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5 Introduction Before installing a left-turn lane (or any other roadway improvement), it is necessary to consider the characteristics of the location where it would be installed. These characteristics guide the practitionerâs decisions about whether to install the lane and what specific design criteria need to be emphasized to optimize the operation of the lane at that location. This chapter sets forth guidelines for establishing and applying decision cri- teria for providing left-turn lanes at unsignalized intersections. These guidelines can be useful to transportation agencies in planning new roadways and upgrading existing facilities. The information in this chapter is closely related to that presented in Chapter 3 because much of the information nec- essary to make the fundamental decision to install a left-turn lane is also used in the subsequent decisions about the details of the design of the lane. Therefore, the practitioner should consider the material in that chapter in conjunction with the following sections when making decisions early in the pro- cess of planning and designing a left-turn lane installation. Assemble Basic Information There are some common reasons that practitioners consider the installation of a left-turn lane to improve an intersection. ⢠Speeds are too high to safely make left turns to or from a particular roadway. ⢠There is a trend or pattern of crashes involving left-turning vehicles, or rear-end or sideswipe/weaving crashes as through vehicles interact with queued vehicles. ⢠Drivers have to wait a long time to make a left turn. ⢠There are a high number of left-turning vehicles. The thresholds that practitioners apply to these situations (e.g., what speed is âtoo highâ) can vary depending on local or state guidelines, previous experience with left-turn lanes in that area, and input from local stakeholders. In addition, the per- ception of these issues (whether real or imagined) also can ini- tiate a review of an intersection, particularly when it comes to speed and/or crashes. As a result, basic information needed to assess the validity of those issues is commonly collected at the start of the planning process. In addition to the items discussed below, information also is needed regarding other conditions in the area, such as how left turns are treated at other locations along the corridor in question and the spacing between a loca- tion under study and upstream and downstream traffic signals. The basic information needed for use with the developed left-turn lane warrants includes: ⢠Development (rural or urban/suburban), ⢠Number of lanes on the major roadway (two or four), ⢠Number of approaches (three legs or four legs), ⢠Peak-hour left-turn lane volume (left-turn vehicles per hour), and ⢠Major roadway volume (vehicles per hour per lane). Other information traditionally used in traffic engineering studies includes: ⢠Volume, ⢠Speed, ⢠Crashes, and ⢠For selected locations delay and/or gap acceptance. Basic Information The basic geometry of the intersection needed for use with the warrants is the number of lanes on the major roadway and the number of approaches to the intersection. The num- ber of approaches and the development type (rural or urban/ suburban) are included in the warrants because the crash prediction methodology used to develop the warrants varied by these features. Rural crash prediction equations vary by C h a p t e r 2 Planning and Design Process
6number of lanes on the major roadway, so the warrants for rural highways also vary by number of lanes. Volume The peak-hour left-turn volume and major road volume are needed for use in the left-turn warrants. Quantifying the overall traffic volumes at a candidate intersection can provide a better understanding of the condi- tions at a site. An aggregate volume count can be conducted with a speed study if automated traffic counters are used. The two studies can then cover the same amount of time and be reviewed for patterns over time of day or day of the week. However, it is also important that these volumes are identi- fied not only by time of day, but by turning movement. A com- prehensive turning-movement count quantifies the number of vehicles on each leg of the intersection that turned left, turned right, or proceeded straight through the intersection. These turning-movement counts must be done manually or be collected by video and later manually reviewed. While this type of count is more resource intensive, it provides the infor- mation needed to know what the left-turn demand is on each approach, and it also identifies the opposing traffic volumes through which turning drivers must complete their turning maneuvers. When analyzing the existing counts, it is impor- tant to consider growth in future left-turn demand associated with plans for future development. If the lane is being considered because of a particular traf- fic generator (e.g., a sporting or concert venue or a seasonal event), then the volume data should be collected when that generator is expected to produce the traffic being considered. Speed A speed study to determine the prevailing speeds at a partic- ular intersection is a straightforward way to evaluate the effect of speeds on turning movements. Commonly, these speeds are collected through the use of automated traffic counters, usu- ally deployed at selected spot locations on each approach (and perhaps departure) leg of the intersection. Alternatively, radar or laser speed devices can be used to collect a sample of spot speeds for a particular period of time, such as the peak hour. Detailed procedures on conducting spot-speed studies can be found in other sources (2). If concerns exist about chang- ing conditions, speed data can be collected on several days or through the course of a week to identify speed patterns over time. An entire week allows the practitioner to see trends on weekdays versus weekends, day versus night, and other time- sensitive comparisons that could influence the decisions related to the left-turn lane. The result of such a study is a spreadsheet of data show- ing basic speed statistics (e.g., average, minimum, maximum, 85th percentile) for the entire study period as well as other divisions of time (e.g., 24-hour periods, 6-hour periods, 1-hour periods for peak times). These statistics help illustrate patterns and trends in operating speeds at the intersection, which can be used to make informed decisions about the design of the turning lane. Crash History If an intersection has a problem (or a perceived problem) with left-turn-related crashes, then a crash study is necessary to determine the extent of the problem. A review of recent crash reports can provide insight into the nature of the prob- lem and how a left-turn lane may be a suitable countermea- sure. For example, if crashes are occurring predominantly on one leg of the intersection, then it may be necessary to install a left-turn lane only on that leg, rather than on multiple legs. Left-turn-related crashes may include rear-end crashes at or near the intersection or driveway as well as sideswipe crashes, especially for multilane streets. Often a crash study considers the previous 12 months, though a 24- or 36-month study may be useful, particularly if the practitioner desires to estimate the effects of adjacent development or other recent changes at that location. For example, if crashes increased after the completion of a new housing development, then the intersection may need a left- turn lane, a need that did not exist prior to its completion. Depending on the jurisdiction or the classification of the roadway, there may be an electronic database of crash data that can be searched for information. Use of the electronic database can be helpful in searching through a large num- ber of crashes in a short period of time; however, electronic records typically do not contain the level of detail provided by printed copies of the law enforcement officersâ original reports. The full report shows a diagram of the intersection that includes the movements of vehicles and locations of key objects. The report also contains a narrative of the sequence of events leading to the crash, as the officer states it in his or her own words. The details provided by the diagram and the narrative are extremely valuable in identifying patterns related to crashes and crash history at a given intersection. Delay A study of the delay and/or gap acceptance at an intersec- tion can provide useful information to determine if there is a potential problem that a left-turn lane can address. If left- turning drivers must wait a lengthy amount of time to com- plete their turns, it can lead to further delay for the drivers waiting in the queue behind them, whether they intend to turn or travel straight through the intersection. A left-turn lane would provide storage for turning vehicles and remove impediments to through vehicles. A delay study looks at the time it takes vehicles to travel through the intersection under prevailing conditions and
7 compares it to the amount of time it would take the same vehicles to travel through the intersection under free-flow conditions. A discussion of delay and gap acceptance studies for two-way and all-way stop-controlled intersections, as well as the recommended procedure and equations to use, can be found in the Highway Capacity Manual (3). It is important to review delay in the context of crashes. Sites with a left-turn crash problem may have high delay, but sites without a crash problem may also have delay because increasing delay can be a precursor to an increasing crash rate. As drivers wait longer to turn, they can become impa- tient and attempt turns when it is not actually safe to do so. Establish and Apply Decision Criteria While the practitioner is still early in the planning and design process, the criteria for making decisions about left- turn lane installation need to be defined. In other words, what factors are important for determining the characteristics that the lane needs to have? Some of these factors depend on the issues identified in the previous section; whether some combination of these factors is an influence on the site will partially determine what other factors need to be considered when planning and designing the new lane. In addition to the basic contributors, other factors might be: ⢠Access to adjacent development, in conjunction with access management considerations and requirements; ⢠Available right-of-way; ⢠Existing roadway width; ⢠Safety or crash history; ⢠Consistency with nearby intersections; ⢠Sight distance restrictions; ⢠Speed differential concerns; ⢠Pedestrian traffic; ⢠Existing or proposed medians; and ⢠Available budget. Decisions relating to left-turn installation, particularly for a left-turn lane into a new development, may be governed by access management provisions of the agencies with jurisdic- tion. In order to preserve mobility and provide safety for the traveling public, many transportation agencies have estab- lished regulations and programs to manage access to their roadway network. For access to a new development, state and local agencies typically use access permitting to apply access management standards to guide decisions regarding where and what access would be allowed as well as any restrictions to this access. In general, the regulations are more restrictive for major arterials, the roadways intended to accommodate higher volumes and speeds. These factors must then be considered in conjunction with the corresponding geometric design criteria and applicable policies on planning and access management to make appro- priate decisions about specific characteristics of that particular left-turn lane. For example, on an intersection approach under consideration for a left-turn lane, there is a driveway upstream of the intersection where left turns also commonly occur. Vehicles turning into that driveway may also queue in the same through lane and cause confusion both to through drivers and to drivers intending to turn left at the intersection. In order to improve operations for through vehicles and reduce the likeli- hood of delay or rear-end crashes as through vehicles approach queued vehicles turning into the driveway, it may be desirable or preferable to provide a left-turn lane at this location as well. The spacing between the intersection and driveway will be a major consideration in establishing the appropriate treatment. An option may be to extend the left-turn lane upstream from the intersection to the driveway to provide storage for all left- turning vehicles. However, it may also be prudent to completely restrict left-turn access to that driveway through the use of a raised median, where possible, in order to reduce confusion among left-turning drivers; prohibiting turns into the drive- way would prevent the formation of two intermingled queues (one for the driveway and one for the intersection) within the same lane, and it would eliminate the subsequent potential for âjockeying for positionâ within the left-turn lane and the adja- cent through lane as drivers turning at the intersection try to avoid the queue for the driveway. The practitioner should identify and list all such influ- encing factors and document them as decisions are made throughout the planning and design phases of the installa- tion. The decision flowchart shown in Figure 1 is an example YesNo YesNo Yes No No Yes Are left-turn movements permitted at this location? Do not install Do access management guidelines require a left- turn lane? Install if spacing is adequate Does providing a left-turn lane achieve design consistency? Install Would installing a left- turn lane produce reductions in crashes and delay that exceed the costs of the lane? (See Table 1 or Table 2 for warrants.) Do not install Install Figure 1. Example decision flowchart for installation of left-turn lane.
8of the steps a designer could take to determine whether a left- turn lane is appropriate for a particular location. Where there are no applicable access management guidelines, adequate spacing and design consistency are both essential require- ments to consider. Apply Left-Turn Lane Warrants Warrants After compiling all of the relevant information pertain- ing to a particular intersection, it is necessary to determine whether that information indicates that a left-turn lane is indeed necessary or beneficial. Left-turn lanes can reduce the potential for collisions and improve capacity by remov- ing stopped vehicles from the main travel lane. The recom- mended left-turn lane warrants developed based on the NCHRP Project 3-91 research (1) are: ⢠Rural, two-lane highways (see Table 1), ⢠Rural, four-lane highways (see Table 2), and ⢠Urban and suburban roadways (see Table 3). Table 1 also present warrants for a bypass lane treatment on two-lane rural highways. Given a peak-hour left-turn vol- ume and a particular intersection configuration (i.e., number of legs, number of lanes on the major highway), the tables show the minimum peak-hour volume on the major highway that warrants a left-turn lane or bypass lane. Figure 2 displays the warrants for rural two-lane highways graphically. Figure 3 shows graphical warrants for four-lane rural highways, and Figure 4 shows the recommended warrants for urban and suburban arterials. Technical warrants are an important element of the decision-making process; however, other factors should also be considered when deciding whether to install a left-turn lane, including: ⢠Sight distance relative to the position of the driver and ⢠Design consistency within the corridor. These factors should be considered in conjunction with the numerical warrants. For example, if volumes indicate that a left- turn lane is not warranted but there is insufficient sight distance at the location for the left-turning vehicles, then the left-turn lane should be considered along with other potential changes (e.g., remove sight obstructions, realign the highway, etc.). Source of WarrantsâBenefit-Cost Approach A benefit-cost approach was conducted as part of NCHRP Project 3-91 (1) to determine when a left-turn lane would be justified. Economic analysis can provide a useful method for combining traffic operations and safety benefits of left-turn lanes to identify situations in which left-turn lanes are and are not justified economically. The development steps included: ⢠Simulation to determine delay savings from installing a left-turn lane, ⢠Crash costs, ⢠Crash reduction savings determined from safety perfor- mance functions available in the AASHTO Highway Safety Manual (Chapter 10 discusses rural two-lane, two-way roads; Chapter 11 discusses rural multilane highways; and Chapter 12 discusses urban and suburban arterials) (4), Table 1. Recommended left-turn treatment warrants for rural two-lane highways. Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Two- Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Bypass Lane Three-Leg Intersection, Major Two- Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Left-Turn Lane Four-Leg Intersection, Major Two- Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Bypass Lane Four-Leg Intersection, Major Two- Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Left-Turn Lane 5 50 200 50 150 10 50 100 < 50 50 15 < 50 100 < 50 50 20 < 50 50 < 50 < 50 25 < 50 50 < 50 < 50 30 < 50 50 < 50 < 50 35 < 50 50 < 50 < 50 40 < 50 50 < 50 < 50 45 < 50 50 < 50 < 50 50 or More < 50 50 < 50 < 50
9 Table 2. Recommended left-turn lane warrants for rural four-lane highways. Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Four-Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Left-Turn Lane Four-Leg Intersection, Major Four-Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Left-Turn Lane 5 75 50 10 75 25 15 50 25 20 50 25 25 50 < 25 30 50 < 25 35 50 < 25 40 50 < 25 45 50 < 25 50 or More 50 < 25 Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Urban and Suburban Arterial Volume (veh/hr/ln) That Warrants a Left-Turn Lane Four-Leg Intersection, Major Urban and Suburban Arterial Volume (veh/hr/ln) That Warrants a Left-Turn Lane 5 450 50 10 300 50 15 250 50 20 200 50 25 200 50 30 150 50 35 150 50 40 150 50 45 150 < 50 50 or More 100 < 50 Table 3. Recommended left-turn lane warrants for urban and suburban arterials. (a) Three Legs (b) Four Legs Bypass lane warranted 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Three Legs, Two Lanes on Major Left-turn treatment not warranted Left-turn lane warranted 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Four Legs, Two Lanes on Major Left-turn treatment not warranted Bypass lane warranted Bypass lane warranted Left-turn lane warranted Figure 2. Recommended left-turn treatment warrants for intersections on rural two-lane highways.
10 ⢠Crash modification factors available in the AASHTO High- way Safety Manual (4), and ⢠Construction costs. For rural conditions, different safety performance func- tions are provided for two- and four-lane highways and for three- and four-leg intersections. For urban and suburban arterials, prediction equations are provided for three-leg and four-leg intersections. Separate urban and suburban predic- tion equations are not provided based on the number of lanes on the major road approach. The prediction equations are not a function of speed limit; therefore, the developed war- rants also are not a function of speed limit. A range of values was used in the benefit-cost evaluation to identify volume conditions when the installation of a left- turn lane at unsignalized intersections and major driveways would be cost-effective. Plots and tables were developed that indicate combinations of major road traffic and left-turn lane volume where a left-turn lane would be recommended. War- rants were developed using the following: ⢠A range of values for the economic value of a statistical life, ⢠Crash costs based on values in the Highway Safety Manual, ⢠A range of construction costs, and ⢠A benefit-cost ratio of 1.0 and 2.0. The research team suggested a benefit-cost ratio of 1.0 along with the mid-range economic value of a statistical life and moderate construction cost to identify the warrants for a left-turn treatment. For urban and suburban areas, that is a left-turn lane. For rural areas, that is a bypass lane. Benefit- cost ratio of 2.0 has been argued as being a more practi- cal value to use to offset the potential variability in other assumptions. The warrants based on a benefit-cost ratio of 2.0 were selected for a left-turn lane on rural highways. These values were similar to the warrants that resulted when the lower crash costs based on older Highway Safety Manual costs were used. Left-turn lanes can reduce the potential for collisions and improve capacity by removing stopped vehicles from the main travel lane. Left-turn lane warrants were developed as part of NCHRP Project 3-91 using an economic analysis procedure for rural, two-lane highways; rural, four-lane highways; and urban and suburban roadways. The methodology presented in the NCHRP Project 3-91 report (1) could also be used if a transportation agency has available local values for delay (a) Three Legs (b) Four Legs 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Three Legs, Four Lanes on Major Left-turn lane not warranted Left-turn lane warranted 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Four Legs, Four Lanes on Major Left-turn lane not warranted Left-turn lane warranted Figure 3. Recommended left-turn lane warrants for intersections on rural four-lane highways. (a) Three Legs (b) Four Legs 0 5 10 15 20 25 30 35 40 45 50 0 50 100 150 200 250 300 350 400 450 Le ft- Tu rn V ol um e (ve h/ hr /ln ) Major Arterial Volume (veh/hr/ln) Urban and Suburban Arterial, Three Legs Left-turn lane warranted Left-turn lane not warranted 0 5 10 15 20 25 30 35 40 45 50 0 50 100 150 200 250 300 350 400 450 Le ft- Tu rn V ol um e (ve h/ hr ) Major Arterial Volume (veh/hr/ln) Urban and Suburban Arterial, Four Legs Left-turn lane warranted Left-turn lane not warranted Figure 4. Recommended left-turn lane warrants for intersections on urban and suburban arterials.
11 reductions due to the installation of a left-turn lane, crash frequency or crash predictions, crash reduction factors, crash costs, and/or construction costs. If crash and/or delay data are available for a specific location, the benefit-cost method as described in the research report can be used to evaluate the potential benefit of installing a left-turn lane at a specific location. The available crash data should be combined with the crash predictions for the site using an empirical Bayes (EB) approach. Both the crash prediction and the EB proce- dures are discussed in the Highway Safety Manual (4). The EB technique is properly exercised by statisticians who have familiarity with this method and interpretation of its results. Highway agencies that desire to use this method but do not have personnel with relevant EB experience should consider employing the resources of a consultant who is experienced in the use of the method. Prepare Designs Once the decision to install the left-turn lane has been final- ized, and the planning process has been completedâconsidering all of the important contributing factors in the placement of the left-turn laneâdesigns for the specific dimensions of the lane must be prepared. Depending on the characteristics of the inter- section, it may be appropriate to prepare more than one design option and compare their relative strengths and weaknesses. Alternatively, individual design elements can be discussed and evaluated as part of an overall design plan. Either way, the ele- ments comprising the design need to be created according to accepted geometric design principles that account for factors such as design speed and design vehicle, sight distance, storage area, deceleration area, grade, and channelization. These prin- ciples and others are discussed in Chapter 3.
