Those roads may bear those names - but may not have been built by the Allagans themselves - they may have been built by civilisations that came later, in an effort of trying to recapure some of the Allagan glory by naming the roads thusly.

If you want a real life example, you only need to look back to the height of the Roman Empire.
If you were under the influence of the Roman Empire back in the 4-500's, you got awesome road ways and market systems etc - advanced for that time period.
Then the roman Empire collapsed - all the people that remained were unable to match and build what the Romans had been capable of - they were literally living in the remains of a past empire, watching things decay that they couldn't build themselves, or even know how to fix. It took another 1000 years before people were able to come back to a stage that the Romans had been at previously - and they were constantly trying in those years to rebuild, they just didnt know how.