I don't think it need to be any more complex than that

The government needs to expropriate the owners of the land necessary for the building of the highway. See below the legal meaning of "emprise" in French

1. Emprise de terrain. Action de prendre par expropriation, spécialement des terrains indispensables à l'exécution de travaux d'intérêt public. Aucun projet de vue nécessitant une emprise quelconque sur l'hôtel de l'ambassade russe n'a jamais été adopté par l'administration municipale de Paris (Monit. univ.,6 juin 1868,p. 782, 2ecol. ds Littré).

2. P. méton. Surface de terrain acquise par l'administration pour la construction d'un ouvrage (p. ex. une route ou une voie ferrée). La marmaille, passant par les brèches de l'emprise, se poursuivait le long du caniveau d'assèchement de la voie (Hamp, Vin de champagne,1909, p. 91).Le chemin de fer de l'ouest (...) étalait ses membres, déclarait ses emprises, tirait des fusées de rails (Duhamel, Notaire Havre,1933, p. 54).

Note that in the context of highways, construction and planning, "emprise" does not have the meaning of "expropriation" that it would in a more general legal context. Though it might be translated as "footprint" for other sorts of construction project, for highways it should be translated as "reservation" (US), "reserve" or "reserved land" (UK) (because it actually includes more than the simple "footprint" of the highway). This is, however, less straightforward in British English than in US English, because we refer to e.g. the strip down the middle as the "central reservation", so "reserved land" is clearer.

Libération, in this context, is "acquisition".

dsewebapps.dse.vic.gov.au/.../Frankston+C99+Explanatory+Report+Exhi...
near Western Port Highway (under the responsibility of the City of Frankston) to a ..... The acquisition of the reservation for the proposed future freeway upgrade ...



--------------------------------------------------
Note added at 19 hrs (2015-03-01 11:23:24 GMT)
--------------------------------------------------

http://www.legislation.gov.uk/ukpga/1980/66/contents

--------------------------------------------------
Note added at 19 hrs (2015-03-01 11:26:29 GMT)
--------------------------------------------------

www.sabre-roads.org.uk/wiki/index.php?title=Telford
20 Jul 2014 - 1 History; 2 Routes; 3 Highways Management; 4 Named Junctions ... the reserved land, often with full-width bridges for any future expansion. ... The only road maintained by the Highways Agency through the town is the M54.

--------------------------------------------------
Note added at 19 hrs (2015-03-01 11:32:04 GMT)
--------------------------------------------------

I have realised that the correct, specific term for the UK is actually not "reserved land", but "dedicated land".

www.highwayengineer.co.uk/section_278.htm
Depending on which Highway Authority is responsible for the roads that are ... and any land to be conveyed to the Local Highway Authority (dedicated land) ...

issuu.com/waterwaysassoc/docs/waterside_paths_10_11.../62
4 Nov 2012 - A Local Highway Authority can also designate by order any public ... horse riding and/or cycling on the dedicated land or existing access land.

www3.hants.gov.uk/roads/highways.../extinguishment-highwayrights.ht...
8 Nov 2011 - By formal dedication or conveyance to the Highway Authority for highway ... Section 239 Powers for the Highway Authority to acquire land for ...

I think this general vesting declaration (following a. NT or notice to treat under the CPO or compulsory purchase order) is used in order to quickly acquire easement. Note that the title/ownership to landunder the surgface road often remains with the owner thus, if the land is not subsequently used for the purpose acquired then title easement or restrictive covenant rights may not be extinguished or may revert.

Many public highways in the UK have a private owner. That is, someone can prove "title" to them, either by being the registered owner or by having conveyances showing exactly how the land has been bought and sold over a long period of time. Such ownership in no way affects the public highway rights, since the relevant "highway authority" (usually a local authority or the Highways Agency in England and Wales, or Amey Highways in Scotland) is deemed to own the surface of the highway, despite someone else's ownership of the land it passes over or under.
http://en.wikipedia.org/wiki/Highway_systems_by_country
"Many public highways in the UK have a private owner. That is, someone can prove "title" to them, either by being the registered owner or by having conveyances showing exactly how the land has been bought and sold over a long period of time. Such ownership in no way affects the public highway rights, since the relevant "highway authority" (usually a local authority or the Highways Agency in England and Wales, or Amey Highways in Scotland) is deemed to own the surface of the highway, despite someone else's ownership of the land it passes over or under. However, the understanding in some places is that if the road or highway is ever permanently closed and demolished, then complete control of the strip of land on which it lies reverts to the actual owner of the land. In other places, the word "easement" is used. The governmental authority has an indefinitely long easement to use the strip of land for a road, but if the road vanishes, so does the easement that goes with it.

http://www.inhouselawyer.co.uk/index.php/real-estate/10561-c...
"...A notice to treat states that the authority wishes to treat for the purchase of the interest concerned, and demands details of the affected interest and of any claim....8 Where a notice to treat and a notice of entry have both been served, then the acquiring authority may enter and take possession. ... in due course, compensation is either agreed or determined by the Upper Tribunal (Lands Chamber), then title is conveyed to the acquiring authority. 
In the case of a GVD, the effect of a vesting declaration made by an authority or promoter is that title, with certain exceptions, vests in that authority on the vesting date. That date fixes the date by which compensation falls to be assessed, and it is also the date on which the authority is entitled to take possession.9 Accordingly, no further conveyancing is required or takes place. The GVD procedure is appropriate in redevelopment schemes where the acquiring authority requires title at an early stage to enable interests to be granted and finance raised in order to develop the scheme.
.."

https://www.gov.uk/government/publications/acquisition-of-la...

http://www.legislation.gov.uk/ukpga/1980/66/section/250

https://www.gov.uk/government/publications/your-property-and...

http://articles.rollingsons.co.uk/2014/01/compulsory-purchas...
Under Parts II and IV and Schedule 1 the 1981 Act, local authorities and other public bodies can acquire land by means of a general vesting declaration (GVD). The provisions apply both to registered and unregistered land.

A GVD is simply a statutory procedure through which a local authority can acquire the ownership of land and property within a limited time frame, generally around 3 months. Once completed the local authority acquires both the title to the property and rights of possession...A general vesting declaration is effectively a two step procedure...Firstly the local authority intending to acquire the property, the Acquiring Authority, must serve a prescribed statement upon the landowner. It must state that the property will vest in the Acquiring Authority following the vesting declaration at the end of the period referred to in the GVD. That period must not be less than two months....The second part of the procedure is the serving of a notice that the GVD has been made. This must state the date upon which the Acquiring Authority will actually take possession of the property which cannot be effective until at least 28 days from the GVD.

Assuming this is for UK? some other terms here
http://en.wikipedia.org/wiki/Eminent_domain

As in here :
http://archive-edu-2012.com/edu/j/2012-11-14_657865_38/Clear...

here
http://www.hounslow.gov.uk/isleworth_brentford_character_app...

and here:
http://www.actpla.act.gov.au/__data/assets/pdf_file/0008/311...

"l'emprise" is the corridor land on which a transport infrastructure (railway, motorway, etc.) is to be built