Every transportation technology has both inherent and perceived strengths and weaknesses. With this in mind, and the talk about urban aerial trams and gondolas getting new attention in Seattle, both from long time blogger Matt Gangemi (Matt the Engineer) and two weeks ago from Matt Roewe at the City Builder happy hour, I wanted to start the discussion on STB by talking about the basics, strengths and weakness of aerial trams and gondolas. I’m not an expert on design or construction, but their characteristics can be defined along the same metrics as other modes of transit.
I see this as a critical starting point for any serious discussion about these technologies, because (let’s be honest) most people find the idea of urban aerial trams and gondolas far fetched.
I’ll start off with the basics about the technologies. First, while there are technical differences between aerial trams and gondolas*, the importance of these differences from a rider’s perspective comes down to two things, frequency and stations. There are differences in speed, but I’ll explain why this isn’t very important.
Aerial trams fall into a family of technology much like elevators, with two counterweighted objects, permanently connected by a cable. When one goes up and the other goes down. Elevators, funiculars, and aerial trams all operate on this principle. Portland has an aerial tram, not a gondola. This fixed, counterweighted design means that the maximum frequency of the service is inherently limited by the time it takes the tram to get from one terminal to the other (cycle time). As the length of the aerial tram increase, the maximum frequency decreases. Gondolas do not have this inherent limitation. For more details go here.
Next, while both technologies can technically have stations along the route, the permanent cable connection of aerial trams, between the cab and cable, means that in practice the only viable place to have a mid-system station is exactly half way between the terminals. This is not the case with gondolas which have cabs that are able to detach from the continuously moving propulsion cable. This means that there can be multiple stations, with station located anywhere along the line.
Finally, while speed is always important, the relative short distances these systems typically cover in urban areas, means that the very high frequency of gondolas (every 1 or 2 minutes) in practice trumps the speed advantage of aerial trams. Additionally, gondolas will result in much more consistent travel times for riders, with roughly equal or lower travel times than aerial trams. Travel time consistency, a result of high frequency, is much more important for short trips because wait time quickly makes up a much larger portion of the overall travel time for a rider.
Here’s an example. Assume aerial trams have an average speed of 10 mph** and are roughly 20% faster** than gondolas. The total travel time on a 1-mile aerial tram would take between 7 and 14 minutes, due to the 0 to 7 minute wait time, with an average of 10.5 minutes (6 minutes traveling + 1 minute docking, unloading, loading, and departing, resulting in a cycle time of 7 minutes). The same trip with a gondola would take between 8 and 9 minutes, with a wait time of 0 to 1 minute, resulting in an average of 8.5 minutes (7.5 traveling + 0.5 minute loading/departing). This means that only in circumstances in which a tram rider waited less than 1 minutes would they get to their destination faster than a gondola rider. If a tram rider waited more than 2 minutes, all trips on a gondola would be faster.
In another post I’ll discuss the strengths and weaknesses of gondolas compared to earth bound modes of transportation, and how it plays out in the Denny Way corridor.
** These numbers are at best informed guesses and only intend to illustrate the point of the example.