
Reversible lanes may postpone or eliminate the need to widen a facility.
Reversible lanes are a relatively low-cost treatment compared to the cost of physically widening a facility.
Table 135 summarizes of the issues associated with reversible lanes.
Table 135. Summary of issues for reversible lanes.
12.4.2 Provide Variable Lane Use Assignments
The concept of variable lane use treatments at signalized intersections is similar to that of the reversible lane but is typically applied locally to a single intersection. Variable treatments change individual lane assignments at a signalized intersection by time of day and thus can be used to accommodate turning movements with highly directional peaking characteristics.
Issues to consider when implementing variable lane use signs include:(50)
Signal phasing requires special attention when using variable lane use signs. While not necessary for all variable lane use operations, split phasing allows any legal combination of lanes to be implemented, provided that the other factors cited above are accommodated. Other techniques that could be used include variable left-turn phasing treatments (e.g., protected-only operation during some times of day, and protected-permissive operation during others).
Figures 125 and 126 provide examples from Montgomery County, MD, where variable lane use signs have been provided for additional left and right turns, respectively. These signs have been employed in conjunction with advance variable lane use signs provided several hundred feet before the intersection. The signs are compliant with the MUTCD, which allows changeable message signs to use the reverse color pattern when displaying regulatory messages (sections 2A.07 and 6F.52).(1) They are reported as being well received by the public and effective in reducing peak-period queuing.(50)
Table 136 summarizes the issues associated with variable lane use.
Table 136. Summary of issues for variable lane use.