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lila (Civil/Environmental) (OP)
21 Nov 02 22:26
What width of offset shall be used for Taper length calculations? Is it the width before the construction zone or after the construction zone?
dellbs (Civil/Environmental)
22 Nov 02 11:47
If you are calculating the taper length to tie your new roadway into an exisitng one, then the width to use would be how much wider is your new pavement than the existing. For example, if you widened a 22 foot wide road to 32 feet, the change in width for each lane would be 5 feet (11 feet to 16 feet). If you were required to use a 25 to 1 taper to tie into your existing, then the taper length would be 25x5 or 125 feet.

Hopefully I understood what you were asking (?).

David
lila (Civil/Environmental) (OP)
22 Nov 02 18:37
Thanks for response.
I guess I have to use a formula for taper length L= (w.s.s)/60 for speed limit of 40 mph. Actually, three 11 foot lanes are going to be widened to three 12 foot lanes. In that case, what is width of offset(w) shall I have to take? whether 1'((36-33)/3). In that case taper length will be around 27 feet. Someone is saying that w should be taken as 12 feet. In that case taper length is going to be about 320 feet . There is a lot of difference. Clarification is highly appreciated.
Thanks
Lila
dellbs (Civil/Environmental)
26 Nov 02 11:24
Seems like your offset is only 1.5 feet per side, assuming your centering the roadway.

WSDOT recommends length of transition taper equals the tangential offset width times the design speed (L=VT). This formula is for lane narrowing on roads woth speeds of 45 MPH and higher. They recommend at least a 1:25 taper fro speeds less than 45 MPH. For lane widening they recommend 1:15 or flatter. Since you have both widening and narrowing going on, I'd use the 1:25. Or if you want to be a liitle more consevative use the L=VT formula, this gives a 60 foot taper on each side. Of course there's nothin wrong with stretching it out, except cost.

Check your state DOT for some guidelines or the jurisdiction your project is located in. Using 12 feet seems to be a bit excessive.

Hope this helps.

David

David Lee
www.lbs-inc.com

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