I am really guessing here because admiteddly I've never seen the word 'creces' used in this context and I think that I'd be asking for help too in your shoes, but from the context I can imagine that what it means is that sidewalls or retaining walls must be built to offer a sufficient level of protection on the sides of the highway. It's hard to find anything to back my theory up because of the amount of other uses of the word 'creces' but 'creces' conjures up the image to me of something rising from the ground, which in this context is likely to be a ridge or a berm.

See:
Civil engineering use [edit]Typical earthworks include roads, railway beds, causeways, dams, levees, canals, and berms. Other common earthworks are land grading to reconfigure the topography of a site, or to stabilize slopes.

Berm has been adopted as a wider term usually used to describe a physical, stationary barrier of some kind. For example in modern highway construction, a berm is a noise barrier constructed of earth, often landscaped, running along a highway to protect adjacent land users from noise pollution.
http://en.wikipedia.org/wiki/Berm

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Note added at 8 hrs (2013-05-29 08:59:22 GMT)
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Having read your note, I think you're right in that 'berm' is perhaps too permanent-sounding for the context. On balance, I think that 'retaining ridge' is good. I have seen one (but only one) instance of 'creces' being used to mean 'ridge' on linguee but it didn't seem to be enough to confirm my suspicion. It is here:
Sin compensación de radio la punta teórica (P) recorre la trayectoria programada (figura central) dejando creces de mecanizado. fagorautomation.esfagorautomation.es Without radius compensation, the theoretical tip (P) travels the programmed path (center figure) leaving machining ridges. fagorautomation.esfagorautomation.es

I've just seen too that I spelled "admittedly" incorrectly too!

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Note added at 8 hrs (2013-05-29 09:10:46 GMT)
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Yes. Fagor make machine tools so the context is not even close but it is at the very best minute substantiation for 'creces' being able to be translated as 'ridges' or at least as something other than 'grows' etc.