Where streets are sufficiently wide to permit their use, median left-turn lanes will be found to have many advantages in the handling of traffic.
E 664.1 General
The purpose of any separated special turn lane is to expedite the movement of through traffic, to provide for and permit the controlled movement of turning traffic, and to promote the safety of all traffic.
At intersections where left turns are permitted, left-turn median channelization will reduce accidents of the following types:
- Opposing sideswipe
- Head-on
- Left-turn
- Rear-end.
Design considerations or standards for rural areas may be highly inappropriate in the slower-speed, highly congested urban areas.
E 664.11 Design Factors
There are many factors or variables to be considered in the determination of the geometric characteristics of left-turn lanes and in their design. The basic factors are:
- Storage length
- Deceleration
- Through-traffic queue
- Physical geometric limitations
- Vehicle composition
- Topography
- Required parking restrictions
- Projected land use.
Variable combinations of the above factors may determine the final design length of the left-turn lane.
E 664.12 Storage Length
The following are some of the criteria used to determine storage length:
- A median lane should be of sufficient length to store the number of vehicles that are likely to arrive during any one interval of time in which left turns cannot be made. The interval of time generally used is one minute. For a peak-hour left-turning volume of N vehicles, the average number of vehicles arriving per minute is N/60. For some intervals of one minute, the rate may double the average of N/60 and may be N/30. Assuming a rate of N/30 and allowing 25 feet for each arriving vehicle waiting to turn left, the required storage length is:
- Turning vehicles per hour ..........30 60 100 200 300
- Required storage length. feet 25 50 100 175 250
Turning volumes of 200 vehicles per hour usually require design capacity and traffic signal control studies where the volume of opposing through traffic during the same hour exceeds 800 vehicles.
Based on peak-15-minute left-turn volume:
L= Vol. x 2
15 (or cycles/15min.)
Based on peak-hour left turn volume
L= Vol. x 2
Cycles/hr.
Rule of thumb, based on 80-second cycle:
L= 1 ft.
Left-turn vehicles per hr.
E 664.13 Deceleration
Preferably, the length required for deceleration should be in addition to the length required for storage, but for practical design purposes there can be a reasonable overlap. Assuming the approach speed at the entry to the left-turn lane to be 70 percent of the posted speed, the following list provides some lengths required for deceleration while braking at a comfortable rate:
70% x 35 mph = 25 mph.......130 feet required
70% x 40 mph = 30 mph.......170 feet required
70% x 50 mph = 35 mph.......200 feet required
E 664.14 Through-Traffic Queue
Vehicles will queue out of a left-turn lane of insufficient length and block the left-hand or number one through-traffic lane. Also, through traffic will often queue back and block the entry to the left-turn lane. The length of a through-traffic queue is very difficult to calculate because of the variables involved. For example, the length of the through-traffic queue will depend upon the difference between the capacity of the signalized intersection under consideration and the capacities of adjacent signalized intersections, which in turn may be variable. Furthermore, varying input volumes are generated by side streets and driveways. Starting delays, pedestrians walking at slow speeds, and bus operations all contribute to the traffic queue problem. Generally, the longer a traffic signal cycle, the longer the traffic queue becomes. Consideration of through-traffic queues is best accomplished by actual field observations of existing facilities.
E 664.15 Physical Geometric Limitations
Depending upon the traffic operation to be established upon a given street, physical geometric limitations may influence or dictate the design length or width of left-turn channelization. These limitations, which are particularly characteristic of existing right of way in urban areas, may consist of closely spaced intersections; reduced roadway width (due to the unavailability of additional right of way); required accessibility to mid-block parking lots or driveways; etc. In some instances, physical limitations at a given intersection may preclude the establishment of opposing left-turn lanes.
E 664.16 Vehicle Characteristics
The determination of the length and width of left-turn lanes may be quite different if, as in a heavily industrialized area, many of the vehicles consist of large trucks or trucks and trailers. The 25-feet-per-vehicle allowance may not be an adequate standard for length, or the usually acceptable 10-foot width of lane may be inappropriate.
E 664.17 Topography
The gradient of a street can exert some influence on comfortable braking rates. A little extra allowance should be made for downhill deceleration. Also, vertical curvature has a distinct effect in the determination of the length of a left-turn lane. Often, a left-turn lane should be lengthened so that the transition into the left lane does not occur at the summit or the downhill side of a vertical curve.
E 664.18 Required Parking Restrictions
The installation of left-turn lanes will often necessitate curb parking restrictions or prohibitions. The feasibility of the imposition of these parking restrictions will depend upon the property developments at the site and require very thorough investigation along with sound engineering judgment and supporting data, since they will usually generate public and legislative complaints.
E 664.19 Projected Land Use
Consideration should be given to the projected land use of surrounding areas. The development of new residential, commercial, or industrial subdivisions, freeway construction, zone changes, etc., will generate additional traffic and turning movements. Anticipation of this future need may warrant present provision for additional left-turn-lane capacity.
E 664.2 Left-Turn Lane Layout
Figures E 464.2A, E 464.2B (Plate I), and E 464.2C illustrate medians with left-turn layouts, where parking may or may not be restricted. Figure E 464.2B illustrates three methods of laying out a left-turn lane. This type of layout has special application for streets without median strips and those where parking restrictions are to be minimized. The first of these methods is recommended over the other two because the installation of an approach tangent provides better turning characteristics for entering the left-turn lane. Moreover, it also allows for possible future expansion of the length of the left-turn lane. The second and third methods require the least amount of overall length. See also Figure E 464.2B (Plate H).
The following subsections summarize the individual geometric design portions of a left-turn lane. (Refer also to the State of California, Department of Public Works, Division of Highways Planning Manual, Part 7; and section 7-406 and Figures 7-406.4A and 4B and Table 7-405.5.)1
E 664.21 Transition
This transition has been referred to as transition (State), taper transition, or approach transition. Its length may be determined from the following two formulas, Length (L) being Design Speed x Transverse Offset:
L = VW where V = Design speed in miles per hour
W = Width of median lane in feet (widening on one side of highway)
L = 
where V = Design speed in miles per hour
W = Width of median lane in feet (widening on both sides of highway)
Rule of thumb:
30 mph — Use 30-to-1 taper
35 mph — Use 35-to-1 taper
40 mph — Use 40-to-1 taper, etc.
The City of Los Angeles utilizes a 30-to-1 taper as a minimum.
E 664.22 Approach Tangent
The approach tangent may vary; a recommended minimum would be 40 feet.
E 664.23 Taper
This taper has been referred to as taper (State), transition, reversal, reverse curve transition, and reverse taper (County). The City utilizes simple reverse curves of 175-foot radii and a taper length of from 60 to 90 feet. Occasionally a minimum length of 50 feet is employed. This is usually the shortest length that will permit mechanized painting equipment to be used.
E 664.24 Length
The length of a left-turn lane measured from the legal stopping point at the intersection end of the added lane to the beginning of the taper may range from a minimum of 10 feet to a maximum of approximately 500 feet. Use of a maximum length of 350 to 400 feet is recommended. If there are over 300 left turns per hour or the calculated lane length exceeds 300 feet, consideration should be given to an optional turn lane or a double-lane left-turn pocket. The City also, on rare occasions has employed lengths ranging from 400 to 550 feet. In general, the length of a left-turn lane should be determined solely upon the basis of its individual requirements.
E 664.25 Width
The width of a left-turn lane that may be used is as follows:
Absolute minimum width = 9 feet
Desirable minimum width = l0 feet
Standard width = 11 feet
Maximum width = 12 feet
Gutter clearances may also be added to the above values. If the left-turn lane is confined between raised concrete islands, its minimum width should be 16 feet to allow for vehicle breakdown.
After construction or striping of median left-turn lanes, it can be determined through field observation whether left-turn lanes are effective in accommodating all turning movements. Where feasible, changes can be made by removing and reconstructing the median curb or by erasing the existing lane striping by sandblasting, then modifying it to the new required length.
E 664.3 Non-intersection Left Turns
It is sometimes permissible or even desirable to allow left turns other than at intersections. This may be done at mid-block or other points of entry into private property such as parking lots, gas stations, etc. See Figures E 464.2A and E 404.2B (Plate I).
The advantage of permitting non-intersection turns is that it reduces the congestion and conflict at intersections. This is particularly important at busy intersections. However, it should be noted that the conflicts are not necessarily eliminated; they may be merely transferred from the intersection to the mid-block or other points.
Footnotes
- The text in parenthesis is from the legacy Street Design Manual text and is no longer in use.
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