## Chapters 18 & 19

Safety performance functions, development (calibration, crash modification factors [Part C], crash proportions), IHSDM and other software

Moderator: khardy

### Chapters 18 & 19

Has anyone used the freeway chapters yet? We just got a copy and have a question about multi-lane collector-distributor ramps. The method seems to limit the analysis to 2 lanes. Our site has C-D roads ranging from 2-5 lanes wide. Is there a way to model this condition?

Thanks,

Andrew
Andrew_Schlichting

Posts: 2
Joined: Mon Jun 06, 2011 3:07 pm

### Re: Chapters 18 & 19

You are right about the limits of the number of the lanes for C-D roads (actually the limit is 1 lane for rural and 2 lanes for urban). However, similar to the freeway segments number of the lanes (Chapter 18), you should be careful to only count the Thru lanes and not the lanes on Entrance and Exit ramps. With that in mind may be some of the C-D roads that you have fit in the limits.

Posts: 17
Joined: Thu Dec 02, 2010 11:49 am

### Re: Chapters 18 & 19

Thanks for your reply. Maybe I don't quite understand how the number of lanes is defined for CD roads. I'm analyzing two collector distributor roads as part of the existing conditions of the project:

If I'm understanding you correctly, the first section would be a 1 lane CD with a lane added then dropped and the second section would be a two lane section with a lane add and a lane drop.

I think our proposed condition is still outside of the model's limits, at the widest it is 5 lanes wide and carries north of 50,000 vpd.

Am I looking at this correctly?

Andrew
Andrew_Schlichting

Posts: 2
Joined: Mon Jun 06, 2011 3:07 pm

### Re: Chapters 18 & 19

Figure 1 shows a 1-lane C-D road. Since the gore-point to gore-point is so short the ramps do not form any add/drop lane situation.
Figure 2 shows a 3-lane C-D road that becomes 2-lane (again, the gore-point to gore-point is so short the ramps do not form any add/drop lane situation). So, part of it that has 2 thru lanes can be evaluated.
The proposed design is at least a 3-lane C-D road and cannot be evaluated by Chapter 19 models.
The 2-lane entrance and exit ramps are 0.48 mile away (gore-point to gore-point). The criterion for a ramp lane being considered as an add/drop lane in C-D roads is 0.3 mile (1600 ft). However, one interpretation can be that because here we have two ramps (one entranceand one exit) then 0.6 mile become the criterion. So, based on this interpretation if the main number of lanes were 2, this scenario could be evaluated. I need to ask the model developer if this interpretation is correct, though.

Posts: 17
Joined: Thu Dec 02, 2010 11:49 am

### Re: Chapters 18 & 19

I need to correct myself on the ealuation of the proposed design. Since the gore-to-gore distance is more than 0.30 mile this will not be a weaving section but a lane add followed by a lane drop. Therefore, it is a 5-lane C-D road for the entire gore-to-gore section that cannot be evaluated by the models in Chapter 19..