On speed limits and lane counts on highways

16 Jun 2021

I’ve recently moved out of Paris due to soon ending my wonderful journey with Datadog. Thanks to my parents helping me out, it was much easier than doing it all alone. Still, this was a much more logistically complicated process than I initially thought:

1. I couldn’t travel by airplane, as the amount of luggage to transport would cost me an arm and a leg. I didn’t want to leave my luggage to be transported by a moving company, as I had a few very important personal items.
2. COVID is still a thing, meaning - tests. Coordinating taking a test within 72h of arrival to France (travelling through Germany) required a bit of planning

Needless to say, while driving / riding a car for > 1700 km one-way (Warsaw <-> Paris), you have lots of time to think. And think I did. “What about”, you ask? Naturally, about highways, which constituted probably > 75% of the total distance travelled by us.

A couple of my observations:

1. Significant parts of German highways (“autobahns”) have no speed limits, no strings attached. No such thing in Poland and France.
2. Where speed limits exist, there are noticeable differences between countries (Poland having the highest limit of 140 km/h, France and Germany both having 130 km / h)
3. The distribution of speed limit areas changes between countries (it seems there are more in Germany than in Poland)
4. Polish drivers seem to obey speed limits more rarely than German and French drivers (purely based on my observations - perhaps this is because Poles are rarely fined due to many fewer speed cameras, and the fines being lower?)

Given all the above, you’d think that getting from point A to B in Poland would be the fastest of the three countries. Turns out, you would be wrong. After nearly 30 h spent on highways in all these countries, here are my observations and hypotheses that explain this apparent paradox:

• German and French highways primarily have 3 lanes, while Polish ones have 2. This is a surprisingly big deal, much more important than any speed limits. Why? Highways are the primary “arteries” through which heavy trucks transport goods. These trucks are slow compared to passenger cars, and so they stick to driving on the rightmost lane. Here’s what happens depending on the number of lanes:
  • On a 3-lane highway, this leaves the middle lane for medium-speed passenger cars, and leftmost lane for high-speed cars and overtakers
  • On a 2-lane highway, the only lane left for passenger cars (both medium and high-speed) is the leftmost one.

Now, what happens if a truck decides to overtake another truck, or if a particularly slow passenger car stays on the left lane? On a 3-lane we’re fine. This still leaves the leftmost lane free. On a two-way, we have a problem. All the cars on that lane will be speed-limited by the slowest-moving car, with no way of overtaking.

This phenomenon was extremely obvious when 3-lane highways in Germany temporarily narrowed to 2 lanes - the whole traffic slowed down by ~ 20 - 30 km / h on average, regardless of the speed limit. This seems to me like the biggest factor behind why driving on Polish highways will be slower than on German ones, despite a higher speed limit.

• While German speed limits seem more frequent than Polish ones, they are introduced gradually, while the Polish ones - more abruptly. E.g. if there is a sharp turn, the speed on a German highway will be gradually reduced to 120 -> 100 -> 80 or similar. In Poland, there is typically only one speed limit reduction, right down to the lowest speed. This makes “fast traffic jams” on the Polish highways, which contributes to slowing down the overall traffic.

• German highways are famously smooth. This means that no speed reductions are needed before surface irregularities such as holes.

It’s clear there was a lot of clever planning done when designing these highways in Germany, and this really contributes to making it a pleasure to drive on them.