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When widening a road, why does construction seem to inevitably include a "smoothing" out of the curves on the road, essentially making it more straight?

I've seen this personally on roads where I travel, and you can even catch a glimpse on Google satellite imagery of roads that are in the widening process: Google overlays a "ghost" lane to show where the new lanes of the road will be. It's faint, but you can see an example here on a road near Flowery Branch, GA:

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Is this the engineers' way of correcting previous mistakes, or a response to devleopment that's sprung up around the road since its initial construction, or safety reasons? Or something else entirely?

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    $\begingroup$ Some older roads were sometimes made without being designed. Generally they followed the natural topography, providing gradients weren't steep. I know of one major highway build in mostly mountainous terrain during the 1930s Depression which was build like this and the road making contractor was paid according to the length of the road. To get more money, the contractor made the road windier than needed to make the road longer. $\endgroup$
    – Fred
    Aug 23, 2018 at 13:54
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    $\begingroup$ @Fred it depends who designed the "old" ones. In the UK, several of the road routes designed and built by the Romans 2000 years ago are still in use, because they are generally "a straight line from A to B." Sometimes the ancient British road has also survived, because it meandered around the contours of hills with the minimum vertical gradient, and still connects the small towns and villages along its route. I used to live close to the longest stretch of perfectly straight road in the UK - just over 23 miles, and originally built by the Romans. $\endgroup$
    – alephzero
    Aug 23, 2018 at 21:55

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A wider road will have faster average traffic speeds, because faster vehicles will be hindered less by slow ones, with more lanes available for overtaking.

So longer visibility distances are needed for safety, and hence more gradual curves and more gradual changes of gradient (no "hidden spots" over the brow of a hill, etc).

Note, for most "fast" roads traffic speeds are not limited at all by the radius of the curves, except perhaps in very severe weather conditions (snow and ice, standing rainwater, etc). Aside from breaking the law and the hazards posed by other vehicles, there would be no problem in driving a modern vehicle along the road way above the legal speed limit.

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I am familiar with this effect; with careful observation you can count at least three rounds of road-straightening on highway 17 between the cities of San Jose and Santa Cruz in California.

The first road was built over 100 years ago and carried horse-drawn wagons. At the time, moving large volumes of earth and rock was more expensive than surfacing the road and so it followed sharp curves in the mountainsides instead of being extensively built on fill from nearby road cuts.

Then came automobiles and more and faster traffic, and when widening the original stage road the sharpest turns were rounded off to carry faster vehicles and cut-and-fill was used so the mountain contours didn't have to be closely followed.

At each step in this decades-long process, the cost of earthmoving fell and the cost of pavement rose, and so it became economical to use more and more cut-and-fill in conjunction with straighter and straighter road paths.

The stage road is now a 4-lane freeway which still has a few somewhat sharp turns in it but much of it is built by cut-and-fill and can be traversed at 50 to 60 MPH.

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    $\begingroup$ Or re-phrased as "over time we have less respect for the land - first we used to work with it and now we force it to work with us"... $\endgroup$
    – Solar Mike
    Aug 22, 2018 at 22:35

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