It is thought that, in the case of a flood, we would not be able to evacuate the Randstad in time. The Randstad is a megalopolis with 7.2 million inhabitants, covering Dutch cities like Amsterdam, the Hague, Utrecht and Rotterdam. Studies indeed confirm that, in the case of a coastal flooding, an evacuation in the case of coastal flooding is not possible, even with a two-day lead time. But the Dutch have one special trait, nearly everybody knows how to ride a bike.

The concept of the intractable evacuation of the Randstad is based on an analysis of our traffic capacity. The real computations are based on simulations but the problem is easy to reproduce with just pen and paper.

The first question is, how many people need to be evacuated. Let's say we evacuate the bigger Den Haag area and Rotterdam area, which comes down to 1 + 1.2 = 2.2 million people. The Randstad also includes areas like Utrecht and Amsterdam, but these are less prone to coastal flooding.

What if everybody takes the car? How many highway lanes do we need? Assuming everybody adheres to the "keep 2 seconds distance rule" and drives 100km/h we can derive that the distance between 2 cars is 55m (100km/h * 2s). If we add to that the length of a car and round it up we have 60m per car and thus about 16 cars per km. If everybody drives at 100km/h, it will take 36 seconds for a section of 1km to fill up with new cars. So we have 16 cars per 36 seconds and can therefore evacuate 1600 cars per hour per lane. This number is not that different from measured "production". In reality the capacity can be higher because not everybody adheres to the 2 second rule and becomes lower due to stormy weather. The average number of cars per inhabitant in the Netherlands is approximately 0.5 so let's assume 2 occupants per car.

If order to evacuate everybody in 8 hour window, we need a total number of 86 lanes to get everybody out. Even with measures like lane reversal, which one should be prepared for in the case of an evacuation, we simply do not have enough lanes to get everybody out of the cities. This is one of the main reasons why the "vertical evacuation" strategy is in place.

The biggest bike ride ever, can it be done?

But what if we evacuate by bicycle? Would we not be able to evacuate more people? Let's go through the numbers. A bike needs about 3 meters of space and you can fit 4 bicycles next to each other in 1 highway lane. So that gives us 1300 bicycles per lane per km (1km / 3m * 4 bicycles per lane). Let's assume an average of 13km/h which results in 4.6 minutes before a 1km stretch fills up with new bicycles. That results in a total number of bicycles of about 17000/hour. This comes down to a requirement of 16 lanes for an evacuation in 8 hours.

So we can fit about 100 times as many bicycles on a stretch of highway but they travel 10 times slower than cars, which results in 10 times more vehicles per hour. Of course cars have some advantages compared to bicycles. You can take more belongings and your pets, and are shielded from the elements. But bicycles are also convenient, most important is that traffic jams resolve themselves faster because a bicycle can move left and right. When a coastal flood will occur it will be most likely be a North-Western, which for cycling away from the Randstad means a convenient tailwind.

A bicycle evacuation might be possible, but it requires careful planning. One has to take into account aspects like how to transport the people that can't cycle, are too old to cycle, have a flat tire. Evacuations are a hazardous undertaking, even more so when you send 2.2 million people on a 5 hour bike ride in bad weather. Should we use the next "car-free sunday" for an evacuation experiment? What would you do, take the bike, the car, or the train?

Fedor Baart & Anna van Gils

Images are from:

Eindrapport, Veiligheid Nederland in Kaart

Houston, TX, September 21, 2005, Traffic on I 45 during evacuation for Hurricane Rita. Ed Edahl @ wikimedia

Still-Leben Ruhrgebiet im Essener Zentrum am Ruhrschnellwegtunnel. Fahrrad-Stau hinter einem Tunnel. Barabbas @ wikimedia



