Change the Navigation cost for Roads created with the Landscapespline tool

How can I change the Navigation cost for Roads created with the Landscapespline tool? I tried changing the RoadMesh to a DynamicObstacle and adding a NavArea with lower cost to it. However, now when I start the Project I get ensure condition failures from the SplineMeshComponent:

bool USplineMeshComponent::DoCustomNavigableGeometryExport(FNavigableGeometryExport& GeomExport) const
{
    // the NavCollision is supposed to be faster than exporting the regular collision,
    // but I'm not sure that's true here, as the regular collision is pre-distorted to the spline

    if (GetStaticMesh() != nullptr && GetStaticMesh()->GetNavCollision() != nullptr)
    {
        const UNavCollisionBase* NavCollision = GetStaticMesh()->GetNavCollision();

        if (ensure(!NavCollision->IsDynamicObstacle())) // This ensure fails
        {
...

void USplineMeshComponent::UpdateBounds()
{
    Super::UpdateBounds();

    CachedNavigationBounds = Bounds.GetBox();

    if (const UStaticMesh* Mesh = GetStaticMesh())
    {
        if (const UNavCollisionBase* NavCollision = Mesh->GetNavCollision())
        {
            // Match condition in DoCustomNavigableGeometryExport
            const FBox NavCollisionBounds = NavCollision->GetBounds();
            if (ensure(!NavCollision->IsDynamicObstacle()) // This ensure also fails
                && NavCollision->HasConvexGeometry()
                && NavCollisionBounds.IsValid)
            {
                const FBoxSphereBounds NavCollisionBoxSphereBounds(NavCollisionBounds);
                CachedNavigationBounds = ComputeDistortedBounds(GetComponentTransform(), Mesh->GetBounds(), &NavCollisionBoxSphereBounds);
            }
        }
    }
}

Can you get the length of the splines and use that in the cost algorithm?