Questions for wawa

[sushi]

Or any other traffic engineer type

Why are pedestrian countdowns different? Is it intentional? Are they short in NOVA b/c they hate pedestrians? Or are they super long in Hawaii b/c island time? They may both say 20, but that 20 means different things.

What about entry ramps? Shouldn't there be an industry standard? Why do they vary by location? Do you have an opinion on the lights for ramps?

[zelda25]

Dec 16, 2015 11:10:26 GMT -5 sushi said:

Or any other traffic engineer type

Why are pedestrian countdowns different? Is it intentional? Are they short in NOVA b/c they hate pedestrians? Or are they super long in Hawaii b/c island time? They may both say 20, but that 20 means different things.

What about entry ramps? Shouldn't there be an industry standard? Why do they vary by location? Do you have an opinion on the lights for ramps?

Are you talking about the highway ramp meters here? I know that in Portland, the timing on those is variable and depends on the flow of traffic at any given time.

[wawa]

I'm going to take these in reverse order -

ramps - we're talking about highway ramps? Not ped ramps? I'm going to assume highway based on your lights question.

lights for ramps - you're talking about ramp metering? Like where there is a traffic light stopping you on the leadup to the highway and then releasing people one by one? It works great in theory. I can model the hell out of that and it's beautiful. Success in implementation depends on a lot of things - mainly education and local driver behavior. And the right balance of mainline vs. merge traffic. People have to comply with the light for one, and then actually accelerate when faced with a nice wide open ramp in front of them, and then the main traffic stream has to not be filled with tailgating assholes, and the merging traffic has to know how to actually merge into flowing traffic (i.e. match speeds, move into a hole that isnt' the size of texas). So...short answer is: it depends. they work great when they work but they aren't an across the board solution.

Industry standards for entry ramps - there are. he AASHTO green book is the highway design bible. Acceleration lane lengths, lane widths, tapers, allowable curve radii - there are standard minimums for all of those things. Anything on the interstate system that doesn't comply with design standards requires a waiver from FHWA and you need a good reason and proof that it'll still work ok. Usually the same thing applies to state roads, and county roads, city roads, etc (i.e. states have standards for their own roads, counties have standards for theirs. All of these standards are usually based on AASHTO).It's just that roads have been built over many many years and design constraints are different in every location. Sometimes there wasn't room to build things properly. Sometimes there wasn't budget. Sometimes it was designed so long ago that the guidelines were different. (or didnt' exist) So you get a mishmash of results.

Ped Clearances - there are standards for this (this one is governed by the Manual on Uniform Traffic Control Devices - MUTCD), but exactly how they're implemented varies by jurisdiction. The standard is that you must provide enough time during the flashing don't walk for a pedestrian to cross the traveled way with the specified walking speed (3.5 ft/s). It's called the ped clearance interval. What you've got on either side of that (walk time, buffer time) is variable. This graphic lays it out pretty well. But yeah, those 20 seconds might mean that when they're up will be run over, or they might mean that you have the full yellow plus all-red which could be more like 6 seconds. Different jurisdictions treat it different ways.  (eta:  or maybe you have 1-2 seconds for the all-red clearance)