His remarks followed reports indicating that the Executive is expected to approve the project plans on Thursday.
Mr McCrossan has spent nearly a decade campaigning for road safety works.
He said: "This is a momentous day for this entire area and first and foremost my thoughts are with the families of everyone who has lost a loved one on this road. This has been a very long journey – far too long – and their strength and courage has been a constant source of inspiration and we would never have seen this delivered without them.
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"The campaign to build the A5 has been my number one priority throughout my time in politics and I have carried on the efforts of my SDLP predecessors in working with everyone to see these important upgrades carried out. Campaigners deserve huge credit for facing down every obstacle and making it absolutely clear they would not stop until people could safely travel on the A5.
"This must be a watershed moment – it cannot be another false dawn. The A5 is key to the future of this entire region, not only in saving lives and preventing injuries, but in creating economic opportunities by making our towns and cities better connected, bringing much needed investment and jobs.
"We must also have assurances from the Executive and Minister that the A5 will be delivered in full. We have seen repeatedly in the past how phased projects of this nature have stalled or been beset with delays. After such a long wait we need work to start – and finish – as soon as possible and I will not stop pushing this issue until these improvements are delivered and this road is fully built."
