DaveG hello.
I did some testing and these are the notes I made at the time.
Graneries and market buyers seem to operate on an x-y grid system.
A market buyer will use a granery only when both the x and y distances are less than 38 tiles. I think the distance is from the nearest tile on the market to the nearest tile on the granery. It makes a difference dependant upon which side of the road you put them. The distance by road is immaterial. I had a buyer going several hundred tiles around Lugdunum.
Market buyers get food, if available in the order - wheat, veg, fruit, meat, (preferred order).
She will continue to get food in that order until she has her minimum stock of each, 600 wheat and 400 of the others.
She will get sufficient food to bring the market up to its maximum stock, 800 wheat and 600 of the others. The amount she collects depends on the markets stock at the time she reaches the granery (telepathy?)
She will get wheat if she is below her minimum stock even if the granery is a long way away and only contains 100 units. She will ignore a nearby granery full of a less preferred food type.
A market buyer goes to a granery for one specific food type. If the granery does not have that food the buyer disappears on reaching the granery aand reappears at the market as soon as a required food type appears in a granery. Sometimes immediately after she disappears.
Market traders distribute food in the same order and people eat it in the same order.
A market trader will not give out veg etc. until she has run out of wheat.
The people in the houses will not eat veg etc until they have run out of wheat, balanced diet?
Low grade pleb housing, e.g. small casa take 4 units of food per occupant when a market trader passes and they are below their "minimum stock" of 4 units per person.
For example a small 2by2 casa with 68 occupants will take 4 x 68, 272 units when its stock falls below 272 units giving it a normal maximum stock of 543 units or 8 units per occupant, 16 months supply. Things get a bit complicated when two market ladies with different food types provide the same house simultaneously.
Conclusion, if you have housing blocks using different food types do not have a road connection between them.
If you want to evolve grand insulae build up a good stock of the second food type and make sure that the markets and houses are well stocked with food before you make a road connection to the second food.