Minnesota's Road Design Manual sets the default left turn lane at 300 feet long with a 15:1 taper into it (180 feet to taper a 12 foot lane). If you look around Minnesota, the vast majority of turn lanes follow this default. Unfortunately, those exact dimensions are wrong almost every single time. They're either not enough or too much.
MnDOT funded a study to get at this issue and they put together a technical report last year – Design of Turn Lane Guidelines. The steps to designing the proper turn lane length are:
- Collect Data (design traffic volumes, speed, % heavy trucks, grades, etc)
- Determine Facility Type (rural, urban, suburban, traffic control)
- Calculate Turn Lane Demand (store 2 minutes of traffic at a stop sign or traffic stacked in one red cycle at a signal)
- Calculate Turn Lane Design (Design the storage and taper based on above)
- Document (cover your tail – file away your calcs, conclusions, and recommendation)
The document does a good job of laying out the design process and even provides several example cases to show the design process in action. A lot of reference tables are provided to help you quickly make all of the potential adjustments away from the default 300 feet/ 180 feet. I hope designers move away from the default and think through the appropriate turn lane geometry for their specific situation.
