Turn Lane Primer: 8 Considerations to Implement a Turn Lane
Turn lanes are a geometric feature that allow for a left or right turning movement to an intersecting road or driveway to occur outside of the through lane movements. It is a fact that turn lanes provide safety and capacity benefits for all vehicles. The Federal Highway Administration conducted a research project, Safety and Effectiveness of Intersection Left- and Right-Turn Lanes, which concluded crashes decreased ten to 44 percent when a left turn lane is added and four to 27 percent when a right turn lane added. Similar gains in capacity can be achieved with a turn lane, depending upon the exact characteristics of the traffic flow.
While the benefits are undeniable, should turn lanes be installed at every intersection? The flip side to the vehicular traffic benefits are negative impacts with longer crossing distance and greater exposure for pedestrians, potentially increased conflicts for bicyclists, and greater delays at a traffic signal for pedestrians and bicyclists at signalized intersections (such as if left turn phasing is added). As we strive to become a greater multi-modal society, how does one decide when a turn is justified?
As engineers, we usually want clear lines of when or when not to act. In past additions, the Highway Capacity Manual provided a rough guideline of 100 or more turning vehicles in a peak hour to justify an exclusive turn lane and 300 or more turning vehicles in a peak hour to justify dual left turn lanes. The latest version steps away from this, instead stating “the potential for a satisfactory LOS in the future would not be sufficient justification by itself for installing the turn lane.” (LOS = Level of Service)
The National Cooperative Highway Research Program also provides some guidance on when to install a left- or right-turn lane in Report 745 and Report 780. Our own Minnesota Department of Transportation (MnDOT) is another agency that provides guidance on when a turn lane is justified on high speed corridors. As you can see though, there is unfortunately not one answer to this question. Like many items in the traffic/transportation discipline, there is a large gray area instead of a simple black and white answer.
For our approach, we generally start with the agency that has or will have jurisdiction over the corridor in question. If they have warrants or other information, it is an easy first step to evaluate the need for a turn lane based on their criteria. Other factors we can then consider include:
1. Functional Classification; high order roads, such as arterials, are often planned with turn lanes to increase the safety and mobility of the thoroughfare.
2. Vehicle Speeds; turn lanes provide a greater safety benefit on high speed corridors (45 mph or higher) eliminating conflicts between through vehicles and vehicles slowing to turn.
3. Capacity Needs; a capacity analysis may indicate the need for a turn lane to provide acceptable levels of service for vehicular traffic.
4. Percentage of Turning Traffic; a high percentage of turning traffic may suggest the need for a turn lane to recognize that movement as a primary travel route.
5. Site Conditions; sight distance limitations, corridor design consistency, roadway obstructions, intersection skews, and/or other characteristics can demonstrate the need for a turn lane.
6. Crash History; a history of right angle and/or rear end crashes can indicate a turn lane will improve safety.
7. Truck Traffic; a high percentage of heavy trucks can need a turn lane due to the slower deceleration and acceleration rates.
8. Railroad Crossings; a turn lane may provide additional stacking or a method for some intersection operations to occur when the thru lane is blocked by a train.
We try to be consistent in how we view the installation of turn lanes, using many factors rather than one or two simple metrics. In the end, however, the use or non-use of a turn lane is likely to come down to engineering judgment. Be sure to use yours wisely.