Lane Narrowing

Allright, so we all know corridors are great for modeling roadways, but what if you have to narrow your lanes? If we simply target our pavement to an alignment, the rest of the subassemblies are parrallel with the original corridor section. In many cases, we don't want that, we want the rest of the subassembly to be perpendicular to our target alignment. Well, that's where Offset Assemblies come into play. Here's our situation, our roadway narrows to provide traffic calming in our subdivision and our curb and gutter and sidewalk flow along with the narrowing. We will need three alignments for this situation, one for the centerline or our roadway and one for each sides narrowing.

We'll need two assemblies for this, one for the standard section of the roadway, before and after the narrowing, and one for the narrowing section itself. The standard section is an assembly like you would have for anything else but the narrowing section needs a special assembly. This assembly has two offset assemblies. These offset assemblies will follow the alignment of the edge of asphalt and the associated profile. If you use an offset assembly, you must have an alignment and profile or they just won't work right.

Here you see the profile views for the three alignments, you'll notice that the Left and Right alignments don't have a design profile yet, we'll make those later. Our centerline alignment has the design profile for our roadway.

Now we need to create our corridor. When we create our corridor, we'll go ahead and add all three of the regions to our baseline, before the narrowing, the narrowing, and after the narrowing. However, since we don't know the elevations for our profile along the Left and Right alignments, we'll uncheck the region for the narrowing so nothing is built in this area for the moment. This will provide us with the beginning and ending elevations for our Left and Right profiles.

This is what our corridor looks like after building it and creating a surface for it. As you can see, there is nothing in the region where the narrowing is.

The next thing we'll do is sample our design surface for the Left and Right alignments. This allows us to design the slope of our curb and gutter based on the corridor elevation before and after the narrowing. We could have gotten this information other ways but I like this way because it makes modifying the design easier. If things change, simply uncheck the region, rebuild the corridor, and you'll see the new elevations you need to connect to.

Now, we go back to our corridor properties and set the targets. The left offset will target the Left alignment and it's corresponding design profile and likewise for the right. The left pavement will also have to target the Left alignment and it's profile.

Once this is all done, here is our finished product. Notice how the curb and gutter and the sidewalk follow along the narrowing?

I hope you got something out of this little lesson. If you have any questions feel free to e-mail me at brianh at ctcivil dot com.