Good morning all,
Apologies, this question is more about geometry theory and practice, rather than a specific software query, but I'm asking anyone I can think of as it's driving me crazy.
To calculate superelevation length, in it's most basic form, the equation is to multiply the carriageway width (or running lane width) by the change in crossfall (I appreciate there's more to it, and there are other ways to write the formula, but I'm simplifying for the purposes of this post). This allows for TD9/93, Para 3.7, ensuring the edge profile does not vary in grade by more than 1%. For motorways the edge profile should not vary in grade by more than 0.5%, therefore a factor of 2 is applied to allow for this.
So far so good. No questions about this bit.
Here's my question: for a long time, I was taught that the formula mentioned above (multiply the carriageway width (or running lane width) by the change in crossfall) applies to single carriageway with a design speed of 85kph or less. For a single carriageway above 85kph a factor of 2 is applied (both use a 131 command in MX). For a dual carriageway of 85kph or less a factor of 1.5 is applied, and for a dual carriageway of greater than 85kph a factor of 3 is applied (using a 132 command in MX). I cannot find any technical document which details this, as it was simply what I was told many years ago. I've since come across many other Engineers from various companies who also go by this method, although it doesn't strictly tie in with TD9, but nobody can tell me where it came from. I could let it go if it was just me, but since I've seen many others using this method it must have come from somewhere!
In summary: Length (L) = Change in superelevation x Carriageway width
Single carriageway ≤85kph = L x 1 (131 command)
Single Carriageway >85kph = L x 2 (131 command)
Dual Carriageway ≤ 85kph = L x 1.5 (132 command)
Dual Carriageway > 85kph = L x 3 (132 command)
Again, I apologise that this isn't strictly the correct forum, but I just want to hear from other highway designers.
Thanks,
Ben
[Off topic] Why are you changing from linear change of crossfall (131) to symmetrical reverse curve change of crossfall (132) - neither applies a factor to the given start and end crossfall values?
I just prefer to use 132 on a dual carriageway for a smoother transition.