To solve Bob's potato transportation problem, we need to formulate a linear programming (LP) model. The goal is to maximize the number of potatoes transported given the constraints on truck and van capacities, costs, and traffic laws.

Let's define the variables:
- \(T\) as the number of trucks used,
- \(V\) as the number of vans used.

The objective function aims to maximize the total number of potatoes transported. Given that each truck can carry 150 potatoes and each van can carry 80 potatoes, the objective function can be written as:
\[ \text{Maximize} \quad 150T + 80V \]

Now, let's consider the constraints:
1. **Cost Constraint**: The total cost of running trucks and vans must not exceed $250. Given that each truck costs $20 per trip and each van costs $12 per trip, we have:
\[ 20T + 12V \leq 250 \]
2. **Traffic Law Constraint**: The number of trucks must be less than the number of vans:
\[ T < V \]
However, since \(T\) and \(V\) are integer variables (you cannot have a fraction of a truck or van), we can represent this constraint as:
\[ T \leq V - 1 \]

To ensure that \(T\) and \(V\) are non-negative integers (since you can't use a negative number of vehicles), we also need to specify:
\[ T, V \geq 0 \]
And because these variables must be integers:
\[ T, V \in \mathbb{Z} \]

Now, let's translate this problem into Gurobi code in Python:

```python
from gurobipy import *

# Create a new model
m = Model("Potato_Transportation")

# Define the variables
T = m.addVar(vtype=GRB.INTEGER, name="Trucks")
V = m.addVar(vtype=GRB.INTEGER, name="Vans")

# Objective function: Maximize the total potatoes transported
m.setObjective(150*T + 80*V, GRB.MAXIMIZE)

# Constraints
m.addConstr(20*T + 12*V <= 250, "Cost_Constraint")
m.addConstr(T <= V - 1, "Traffic_Law_Constraint")
m.addConstr(T >= 0, "Non_Negativity_Trucks")
m.addConstr(V >= 0, "Non_Negativity_Vans")

# Optimize the model
m.optimize()

# Print the results
if m.status == GRB.OPTIMAL:
    print("Optimal solution found:")
    print(f"Number of Trucks: {T.x}")
    print(f"Number of Vans: {V.x}")
    print(f"Total Potatoes Transported: {150*T.x + 80*V.x}")
else:
    print("No optimal solution found.")
```