[AuSsIeThUnDeR36]

After getting the jet pack in the mission The Black Project, I decided to head straight for Las Venturas to collect horse shoes and I was surprised to find LV looked deserted. Hardly any vehicles and just a few peds here and there. It was dead, usually LV is a busy place with lots of activity but it was like a ghost town.
My feeling is that because I was technically still on The Black Project mission that the graphics were more focused on the Area 69, which might explain the lack of vehicles in LV.
Try it out and see for yourself. I don't use mods either this is 100% legit

[Carbonox]

There are occasionally missions where traffic is either completely removed or clearly reduced (in some instances your wanted level remains locked, such as in that very Black Project mission - great opportunity to steal a tank or a Hydra without trouble! You only need to get rid of the jetpack and use a vending machine to gain the ability to enter vehicles again). You should notice the same or a similar effect in other missions that allow you to freely roam around, while being primarily focused on a single area, such as Reuniting the Families earlier in the story.

[˸øåíüêà]

"Interdiction" is an interesting mission. Even though the roads aren't empty, there is a very small chance to see a car until sunrise, apart from the caravan, that pulls up in the very beginning. Don't think this has been explained yet.

[AuSsIeThUnDeR36]

Interdiction is easier by doing the following - when you get the call from Toreno, go to the San Fierro airport and pick up heat seeking rockets from between the fuel cells western side. Then fly the chopper to Toreno's ranch and land in front of the garage. Activate the mission, fly to the ghost town but don't land in the red marker, land away from it. Walk into the marker. Ignore what Toreno says, you already have the rockets! Get back in the chopper and fly up to the cliff. Again, land to the left of the marker or a bit behind it, so your heli won't be in the attack zone. Walk into the red marker. Blow up the attacking helis. Be careful you don't accidentally target your own heli on the ground. Then the cut scene shows the package floating away. Then just take your heli and fly to where it lands in that area near the beach inlet. If you can get there fast enough and land, you will see the package still floating down which is cool. Then just pick up the package and fly back to the ranch. The cool thing with the helicopter is that it will then be available to fly straight to Verdant Meadows for the next mission, saving a lot of time driving by roads