So, as I promised last week sometime (whenever it was that Jon posted about the guy running for senate or something in Pennsylvania) here is my post about determining speeds for roadways...of course, for whatever reason the book gives everything more as a "if you want the speed to be this, then you should design this as..." as opposed to "if you have a roadway that is designed as this, then the safe speed should be..." But I digress.
Ok, #1. There are tons of tables...which I do not feel like scanning in for your betterment...so if you want the tables themselves, go find them from AASHTO (American Association of State Highway and Transportation Officials)...or find the transpo book I have (Transportation Engineering Planning and Design Fourth Edition by Paul H Wright And Norman J. Ashford)
Various parts that go into designing a road:
-Sight Distance (how far can you see in front of you),
-Perception-Reaction distance (how far you travel from when you first see something to when you react to it--Reaction is usually set to 2.5 seconds, but can go up to 3.0 seconds),
-Braking distance (how long you're breaking for),
-number of lanes (number of lanes needed to maintain a certain level of service--otherwise known as "why am I sitting in this traffic jam?")
-width of shoulder (the wider the clear shoulder, the faster people will go)
-slope of hill after shoulder (this determines usage of guardrails)
-other various stuff which I do not feel like typing about (gutter inlets, train intersections, highway intersections, bridge design, type of pavement used, thickness of pavement, etc.)
Sight distance on the road generally just means "how far can you see in front of you?" this is why when it's dark out you don't want to "out drive" your headlights. And thus, probably the reason for Texas having "Day speed limits" and "Night speed limits". Stopping sight distance is the distance you travel from when you first see something to when you actually stop...How fast can you stop the car when you see a hindrance in the road? AASHTO uses "wet pavement" calculations because wet pavement has less friction than dry pavement, thus it takes you longer to stop. So then, the calcuations are as follows:
Perception-Reaction distance: 1.47*(2.5 sec)*V where V is the velocity of the car in mph.
Braking distance: V^2/(30*f) where V is the velocity of the car in mph and f is the coefficient of friction (between 0.28 (for about 70mph) to 0.4 (for about 20mph))
Therefore your stopping sight distance is the distance you travel while reacting plus the distance you travel while breaking...and this is only for level ground, it would be longer for slopped ground.
And thus, the speed limit you select should take into account that your stoping sight distance needs to be shorter than your sight distance--you don't want people traveling on a road at 65mph when they can only see 50 feet in front of them.
Number of lanes calculations: Number of lanes = Directional Design Hourly Volume (DDHV) / Service Flow Level.
DDHV = Annual average daily traffic * K * D
K = percent of AADT that occurs in the peak hour (usually 10 - 20%)
D = Directional factor (percentage of cars on the roadway going the direction of interest) (usually 50-70%)
SFL (service flow level) is taken from a table--calculated as "vehicles per lane" by a percentage of trucks (0-20%) and by terrain (level, rolling hills, or mountains). The better the level of service, the less cars in a lane, and therefore the less likely you are to get stuck in a traffic jam.
Wide shoulders and other types of obstructions:
The wider the shoulder, the faster people can travel safely because brokedown cars can get completely off the road, plus you can use the shoulder for evasive tactics if needed.
The wider the lanes, the faster people will drive because they don't have to worry as much about hitting cars in other lanes.
The steeper the hill off of the shoulder determines the use of gaurdrails--you're more likely to over compensate and try to stay away from the guardrails, plus if you do hit the guardrail, it's supposed to keep you from going over the edge of the mountian.
Dividing a highway makes it safer to travel faster because you're not as worried about being hit by oncoming traffic.
Dividing a small neighborhood street, usually gets you to go slower because you can't necessarily see people or animals crossing the street as easily, especially if trees are added.
Adding cones gets you to slow down because you don't want to hit anything...even if the cones are placed to give you the same amount of space as the lane does.