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Does anyone know why the flyover ramps in a stacked interchange usually have such large vertical clearances? The vertical separation seems much larger than nece

Does anyone know why the flyover ramps in a stacked interchange usually have such large vertical clearances? The vertical separation seems much larger than necessary to accommodate the tallest trucks allowed on the highway. EDIT: There are some more theories here: https://engineering.stackexchange.com/questions/4238/why-are-flyovers-metro-rail-tracks-much-higher-than-traffic https://engineering.stackexchange.com/questions/4238/why-are... Obviously there may be many contributing factors, but I expect one is dominant. This one is most convincing to me: The flyover ramp generally needs to go over some things and under other things. Additionally, for both safety and rider comfort, the ramp needs to be vertically smooth, not having fast drops or rapid climbs. That means ramp A might pass over ramp B by a lot more than the minimum amount because ramp A also needs to pass over ramp C just a bit later, where it does so by a much lower amount. With many overlapping constraints like this, most ramp intersections are much higher than the minimum clearance even though none can be lowered without either (1) making one of the clearances below minimum or (2) forcing a ramp to rapidly climb/descent.

function_seven 2y ago
This comment was posted the same time you asked your question, but I think it's one decent reason: https://news.ycombinator.com/item?id=41303428 https://news.ycombinator.com/item?id=41303428
- jessriedel 2y ago
  It's an interesting theory, but ChatGPT was skeptical, and I can't find anything on the internet that supports it. Thanks for the pointer though.
bob1029 2y ago
Part of it may be due to constraints of the construction process itself (I.e. not disrupting existing traffic) I've never seen one built all at once. It's always a bunch of incremental changes over time.
gosub100 2y ago
I'm completely guessing, but maybe to prevent harmful oscillations from wind currents blowing between the roads?
jeffbee 2y ago
It's because they let the same company design and build the interchange and own the concrete plant. "Design-build" project delivery. It is the same reason why the new HSR projects in California have ludicrous vertical and side clearance compared to other countries with better/cheaper HSR.
- bluGill 2y ago
  design build works well for things they copy often. A design build house is your best deal on new construction. When things are unique costs blow up
- garaetjjte 2y ago
  I don't follow, why they would increase their costs on purpose?
  - kube-system 2y ago
    No clue as to the validity of the above claim, but some contract types expose the customer to actual costs exclusively. e.g. T&M or cost-plus.
  - bluGill 2y ago
    Who is they? The desingers charge to more for harder designs so the more it costs the more they make. Similar for the builders.
kube-system 2y ago
> The vertical separation seems much larger than necessary to accommodate the tallest trucks allowed on the highway. The vertical height of things we want to send down the highway are also sometimes larger than the tallest trucks on the highway. https://bloximages.newyork1.vip.townnews.com/news-journal.com/content/tncms/assets/v3/editorial/1/ba/1ba95baa-9900-5a1c-8142-015ed574ae92/5a8ffc1f0cdcc.image.jpg https://bloximages.newyork1.vip.townnews.com/news-journal.co...
chiph 2y ago
The clearances in Texas are taller than ones I've seen in other states. The national standard is 17 feet, but Texas often uses 18 feet (5.4 meters) or more. Bridges on the East coast interstates sometimes don't even make 17 feet (they often predate many standards). The theories I've heard as to why TXDOT does this include: they wanted sufficient clearance for mobile oil drilling rigs, military vehicles[0], "just in case", and of course, bragging rights. You can zoom in and click on the blue dots to see clearances on major TXDOT bridges here: https://gis-txdot.opendata.arcgis.com/datasets/0126e7969bfb4f6bb92b9b59091c1acb/explore?location=32.641056%2C-96.823570%2C17.64 https://gis-txdot.opendata.arcgis.com/datasets/0126e7969bfb4... [0] The US interstate system wasn't really planned for use by the Department of Defense in time of war, but the bill was named "National System of Interstate and Defense Highways" to help it get passed in Congress. This isn't to say that during a national emergency, the highways couldn't be cleared of civilian traffic. https://highways.dot.gov/highway-history/interstate-system/50th-anniversary/interstate-highway-system-myths https://highways.dot.gov/highway-history/interstate-system/5...

function_seven 2y ago

This comment was posted the same time you asked your question, but I think it's one decent reason: https://news.ycombinator.com/item?id=41303428 https://news.ycombinator.com/item?id=41303428

jessriedel 2y ago
It's an interesting theory, but ChatGPT was skeptical, and I can't find anything on the internet that supports it. Thanks for the pointer though.

bob1029 2y ago

Part of it may be due to constraints of the construction process itself (I.e. not disrupting existing traffic) I've never seen one built all at once. It's always a bunch of incremental changes over time.

gosub100 2y ago

I'm completely guessing, but maybe to prevent harmful oscillations from wind currents blowing between the roads?

jeffbee 2y ago

It's because they let the same company design and build the interchange and own the concrete plant. "Design-build" project delivery. It is the same reason why the new HSR projects in California have ludicrous vertical and side clearance compared to other countries with better/cheaper HSR.

bluGill 2y ago
design build works well for things they copy often. A design build house is your best deal on new construction. When things are unique costs blow up
garaetjjte 2y ago
I don't follow, why they would increase their costs on purpose?
- kube-system 2y ago
  No clue as to the validity of the above claim, but some contract types expose the customer to actual costs exclusively. e.g. T&M or cost-plus.
- bluGill 2y ago
  Who is they? The desingers charge to more for harder designs so the more it costs the more they make. Similar for the builders.

kube-system 2y ago

> The vertical separation seems much larger than necessary to accommodate the tallest trucks allowed on the highway. The vertical height of things we want to send down the highway are also sometimes larger than the tallest trucks on the highway. https://bloximages.newyork1.vip.townnews.com/news-journal.com/content/tncms/assets/v3/editorial/1/ba/1ba95baa-9900-5a1c-8142-015ed574ae92/5a8ffc1f0cdcc.image.jpg https://bloximages.newyork1.vip.townnews.com/news-journal.co...

chiph 2y ago

The clearances in Texas are taller than ones I've seen in other states. The national standard is 17 feet, but Texas often uses 18 feet (5.4 meters) or more. Bridges on the East coast interstates sometimes don't even make 17 feet (they often predate many standards). The theories I've heard as to why TXDOT does this include: they wanted sufficient clearance for mobile oil drilling rigs, military vehicles[0], "just in case", and of course, bragging rights. You can zoom in and click on the blue dots to see clearances on major TXDOT bridges here: https://gis-txdot.opendata.arcgis.com/datasets/0126e7969bfb4f6bb92b9b59091c1acb/explore?location=32.641056%2C-96.823570%2C17.64 https://gis-txdot.opendata.arcgis.com/datasets/0126e7969bfb4... [0] The US interstate system wasn't really planned for use by the Department of Defense in time of war, but the bill was named "National System of Interstate and Defense Highways" to help it get passed in Congress. This isn't to say that during a national emergency, the highways couldn't be cleared of civilian traffic. https://highways.dot.gov/highway-history/interstate-system/50th-anniversary/interstate-highway-system-myths https://highways.dot.gov/highway-history/interstate-system/5...