def limit_exceeded(amount = 1)
      limits.each do |limit|
        limit_exceeded = limit.limit_exceeded(amount)
        return limit_exceeded if limit_exceeded
      end
      nil
    end
def limit_exceeded(amount = 1)
      limits.each do |limit|
        limit_exceeded = limit.limit_exceeded(amount)
        return limit_exceeded if limit_exceeded
      end
      nil
    end
Return the first limit to be exceeded if incrementing by the given amount,
 or nil otherwise. Does not change amount used for any limit.

 @param amount [Integer]
 @return [TrafficJam::Limit, nil]