I don’t recall exactly when I first heard Professor Mahmassani’s name, but it was likely in the fall of 1999, shortly after I began graduate school at the National University of Singapore. My advisor, Professor Der-Horng Lee, had also just arrived in Singapore, and—perhaps because I was his first and, for a while, only student—we spent a great deal of time talking that fall. Having earned his PhD in transportation and worked in the U.S. for several years, he seemed to know the field exceedingly well. He was also a gifted storyteller, and one of his specialties was to share anecdotes about academics. A quarter century later, I no longer remember the details of those conversations, despite my fond memory of them. What I do remember clearly is that Hani’s name came up often—as a leading authority in dynamic network analysis, the creator of DYNASMART, and a charismatic scholar known for his sharp wit and formidable intellect.

Those stories and names—especially Hani’s—left an indelible impression on me and ultimately played a role in my decision to pursue an academic career myself.

I must have seen Hani from afar at various conferences many times in the early 2000s, when I was a PhD student at UC Davis.  Yet, I have never worked up the courage to introduce myself.  For a young student, having a word with him at those events was no easy feat—it was a fierce competition that required social skills and persistence to win.

The first time I actually spoke with him was at the INFORMS annual meeting in Pittsburgh in the fall of 2006, right after I joined Northwestern University as an assistant professor.  I caught up with him after a session we both attended.  Upon learning that I was new to the profession, he immediately offered advice on how to navigate the tenure process.

By now I’ve forgotten most of what he said except the part about the importance of getting the NSF CAREER award, likely because I never got it (sorry, Hani). What has stayed with me to this day, however, was how gently and candidly he spoke to me.  I remember thinking afterward: Wow— this isn’t the imposing figure that I had admired in awe from a distance, but a humble and empathetic mentor, someone you could trust and turn to for guidance.

As fate would have it, just a few months after we first met, Hani came to interview for the William Patterson Chair Professorship.  On the day of his seminar, I ran into the Dean of Engineering, Julio Ottino, in the hallway.  He casually asked for my opinion about Hani’s candidacy and said something to the effect of, “I heard he’s like a god in your field.” I nodded without the slightest hesitation.

Hani arrived in Northwestern to join my department in the summer of 2007.  Soon after, he also assumed the role of the director for the university’s renowned Transportation Center.  His leadership at that critical juncture transformed the center—from an endangered entity into a thriving institution that truly lived up to its promise: a world-class think tank and laboratory for cross-disciplinary transportation research and education that matter.

It took me months to accept the seemingly surreal fact that Hani—someone everybody in the field knows and talks about, whose name I had mostly encountered in books and papers—was now a colleague who sometimes sat next to me at faculty meetings. It will take much longer to come to terms with the gigantic void he has left behind.

There is no need for me to tout Hani’s legacy in transportation research, industry, government agencies, and the broader scientific community. A short piece of personal reflection cannot do it justice anyway. As a colleague who had the fortune to observe him up close, it’s not hard to see why Hani accomplished all that he did.

Hani has one of the most brilliant and inquisitive minds that I’ve ever known. He was always open to new ideas and able to grasp their essence as quickly as anyone in the field. That’s why his expansive research portfolio—now including autonomous vehicles, tele-work, urban air mobility, machine learning and AI—remains cutting-edge, often ahead of his much younger colleagues, myself included.

At PhD prospectuses and defenses—where our intellectual interactions most often took place—he always asks the most penetrating questions, the kind that could only come from someone with a profound understanding of the topic at hand.  It was hard to walk away from those encounters without feeling that he knew almost everything.

What amazed me the most was that, even in his 60s, Hani could still be animated by new ideas like a graduate student.  It was not uncommon for our discussions during PhD defenses to be carried away by one of Hani’s bursts of human ingenuity, leaving the thesis defender standing by in relief and amusement.  As far as I could tell, his passion for developing DYNASMART—the software he began almost 40 years ago—never wavered in the slightest.

Hani is also one of the most hard-working academics that I’ve ever known.  He maintained an excruciatingly busy schedule that includes a tremendous amount of travel.  When I was an assistant professor, I once complained to Hani that I could not concentrate on writing proposals because I had to teach at the same time. Hani reply with a smile, “Marco, you have to multi-task.“  Since then I’ve taken that advice to heart—but I know I could never be as good a mulit-tasker as he was.

Hani was famous for sleeping only four hours a day, typically going to bed in the early morning hours. I once read that there is a genetic mutation that allows a person to function well on much less sleep than normal—about six to seven hours a night. Roughly one percent of people have that mutation. I have no doubt that Hani was a one-percenter. Still, a four-hour-a-day sleep schedule, sustained over decades, must have taken a toll on the body.

In any case, because of Hani’s unusual hours, many of his associates learned to get his attention by writing to him after midnight—because that’s when he replied to emails. When I was younger, I occasionally took advantage of this trick. It worked nearly every time, though in recent years, I could no longer keep up with him.

The busy life Hani led meant that he could sometimes be hard to reach by email. A non-trivial proportion of the messages I sent him never received a response. For a while, I found this spontaneity to be an annoying inconvenience. But eventually, I came to realize that—like every human being—Hani also had only 24 hours in a day. Yes, he slept three hours less than most people, but even that wasn’t enough to share with everyone who wanted a piece of his attention.

The truth is, all my important requests were answered in a timely manner. More importantly, whenever he did respond, he made the time count: he would sit me down and ensure I was treated with warmth, respect, and care. In those moments, he made me feel as if I were the center of his attention.

In retrospect, every milestone I achieved at Northwestern owes something to Hani.

• He was a co-PI on my first NSF grant, a critical component of my tenure package.
• He guided me through the tenure process. We had lunch together at one of his favorite Evanston restaurants—whose owner Hani knew personally (he had great taste in food and restaurants, by the way)—to discuss who should write letters for me (and who should not). As always, he insisted on paying.
• After my daughter was born in 2010, he surprised me with a nicely wrapped gift one day when I showed up at the Transportation Center for a meeting.
• In 2016, when I was debating whether to go up early for promotion to full professor, it was Hani who reassured me that I was ready and promised his full support, without reservation.
• Most recently, when I was tapped to be the next department chair, Hani was among the first who congratulated me and expressed his confidence. He met with me for an hour to offer a wide range of advice in late May, about a month before his passing. That was the last time I saw him.

As I write down this long and woefully incomplete list of things Hani did for me, I can’t help but ask myself what I ever did for him. The answer is: almost nothing. I didn’t even buy him a single meal—and I can’t count how many times we dined together. I had never given it much thought, but I realize now that I had quietly assumed there would be time to return the favor. After all, we were colleagues; there would always be another opportunity, right?

Now that he’s gone, the thought that I will never have the chance to buy him a meal—even just as a small token of gratitude—makes me feel incredibly sad and powerless.

Yesterday, the Transportation Center organized an informal gathering in Hani’s memorial. I sat at a table with my colleagues Joe Schofer and Pablo Durango-Cohen.  We chatted about how the sudden death of someone close to you sheds light on the fleeting fragility of life and the importance of living in the moment, not in the future.

Living in the moment means to do what feels important to you now, whether that’s writing an essay that speaks your truth, visiting a faraway place that occupies a special place in your heart, saying “I love you” to your family, or making a trip just to tell an old friend “I miss you.”

The conversation reminded me of something I once read about the Japanese eschatology captured in the word sayonara. Commonly translated as “goodbye,” it literally means “so be it.” To the Japanese, life moves along an unpredictable path. Every moment holds the potential for abrupt change—even death. So they say sayonara not just to mark parting, but to appreciate the moment and its mortality. As John Toland wrote in The Rising Sun, this eschatology gave the Japanese “the strength to face disaster stoically and a calm determination to let nothing discourage or disappoint.”

In every sense, Hani was a living embodiment of the ideal behind sayonara. He was always on the move—full of curiosity, conviction, and fortitude—chasing his dreams while savoring every step along the way.

The fact that Hani lived the fullest version of his life is the small comfort I take as I mourn the premature passing of a giant in our field.

Sayonara, Hani. May you rest in peace.

Marco Nie, July 19, 2025