A barrier, crudely constructed out of tree branches, blocks the road. This presents no real obstacle to travel and the PCs can easily bypass it they so wish.
Beside the barrier is a four-man tent of the kind commonly used by the Kings Army and a small handcart on which is a wooden cage containing several homing pigeons. Setting in front of the tent are three scruffy looking soldiers playing cards and drinking tea. The soldiers uniforms are frayed and dirty but their weapons (crossbows / muskets / swords / spears / whatever) are in good repair.
As the PCs approach the barrier the sergeant greets then and demands, in the name of the King, that they pay a toll to travel the Kings’ Road. Note that the amount demanded should not too excessive, roughly what would be expected for a road toll.
If the PCs pay the toll the soldiers will drag their makeshift barrier clear and allow them to pass, if not they will release one of their homing pigeons but will take no other action unless attacked.
1. Genuine Toll Booth - The soldiers are exactly what they claim to be.
If the PCs refuse to pay the toll they will be arrested as soon as they enter the next town and held until the local magistrate is holding court (in about 3-4 days). The magistrate will then order that they pay a heavy fine for refusal to pay the Kings’ taxes. He will accept neither excuses nor protestations of innocence nor pleas of poverty.
2. Highway Robbery - The soldiers are actually highway bandits running a simple, but highly effective, scam.
By posing as toll collectors their victims normally hand over the loot without a fight, as long as they don’t get greedy of course. Also, they are unlikely to be reported since the victims do not even realise they have been duped.
The pigeons are just wild birds captured to compete the disguise (genuine toll stations always have messenger pigeons).
3. A Test of Honesty - The station is set up by the local authorities (lord / town council / whatever) to test the honesty of approaching adventuring parties (like the PCs).
Those parties who pay the toll (and thereby demonstrate their honesty) will be welcome in the next town, those who do not will find the town gate barred to them.