Restricting road access by vehicle type in ArcGIS Network Analyst

To do this look at the section on Assigning values to restrictions in the Network Analyst page on Types of evaluators used by a network:

Each attribute defined in the network must have values for each source participating in the network. An evaluator assigns values for the attribute of each source. ... In ArcGIS, the field evaluator assigns values to a network attribute from a field of a network source. In addition, ArcGIS offers other types of evaluators that can be used, for instance, constant, field expression, function, and script evaluators.

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Restriction attributes have a Boolean data type. Since a source element can either use or ignore the associated restriction, it can be assigned a constant value of Use Restriction or Ignore Restriction.

Alternatively, restriction attributes can be derived using the function evaluator to compare another attribute to a parameter value. For example, an attribute that models vehicle height restrictions can use the function evaluator to compare the height limit of a road to the vehicle's actual height stored in an attribute parameter. When the expression evaluates to true, the restriction is used on the road; when the expression evaluates to false, the restriction is ignored. The exception to this rule is that anytime either of the operands (MaxHeight or Vehicle Height) have a value of zero, the expression always evaluates to false.