I'm no expert by far, but in coreabilities.xml check
<AbilityBonus InternalName="RoadBuildingAbility">
and replace the record with
<AbilityBonus InternalName="RoadBuildingAbility">
<AbilityBonusType>Unit_Design</AbilityBonusType>
<AbilityBonusOption InternalName="RoadBuilding">
<DisplayName>Road Building</DisplayName>
<Description>Allows the unit to build roads</Description>
<Icon>Ability_PathOfTheGovernor_Icon.png</Icon>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>UnlockAction</Attribute>
<StrVal>BuildRoad</StrVal>
<Provides>Allows the unit to build roads</Provides>
</GameModifier>
<AdditionalTrainingTurns>100</AdditionalTrainingTurns>
<Type>Army</Type>
<AIData AIPersonality="AI_General">
<AIPriority>5</AIPriority>
</AIData>
</AbilityBonusOption>
</AbilityBonus>
Obviously with the cost I use, you'll only be able to afford one road builder in mid game, not before, and then some others after that. No spamming...