How to code banned turns (i.e., left-in/left-out) at intersections

Hi All,

This question is similar to what has been previously asked in other posts, though I am reposting as I do not believe the situation has been adequately settled. Left-in/left-out intersections (right-in/right-out in countries that drive on the right) are increasingly being used to deal with the high-severity crashes that can result from cross-traffic movements at three-leg intersections. The treatment is used to prevent movements that allow side road traffic to enter the intersection in such a way that right-angle impacts with main road traffic are possible (i.e., from right turns). In South Australia (where I am from), we deal with right turn movements by providing a u-turn facility further down the road (see below image). My question is regarding the appropriate coding of such intersections. In a previous post, Luke Rogers has suggested that these be coded as a 3-way intersection with channelisation. While at first thought this sounds appropriate, I believe it fails to consider the primary function of the treatment, which is to ban movements associated with high-severity outcomes. Channelisation does not prevent certain movements and as such a similar intersection that does allow right turns could be coded in the same way, even though the two intersections (right turn banned and right turn allowed) would likely have very different risk profiles. While it sounds odd, I would think that this could be more appropriately coded as a merge lane, as the key function is similar - to facilitate entering the main road without allowing cross-traffic movements at the intersection. The fact that side road entry as well as exit is allowed is, from a safety perspective, less important. Essentially, a merge lane and a left-in/left-out intersection should have more similar risk profiles than compared to a left-in/left-out and a conventional 3-way intersection that allows right turns (irrespective of channelisation). Hence, from a coding perspective, I believe coding this as a merge lane is more realistic with respect to the safety outcome of the intersection design. I’m keen to know other people’s opinions, and especially yours, Luke, as you have an intimate knowledge of the evidence behind the coding.

Cheers, Chris.

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