To survive ~ 15 seconds isn't so easy. When ammo is empty then ammo/weapon should respawn earlier. Can you highlight the "deathzones" and overwork your controls? Left and right is untypical. Weapon switch should be better.
Countertime for vehicles you should change too. When counter reached zero then vehicles should slowdown.
Option for weapon pick up, if a pickup is not so strong than current don't switch. If you have only one weapon with ammo then switch it automatecly on start.