# Speed Limit M.P.H.

How is the speed limit on the road determined? i.e., how is it determined that it’s safe to go 65MPH on the highway on the East bound but only 55MPH on the West bound? On Smith Street you can drive 45MPH but on Oak you can only do 35MPH.

[calvin&hobbes]They drive cars down the road at progressively faster and faster speeds until somebody gets in a wreck, then set the speed limit to be 10 mph slower than that.[/calvin&hobbes]

For highways, they engineer the banks of turns for a given speed, then subtract a great deal for a safety. (Because if you designed a highway for 70 MPH and told people the speed limit was 70, someone would undoubtedly go 75 and go flying off at the curves.) IIRC, interstate highways are engineered for 85 MPH, maybe more where there isn’t too much terrain to go around nor too many entrances/exits.

For city streets, they classify them on several levels and assign speeds of 5, 15, 25, 35, and 45. Some jurisdictions just post the limits at the city line and tell you what type of street has what limit.

Things such as pedestrian traffic, age of pedestrians, number of lanes, parking situation, location of parks, schools, churches, all play a part in the ultimate calculation.

I don’t know about highway speed limits, but where I live, there are some roads where the limit is inexplicably slower than similar roads elsewhere in the area. I have since learned that enough well-connected / big-mouthed people on those streets bitched enough to get the speed limit lowered. I’m sorry, but if a straight road can have a double yellow line down the middle, we do not need to be going 35 mph.

So, relating to the OP, maybe enough people on Oak St bitched about not having the limit raised.

Looking to make sense of traffic engineering? Good luck.

I just remembered a news report I saw a few years back. Seems there’s this really dangerous turn that’s had wreck after wreck; they kept lowering the speed limit and people kept ignoring the signs and getting into accidents. Some bright person put up a sign saying:

Speed Limit
38 MPH

It worked. People saw the sign and thought, “Maybe they aren’t kidding…”

Every road has a design speed and a posted speed. The posted speed is just that, the speed limit you see on the sign. Design speed is the actual speed you can safely travel on that road, due to things like curves, hills, and stopping times.

Engineering practice dictates that the posted speed should be 5 to 10 MPH less than the design speed for the reason that AWB gave, namely, many people ignore the posted speed limit. The 5-10 MPH reduction is only a rule of thumb; the local authorities (usually the town/city council, with input from the police department) can set the posted speed to anything they want. This gives rise to situations like Dire Wolf described, where the town lowers the speed limit on some streets in order to pacify residents.

As for where the design speed itself comes from, it is the result of years of research by an organization called the American Association of State Highway Transportation Officials, or AASHTO. AASHTO compiles statistics on road tests and real-life situations, and along with some basic laws of physics (centrifugal force, reaction times, etc.) determines what is a safe speed for a given geometric configuration. Each individual state then adopts AASHTO guidelines as their own, amended as they see fit. When engineers use the AASHTO guidelines to design roads, they decide in advance what the speed limit should be, then design try to design the curves to meet the minimum radius criteria found in the book.

Reminds me of when I was a Boy Scout in Brewster, NY. Near the campground we used the most, the Park Service posted a Speed Limit 75 MPH sign. Of course, anyone who tried to go over 30 would end up in a tree!

Just like Angeles Crest Highway that winds it’s way through the mountains to the north of Los Angeles. The speed limit on most of it is 55MPH, and even in my sports car, I never have broken 50 on the straightest of straightways. Anyone that tries to go quicker could easily find themselves stopping 4500 vertical feet below.

In Texas, in theory, regardless of design speed, we set the speed limit by doing a statistical survey of how fast drivers go on a stretch of road in the absence of speed limit signs. The speed limit is supposed to be the 85th percentile speed (if that isn’t faster than the limit set by the legislature). So if 85 percent of drivers on a stretch of road were going 60 or slower, for example, then 60 would be the speed limit.

In practice, of course, in Texas, more than 15 percent of the drivers are usually going faster than the legislative limit. But if the 85th percentile speed on a road is 55, we aren’t supposed to set the speed limit lower than that. (It may happen anyway in certain circumstances.)

This may be a problem on older roads, but as far as I know most new roads here are designed for higher speeds than the speed limit is likely to be.

Here’s a URL that talks about some of this stuff: