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To pick a specific example, a flyover that goes over a ground-level road/rail need only be as tall so that the tallest vehicle travelling in the outermost lane can clear it.

Or another, an elevated road/metro rail that runs along a ground-level road need to be as tall as the tallest vehicle.

But why are they made much taller than that?

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    $\begingroup$ tallest road legal vehicle (that isn't exceptional transport) is ~5m add some margin and you are at 6m easily $\endgroup$ – ratchet freak Aug 31 '15 at 13:52
  • $\begingroup$ Without specific measurements, it is hard to answer. How do you know it is "too tall"? $\endgroup$ – hazzey Aug 31 '15 at 14:28
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    $\begingroup$ Here is an example near Austin, TX. There is a flyover at the intersection of highways 45, 130 and 183 that is around 100ft in the air, for no apparent reason. $\endgroup$ – JPhi1618 Aug 31 '15 at 21:27
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    $\begingroup$ @JPhi1618 Well, everything is bigger in Texas. $\endgroup$ – Chris Mueller Sep 1 '15 at 11:52
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    $\begingroup$ @RossV, my example is literally in the middle of nowhere. It's on a tollway whose sole purpose is to bypass Austin and other populated areas to get to San Antonio or Houston faster. In that street view the closer flyover does go over other highways on the beginning and end, but the far road does no such thing. Its just super high for no apparent reason. I believe this goes to the heart of the OP's question. I don't think hes asking about a clearance that is 16ft rather than 14ft. What's the deal with having an extra 30, 40, or 60 feet?? $\endgroup$ – JPhi1618 Sep 1 '15 at 14:14
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If I'm understanding your question correctly, you're asking why a flyover has excess clearance beneath it for traffic below. For example, if the tallest vehicle expected beneath the flyover is 12ft tall, the flyover may have 14ft clearance.

There are many reasons for this, some of them more obvious than others.

  1. You don't want to try to squeeze a 12ft tall vehicle through something 12ft exactly. Vehicles have suspensions which, depending on the exact loading of the vehicle, may have a variance of a couple inches. Thus, a lightly-loaded vehicle could be riding higher than the 12ft nominal clearance and hit the bottom of the flyover.
  2. Occasionally, heavy, oversized loads will need to be transported. Having excess clearance is very beneficial for travel of such items, since their dimensions tend to be all over the place.
  3. Roads have to be resurfaced. Often times, it is more economical to do a simple resurface where a new layer of asphalt is added on top of the existing layer. This reduces the clearance between the road and the flyover. Thus, building in some margin for the clearance is advantageous.
  4. If the road below is expected to be in use during construction of the flyover, the height of the flyover may need to be increased to provide clearance beneath any formwork (falsework) supporting construction of the flyover (thanks @CableStay).
  5. Bridges deflect under load. Generally, the deflection is limited to a certain value that depends on the jurisdiction. However, you don't want a deflected bridge to eat into the minimum clearance, hence some margin is built in.
  6. The height of the flyover could have nothing to do with required clearance. Train tracks are designed to be as flat as possible and have no slopes where feasible. Thus, if the train tracks are, say, 30ft above the road surface below, it would be silly to slope the train tracks down to make a minimum clearance of 15ft at the road level, just to slope the train tracks back up on the other side.

Where I went to middle school, there was a rail viaduct just down the road. There were a couple times where a semi trailer struck the bottom of the viaduct due to inadequate clearance. If I recall, this road had been recently resurfaced, so who knows if the clearance value had been updated afterward. Just something to think about.

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  • $\begingroup$ Though this is also not something actually taken into consideration during design, excess clearance also helps avoid bizarre accidents such as this one which happened last year in Rio de Janeiro where I live. At 0:06 you can see a dump truck with its bed raised (accidentally). And then at 0:25 you see it strike a pedestrian walkway. $\endgroup$ – Wasabi Aug 31 '15 at 14:47
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    $\begingroup$ Depending on construction type (i.e. cast-in-place) additional clearance will be required for falsework if the roadway underneath will have traffic during construction. $\endgroup$ – CableStay Aug 31 '15 at 15:08
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    $\begingroup$ This is specific to complicated, multi-highway, intersections. If long term plans call for one or more additional ramps, it may be better to build a flyover with the clearance it will need in the future. The alternative would be to tear it down in 10 years time, to rebuild a few feet higher. $\endgroup$ – Patricia Shanahan Sep 1 '15 at 8:05
  • $\begingroup$ Taking a complete flyer on what the OP's specific bridges in question are, I suspect no6 is most likely to be the right answer. $\endgroup$ – AndyT Sep 1 '15 at 13:25
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In the United States the public safety standards, designed by the American Association of State Highway and Transportation Officers (AASHTO), require that:

Vertical clearance on State trunk highways and interstate systems in rural areas shall be at least 16 feet over the entire roadway width, to which an allowance should be added for resurfacing. On State trunk highways and interstate routes through urban areas a 16-foot clearance shall be provided except in highly developed areas. A 16 foot clearance should be provided in both rural and urban areas where such clearance is not unreasonably costly and where needed for defense requirements. Vertical clearance on all other highways shall be at least 14 feet over the entire roadway width to which an allowance should be added for resurfacing.

As others have pointed out, it is important to allow enough height for nonstandard loads. In addition, ensuring that all of the bridges in the highway system conform to a certain level of requirements ensures that vehicles can be designed such that they can travel throughout the country without having to check the height of every flyover on their route.

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Looking at the the street view provided by the OP I would say that vertical profile constraints are at play here. The ramp needs a certain vertical clearance near its start and near its end. Rather then have the road return to ground level, the designers may have opted to use one vertical curve over the entire ramp that suited their design needs. The consistent vertical curve should ensure a comfortable ride for the user as well as provide plenty of sight distance to ensure they have enough time to brake if need be.

In addition to what has already been said about required clearance for over sized load, there may be a major industry on this route that routinely ships large/tall objects.

Similar to the need for formwork/falsework clearances, there may be an allowance for future maintenance or rehabilitation work to be performed.

Having done more than a few bridge inspection, I can dream and say they left plenty of clearance for an inspector to work under the bridge in a bucket truck over live traffic without the need for a lane closure! Pipe dream that one!

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