In a recent trip to Utah, I was driving from Salt Lake City to Moab. It is a 230-mile drive, almost all of it on highways. On long road trips like this, I like to put my car on cruise control, engage all driver assist features and have a relaxed drive. In my pursuit of relaxed driving, I am usually slightly below the speed limit and in the slow lane. However, this section of the highway just had one lane in each direction. Soon, I noticed a fast approaching pickup truck in my rear-view mirror. As expected, the pickup driver started tailing my car and, after a while, flashing his headlights - urging me to drive faster. I was in no mood to do any such thing. Luckily, we entered an overtaking zone and I moved over. The pickup driver slammed on accelerator and sped past, gesturing some fingers at me. Nothing new so far.
20 mins later, I reached my destination - Arches National Park. There was a queue of cars waiting to enter. And guess who I found just two cars ahead of me. Our dear friend - the pickup driver! Let’s call him John. All that speeding and here he was entering the park at the same time as me. In my limited driving experience, I have come across quite a few Johns - over-speeding, aggressive lane changes, trying to beat a yellow and a lot of hand gesturing. But they all reach the destination at more or less the same time as me. This begs a question - why did the Johns not reach early even when they sped up? If you go 50% faster, you should reach in 50% less time. Right? While that sounds correct, it isn’t really. Alas! Life is not linear. The rest of the article describes 3 fundamental non-linearities which prevent you from reaching early even if you speed up.
Non-linearity of time Time taken is equal to the distance covered divided by the average speed. So, the relationship between time and speed is inverse, ~not~ linear. It means that going 50% faster will only save 33% time, not 50%. Let’s take an example to understand. Say that your average commute distance is 5 miles. Consider a scenario where most of this journey is on a highway and you are able to maintain an average speed of 60mph. At this speed, it will take 5 mins to complete this trip (5/60 = 0.0833hrs = 5mins). Now, let’s say you are in an extreme hurry and decide to speed up by 50% (i.e. to 90mph) throughout the journey (Disclaimer: You should not do this). In this case, you will reach home in 3.34mins (5/90 = 0.0555hrs = 3.34mins). Instead of saving 50% time (2.5 mins), you only saved around 1.7mins. What a bummer! To add insult to injury, your GPS might just show the total trip time as 4mins. So, while you did reach early, the time saving was not proportional to the effort you put by consistently driving at a high speed. Moreover, most people will not consider time savings of 1.7mins as reaching early as it might be below their just-noticeable-difference threshold.
However, let’s consider another scenario where the commute distance is still 5 miles but this time it mostly consists of city driving. Therefore the average speed is around 30mph. So, your commute time will now be 10mins (5/30 = 0.1667 hrs = 10 mins). Driving at 50% faster (at 45mph), you will complete the trip in 6.67mins instead. A saving of 3.3 mins! The same effort of driving 50% faster leads to more time savings if you start from a slower speed. But, we humans tend to underestimate the time saved by increasing speed when starting from a lower speed. This is called the time-saving bias.
Non-linearity of speed One might argue, “Hey! I am smarter than most humans and I am not affected by the time saving bias. So, by speeding up on city roads instead of highways, I can save a lot of my valuable time”. Not so fast. In the previous section, we assumed that you could consistently maintain a speed 50% higher than posted limited, both in cities and on highways. In the real world, that is rarely the case. Traffic signals and stop signs often eat away all the time saved by speeding. Let’s go back to our example of the 5-mile city commute and account for these. On average, there are 2 traffic signals per mile in the US. So, your trip will have around 10 traffic signals. Let’s say you see a red light at 5 of these signals. Assuming a 1min wait at each red light, 5 traffic signals will end up adding 5mins to your travel time, no matter what. Even if you speed up 50% above the limit between traffic signals, you only need to stop at 2 more red lights for the majority of the time savings to evaporate. This is one of the reasons why speeding up can have no effect effect on the ETA shown on your GPS.
The other key reason is the inability to consistently speed due to traffic conditions or other rule-following drivers. When driving in city or the highway, most people tend to drive close to the speed limit. In the US, many people falsely believe that it is okay to go 10% over the speed limit because there is a buffer. There is none. But even accounting for that, you’d be hard pressed to find drivers going 30-50% over the limit. In a 2018 survey, 82% of drivers agreed that they should follow the flow of the traffic. Therefore, it is hard to consistently speed because you will have to slow down as you approach traffic or other rule following drivers. Overall, these factors prevent drivers from maintaining a higher average speed and reaching early, even if they want to.
Non-linearity of risks The last part of the previous sentence is important. In general, people want to reach early but they do not want to take egregious risks doing so. On the roads, drivers are not actively thinking of risks and rewards. Instead, they rely on experience develop cognitive scripts or “routines which help simplify the driving task”. One such routine is the general knowledge that it is unacceptable to go 20mph above the speed limit. This is aided by the fact that in most US states, over-speeding by 20-25mph is considered reckless driving and could result in severe consequences. So, the risks of increasing speed beyond a certain point are not worth the reward of reaching early. While drivers are not actively making this trade-off analysis, if they see themselves going too fast on a narrow road or as compared to other vehicles, the immediate instinct is to slow down.
As a result of these non-linearities (time, speed and risk), we generally don’t reach early even if we speed up. Coming back to our pickup driver John, all these factors resulted in him not reaching early even though he went faster than me. First, he overtook me by going at around 70mph while I was at 60mph and we were 20 miles away from the destination. Even if he maintained that speed throughout, he would have reached in 17mins as compared to 20mins for me. Not a lot of time saving to begin with. To add to it, the road closer to the national park had a couple of traffic lights and a lot more congestion due to peak season. Inevitably, John had to stop at a red light and slow down for traffic, eating up almost all of his 3min advantage. Lastly, while he was overspeeding, he still had a subconscious risk-reward structure, which prevented him from taking extreme risks. For example, he was tailing me for about 8-10mins. In the meantime, there were very few oncoming vehicles. However, he waited until an overtaking zone instead of taking unnecessary risks by crossing a solid yellow.
So, is the conclusion that you cannot do anything to reach early and should just give in? Not quite. There are a few things which you can do to both actually reach early or at least feel like you have not reached late.
- Using legal ways to go faster than the traffic flow: In my daily office commute, I quite enjoy the bus ride on Seattle highways, where most of the cars are going at 40mph because of congestion but the bus is going at 60mph in the high-occupancy vehicle (HOV) lane. I do feel a bit smug that I get to reach early while not having to drive. But it is not just buses, high-occupancy cars also enjoy the same perk. So, it might be a wise idea to check for alternate commute options which can save you both time and energy.
- Managing expectations: As we have seen that traffic flow and congestion play a huge role in determining your average driving speed. So, even if you speed up for half your journey but get stuck in traffic for the remaining half, you may not save any time, or worse, reach late. So, before starting your journey it might be worthwhile setting the right expectations by checking the ETA on a GPS navigation tool like the maps app on your phone. This will not only give you a rough idea of the trip time but also if there are extenuating circumstances like crashes or road closures which may cause you to arrive later than usual.
- Be aware of the risks involved in speeding: While this may come off as generic advice, but speeding is dangerous. In 2022, speeding was a contributing factor in 29% of all traffic fatalities [Source]. Shaving off few minutes or seconds from your commute time is not worth the risk.
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