Published 07 February, 2020
We measure eight discontinuities of over ten years in the history of longest bridge spans, four of them of over one hundred years, five of them robust as to slight changes in trend extrapolation.
The annual average increase in bridge span length increased by over a factor of one hundred between the period before 1826 and the period after (0.25 feet/year to 35 feet/year), though there was not a clear turning point in it.
This case study is part of AI Impacts’ discontinuous progress investigation.
We investigated bridge span (rather than bridge length, mass, or carrying capacity) because it was suggested to us as discontinuous. We also expect it to be a good metric for seeing technological progress, rather than economic progress, because additional spending can probably add more spans to a structure more easily than it can make each span longer. Span length is also a less ambiguous metric than total length, since it is not always clear where a road ends and a bridge begins.
We gathered data for bridge span lengths from several Wikipedia lists of longest bridge spans over history for particular types of bridge, plus a few additional datapoints from elsewhere. Our data and citations are in this spreadsheet in the tab ‘five bridge types’.
Problems, ambiguities, and limitations of the data and our collection process:
Figure 1-3 show the length of the longest bridge span for five types of bridge over time. If we understand correctly, these include the longest bridges of any kind at least since around 500AD.
To measure discontinuities relative to past progress, we treat past progress as linear, and belonging to five different periods (i.e. three times we consider the recent trend to be sufficiently different from the older trend that we base our extrapolation on a new period).4
Using this method, the length of the longest bridge span has seen a large number of discontinuities (see table below).
|Year opened/became longest of type
|Main span (feet)
|Menai Suspension Bridge
|Great Suspension Bridge*
|Wheeling Suspension Bridge
|Niagara Clifton Bridge*
|George Washington Bridge*
|Golden Gate Bridge
|*Entry was more robust to informal experimentation with different linear extrapolations
Deciding what to treat as the previous trend at any point is hard in this dataset, because the shape of the trend isn’t close to being exponential or linear. The sizes of the discontinuities and even the particular bridges that count as notably discontinuous are not very robust to different choices. In a small amount of experimentation with different linear trends, five bridges were always discontinuities, marked with * in the above table. That the overall trend is marked by many discontinuities seems robust.
The annual average increase in bridge span length increased by over a factor of one hundred between the period before 1826 and the period after (0.25 feet/year to 35 feet/year), though there was not a clear turning point in it. See spreadsheet for calculation (tab: ‘Five bridge types (longest)’)
“Span (Engineering).” In Wikipedia, November 7, 2017. https://en.wikipedia.org/w/index.php?title=Span_(engineering)&oldid=809190532.