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Today I am reading an Algorithms textbook. The author, in order to explain the shortest path between two vertices in a graph, gives an example in the real world:

At one time, there was a speed trap on I-70 just east of the Indiana/Ohio border, but only for eastbound traffic.

In order to understand this sentence, I used Google Maps to find the site of Indiana and Ohio. I found that they have only one common border. And I think this border is an abstract line. So I can't understand what "east of the Indiana/Ohio border" means... Does the author mean "east of the common border"? It's in Ohio state. Why not say "in Ohio state" directly?

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    Because this is more precise. Ohio is rather large. If I said "There's a speed trap on the I-70 in Ohio", you'd have no idea where in Ohio it is. This way you know it's near the Indiana/Ohio border. – jimsug May 22 '14 at 3:25
  • What the author of the passage is saying is "In the past the Ohio state Highway Patrol often stationed an officer in a patrol car within the state of Ohio, near the common border between Indiana and Ohio, for the purpose of checking the speed of cars which had recently entered Ohio from Indiana eastbound on interstate route 70, and for giving speeding tickets to those drivers who were exceeding the Ohio speed limit". Speed limits often change when one enters a new state, and Ohio used to have lower speed limits than neighboring states; thus, they could count on issuing many tickets. – Bob Jarvis May 22 '14 at 13:22
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One detail you might be missing is that I-70 is the name of a major interstate highway which travels across most of the country. I would say that most Americans are at least somewhat familiar with the route.

Another thing that might be tripping you up is the author's use of "just east". "Just" in this case means "exactly" or "directly"- "Just East" can be understood to mean "within a few miles of". If I said "My house is just east of the river" you'd know that my house was very close to the river.

The border between Indiana and Ohio is hundreds of miles long, but by saying "Just east of the Indiana/Ohio border" the author has described a location that most Americans can visualize rather easily.

A map of the United States of America, highlighting interstate I-70, with an arrow pointing to the location described in the text

Another reason this passage might make more sense to Americans is how common the scenario it describes is. Each state has its own police, and it's common for the laws of one state to be slightly different than another state- especially things like speed limits. It's extremely common for police from one state to sit right across the border from another state that has a higher speed limit to catch people who may not realize that the speed limit has changed.

  • "people who may not realize that the speed limit has changed" -- I don't suppose the states trouble themselves to post speed limit signs on the interstate at the state border? – Steve Jessop May 22 '14 at 16:20
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    They do, but of course there are still plenty of people who don't notice or don't care. – evan May 22 '14 at 17:54
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    I've never driven outside a city anywhere near there. Before I looked at your map I didn't remember that I had ever even been on I-70. However, most any American driver would recognize the format of the name. I-<2 digit number> is always an interstate highway. – Loren Pechtel May 22 '14 at 22:28
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    Your third paragraph implies that "just east" narrows down a point on the hundreds-of-miles-long border, but in fact it's the "I-70" part that does that. – Russell Borogove May 23 '14 at 0:58
  • "On I-70 east of the Indiana-Ohio border" could be anywhere east of Indiana. It could be in Maryland. "Just east" implies it's within a few miles. – evan May 23 '14 at 1:08
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For the same reason he didn't just say "on I-70, in America." It's just a matter of additional precision and in this statement it's likely not necessary but just the author adding a bit of flavor to an otherwise very bland text.

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