After writing about turn sizes abstractly, I want to examine some real intersections. First is the standard Kirkland (and elsewhere) residential intersection. Kirkland uses wide residential streets (24, 30, and sometimes even 36 feet) and 25 foot curb radii on them. Parking spots near intersections are rarely used; it’s illegal to park closely and there’s no reason to do so with tons of other parking on almost every street. If we assume that 10 foot lanes lead to turns closely corresponding to the curb radii, then these lane sizes leave an extra 2, 5, and 8 feet. Further assuming that both streets are the same width (so, for example, a 24 foot street meeting another 24 foot street), this translates to possible turning radii increases of 7, 17, and 27 feet! Even the more common middle size pushes that 25 foot curb into the 40 foot classification – suitable for “moderate” speed car turns, or in other words not slow ones. If the cars encroach a bit into the oncoming lane, these numbers get even bigger.
Next is our local favorite, NE 70th St and 130th Ave NE, and specifically the left turn onto northbound 130th. The center turn lane is 12′, leaving some extra space. 130th is 30′ across, adding a lot of extra space. Worse, it’s quite common for drivers making this turn to greatly cut the corner. I’ve seen cars fully on the stop bar. Here’s a drawing (map from Bing) with the curb (black), a fairly proper turn track(blue), and an observed turn track (red). These are intended as tracks of the inside of the turning car.
It’s interesting that when the corner can’t be cut (blue track), even being far from the curb doesn’t allow a larger turn. Experience matches this observation; when a car is fully past the stop line (which is necessary for visibility), this turn must be taken pretty slowly. However, cutting the corner greatly increases the turn size. A very rough measurement suggests that the 30 foot curb radius turns into a 50 foot effective turning radius. Again experience matches this; cars on this path are moving quite quickly. And this is one reason why I prefer to walk up the west side of 130th without a sidewalk over crossing to the sidewalk on the east side.
Last is the site of a Seattle SUV/bicycle collision almost a month ago, Dexter and Thomas:
There was a lot of talk after this one, but for my purposes here I want to draw attention to this image from the video (direct YouTube link for the same video):
This driver is cutting the corner exactly as described above in the Kirkland intersection. This leads to a higher speed, which leaves less time to see the cyclist and less time to do anything about it. It looks like the driver may have sped up and turned more sharply. With more time, stopping would have been a more viable option.
As in the previous post, the simple solution of smaller lane widths for the first scenario is a good one but difficult to achieve politically. The other two examples are abuses of an additional lane. Let’s add a simple pedestrian island to the Kirkland example:
We need to be careful to provide a reasonable but slow left turn path, but with any placement of an island, the red path becomes more obviously ridiculous.
The same thing could be done over in Seattle. Here’s the same image with a simple median:
It would probably need a sign too, but the modification of the vehicle path would be pretty significant. I think even better would be to push the bike cross back from the intersection:
This modifies the vehicle path at least as much and creates a perpendicular crossing of the bike path. It essentially applies the protected intersection design to a left turn rather than the usual right turn.
I’ve never been to this intersection, so someone with more familiarity would need to turn this into a serious design. I’ll add, though, that if these roads are too busy or too fast for a treatment such as these, then the problems and dangers extend far past just this intersection design.