I have been wondering lately how the placement of manhole covers/sewer covers - the kind that are in the street - is decided. I have yet to see any rhyme or reason to it. For instance, on my way to work, I would have to be swerving from curb to curb to avoid them consistently which the local police tend not to like. Then, it’s not like they’re a certain distance apart - I can go blocks without seeing one, then there will be three within three hundred feet. Who decides where these thingies are put??
This is just a WAG, but I’d say they are placed according to where the pipes are that they give access to, and the need to access those pipes. Places where lines meet would be an obvious choice for access as well as places where underground pumps are as well as the occasional access point for pipe inspection and repair. I’d say where they appear in the street is not much of a consideration as long as they appear somewhere that offers access.
City planning engineers, urban designers, state laws, and physics.
For example, water lines are typically on one side of the street and sewer lines on the other. What if you had a leak in both? If they were in the same trench line, you run a much higher risk of contaminating your drinking water supply.
You did know that they’re traffic-rated. right? You can drive over them; you don’t have to swerve around them.
Back when I worked in sewage in the UK, a manhole had to be placed everywhere a sewage line changed direction - there were no curved sewage pipes, just straight ones between manholes. Ergo, there’s a manhole everywhere the lie of the land, or the places to be serviced, requires a turn or a join in the sewage pipes. I don’t know if this is a requirement outside the UK, or for any other services, but it makes a certain amount of practical sense.
Well I can’t remember my university notes right now, but IIRC there are sewage pipes joints that give a little bit of leeway when joining the pipes, a few degrees left or right.
And yes there’s also a manhole at every change in gradient and where two different pipes meet.
Is there a manhole every so often to allow inspection even without a change in gradient or direction?
The utilities decide where they need to be, and the local governments grant the necessary permits for us to dig. Anywhere we (the telcos) have critical junctions for underground cables or equipment, we need a manhole in order to gain access to that equipment.
Why do you have to swerve to avoid them?
They probably occur with more frequency in areas where blockages or flow problems are most likely to occur (i.e. directional changes, gradient changes), and probably less frequently during long stretches of pipe whos geometry doesn’t change too drastically.
…and the round manhole covers are round in shape, so, once removed, they can’t accidentally fall back into the hole.
Triviaboy runs from the room… 
There’s been no reply as to why they are swerved around, but I can imagine that it’s because when they repave the streets they sometimes don’t make the cover even with the new pavement.
This is basically correct. Actual placement depends greatly on who is doing the design, and varies between design firms and cities. Spacing on straight runs can even vary depending on the equipment the city has to do pipeline cleaning, some cities can handle larger spacing than others.
Manhole placement is typically NOT placed according to areas that are more likely to plug. The design engineers believe that their designs are not inherently likely to plug due to their proper design, they have no real idea of the future usage patterns, and they usually have no idea what goes into cleaning the sewers anyway.
The last randomizing factor is that even if the manholes are placed in a logical position related to the original road, 5 to 50 years later, the road will often be moved, widened, etc. The manholes usually stay in place due to the vast expense it would require to move the sewer lines, and Presto! the manholes no longer match the street.
So why aren’t they equilateral triangles? Eh, triviaboy? Eh? Come back!
Manhole covers are placed where there are manholes to cover – Duh!
And manhole covers are round because manholes are round.
Uh, because an equilateral triangle can fall into its hole (hint: the height of an ET is less than the length of its sides).
Of course - how could I have forgotten Cecil’s column on the subject. :smack:
I swerve around them because quite a few of them arn’t flush with the roadbed. I’ve seen some that were more then a few inches above the road (with an asphault ‘ramp’ leading up to it) as well as some that are as much as an inch below the road. It makes a rather unpleasent sound when you hit them, and can’t be to good for the car’s tires, suspension, steering etc…
Believe it or not, I was once asked about this on a job interview.
I got the answer right, but wasn’t hired anyway. 
Ding ding ding - we have a winner!!! On the street I’m driving on, they are in some cases several inches below the street grade and hubby having a conniption because I’m supposedly throwing the front end out of whack at six in the morning is not my recipe for the start of a “good day”.
On my way in to work today, I counted SEVENTEEN of them on the west side of the street within six tenths of a mile. I know my town has been doing some major work with flood control and with other types of infrastructure (burying utilities, sewer work, cable being rerun), maybe this is why there’s so many of 'em.
Thanks, all - I appreciate your answers and help. Just gotta find a new way to get to work, I suppose. 
What
NAMETAG said…plus, being as they weigh in the hundreds of pounds, ya can always roll a round one by yourself.
If you’re driving in an area that gets a lot of snow, the manhole covers are probably intentionally set slightly below grade to prevent plow blades from catching on the rim. If they’re not recessed, a plow can do a whole lot of damage to the pavement by shoving one of those things around.
I think there were a couple like that in the area I was working on - don’t recall there was any required interval for manholes, though. (Of course, most of the sewer pipes were nicely twisty and turny, anyway - manholes every few yards, in some places.)