Welcome back to our new advice column, Ask R. Moses!
R. Moses Answers:
You have a good eye if you've noticed this. It's a perplexing question indeed. Here are a couple potential explanations.
Often, the center turn lane width is determined by the width of something else within the corridor in which it is located. For example, adjacent to the turn lane there may be a two-way left turn lane (TWLTL), which is used far differently than your typical left turn lane and warrants a somewhat wider width. A few reasons for this are to provide a “safety factor” (in theory, TWLTLs could result in head-on collision), or to provide refuge space for a car that is trying to make a left turn from an unsignalized driveway or side street onto a busy road. (They can cross over the first lane, and wait in the center lane until there is clear space to pull into traffic.)
Providing additional width for a TWLTL is not essential, and should be weighed against other needs competing for the pavement width, such as bicycle lanes. When reapportioning an existing roadway to better serve all roadway users, every inch of lateral space is important.
Another possibility is that there may be a raised median elsewhere in the corridor, or some other element that “requires” the curb-curb cross-section be wider at that location. We engineers tend to like consistency, so often times the cross-section width will remain constant, with the center turn lane acting as the “extra space repository.” Unfortunately, this reasoning gets used in areas that do not need wide lanes or the speed that a wide lane will induce. In any case, the use of wider than necessary lane widths induces speeding and makes streets less safe for pedestrians to cross.
One last possibility is that, if you're in a northern region, the center lane is often used as a place to temporarily place plowed snow and then come back and remove it after the streets and parking areas are cleared. As such, a wider center lane can be advantageous. Of course like provisions for fire fighting equipment, one has to question a design predicated on what happens only 20 to 30 days out of 365.
If none of those conditions listed above exist, there is no reason a left turn lane needs to be any wider than 10 feet. It could possibly be narrower, but consider the types of vehicles that might be making the turn on a frequent basis. If city buses, school buses, or larger trucks use the turn lane on a regular basis, 9 feet might be a bit too tight. It will also move you outside of typical design guidelines, so you might have a battle on your hands. Consider how necessary the turn lane is, and what would be the tradeoffs of restriping the street without the turn lane.
Keeping lanes narrower is one of the most cost effective way to reduce speeds on a street, and lower speeds = higher safety. Everything else being equal, I'll always lean toward narrower travel lanes for the traffic calming effect. A new “rule of thumb” is to start with the minimum number of lanes and lane widths for cars to promote safety, and provide the best type of bikeway design that is feasible and serves the community. One caution: If this is a high volume street, providing a bike lane that is too skinny (i.e. 3 feet) may be worse than no bike lane at all.
How would you respond to this question? Jump in with your answers in the comments!
Note: R. Moses is not meant to be professional engineering advice nor should be relied upon as such. Consult your own technical professional before proceeding with your own project.