You don't have to get this fancy. Mark Duncan/AP

Pre-signals are a rare bird of traffic engineering, but they could save bus riders a lot of time.

Why are trains usually more reliable than buses? Mostly because they don’t have to mix with other vehicles. Dedicated rights-of-way—in this case, tracks—help trains move quickly and evenly; they’re not constantly dragged down by car traffic like their rubber-tired counterparts. (Unless it’s a streetcar, a topic for another day.)

But buses can run fast, too, when they’re given a fair shot. The democratizing force is a dedicated lane, which can allow buses to carry upwards of four times as many travelers per hour than a general traffic lane. Separate lanes can also double or triple bus speeds by eliminating delays caused by single-occupancy vehicles. (Just look at what Seattle has done with a few dedicated lanes downtown.)

Not every street that could benefit from a bus lane could fit one, sadly. On streets with only a single lane available in each direction, bus riders have been generally doomed to suffer. But that doesn’t have to be the case, according to Vikash Gayah and S. Ilgin Guler, two professors of civil and environmental engineering at the Pennsylvania State University, if a “pre-signal” is in the mix.

Traffic signals that give buses priority at the intersection aren’t uncommon on arteries with dedicated bus lanes. Pre-signals, on the other hand, are a very rare bird of traffic planning: These traffic lights are placed mid-block—“upstream” from regular signalized intersections, as the engineers like to say—and actively change the flow of traffic before vehicles hit the intersection at all. This helps avoid conflict and delays amid crosscurrents. In the real world, pre-signals are only known only to exist on few multilane roads in the U.K. and Switzerland. But Gayah and Guler recently developed and simulated a strategy that would allow two-lane, two-way streets to get on board, and essentially create a dedicated bus lane out of thin air.

To pull it off, both lanes get a pre-signal facing both directions of traffic. The pre-signal on the lane facing the opposite direction of the moving bus stops the flow of cars, so that a segment of the lane becomes clear for a bus to merge into it. Meanwhile, the pre-signal on the lane in the same direction also pauses its flow of cars, allowing the bus to merge back onto its original lane with the stopped cars behind it. This all happens before the main signal, avoiding conflict with cars in the intersection. “You get a de facto bus lane by utilizing the opposite direction lane to jump the queue of cars,” Gayah said in an interview.

Gayah, Guler, and Monica Menendez of the Institute for Transport Planning and Systems, ETH Zurich, published a paper about pre-signals as intermittent bus lanes in Transportation Research Procedia in 2015. (This was on the grand occasion of the 21st International Symposium on Transportation and Traffic Theory, mind you.)

According to the simulations that the authors ran, the time-savings appear to be significant. Pre-signals can save bus travelers 5 to 7 seconds of delay per intersection at uncongested intersections, and up to 30 seconds of delay at congested intersections. “Even though the undersaturated number appears small, this is per intersection and includes buses that experience zero delay,” Guler wrote via email. In other words, even at low-traffic hours of the day, pre-signals could speed buses up. And 30 seconds of time-savings per intersection at rush hour could easily mean a longish-haul bus rider gets home 10 minutes sooner.

The other benefit of this type of treatment is that it minimizes delays for drivers. The flow of cars is only being stopped in one direction ahead of the main signal, and the intersection still functions normally in all directions once the bus passes through it. That’s important, Guler and Gayah said. Dedicated bus lanes and other kinds of transit-priority signal treatments can actually slow down the flow of traffic for cars, in certain cases. That makes drivers mad, and gives them fuel for arguing against such changes. (Also, for scofflaws who enjoy merging into bus-only lanes where they exist.) “As advocates, we want to prioritize transit improvements when it’s least impactful to cars, since the majority of people are still drivers,” Guler said. “It won’t be beneficial in the long run to make people unhappy, so we take the holistic approach.”

Hardcore bus fans may agree or disagree with that sentiment, but the fact is that cities need more options for running buses faster. The vast majority of U.S. transit agencies are suffering from declining ridership, something dedicated lanes could help with. Based on the academic literature summarized by Todd Litman of the Victoria Transport Policy Institute, “bus lanes that reduce total transit door-to-door travel times by 5-15 percent will by themselves increase urban-peak ridership 2-9 percent.” That’s without other improvements such as expanded routes or a faster fare payment system, Litman wrote. But with urban space as limited as it is, and with drivers are bound to push back on proposals that reduce the number of lanes (or parking spots) they can use, a bit of pre-signal wizardry might be worth testing.

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