Highway medians and calculating curves with Google Maps

I haven’t posted here in a while (year and a half!) but with the demise of functional Twitter and figuring out HTTPS on my site (which … took me longer to do than it should have, sorry to everyone who attempted to enter their credit card to give me all the money and wound up giving it to a Nigerian Prince, but, really, I have a Patreon, it’s for my ski-related podcast but you can certainly click buttons over there) I figured it was time. In any case, I have a new job, I can probably not post about a lot of things, but now instead of long, ramblingTwitter threads, I’ll probably just post long, rambling posts here. Anyway …

Alon wrote a blog post nearly a decade ago about using highway medians for rail, and the differences between the US and Europe. It’s informative and mostly on-point, but I think he missed a couple of nuances and I’m going to go down a rabbit hole about a couple of them, specifically a) that roads are often less curvy than design limitations (with a nifty Google Maps way to measure that) and b) in most cases, European motorways/autobahns do not have wide medians, while North American roadways often do.

First, a story: this summer, I was in Berlin (having gotten there from FRA via 9€ train), and was supposed to take a train to Frankfurt, meet my girlfriend, and take another train (or maybe the same train) to Switzerland for some mountain time. Instead, I got the covid, got holed up for a few days, and walked around Berlin and looked at tram tracks. In a fit of genius, she suggested we rent a car and drive the die Autobahn mit the windows down (she’d been exposed along with me and had had it a few months before, so this seemed like a reasonable precaution) and so we rented a car at BER (which exists!) for the trip south to almost-Basel (much more expensive to return the car in CH than DE).

Was it how I had planned to cross Deutschland? Nein! But it was an interesting trip. My experience driving stick is limited to 20 years ago in New Zealand and a few days practice on her Fit. But once I got into gear in a rest stop, I could handle the autobahn fine (I even made it through a traffic jam, sorry, someone else’s clutch). I topped out at 183 km/h, slower than the ICE but plenty fast with the windows down. Sidebar to the sidebar: driving the autobahn is very civilized. There are rules: no passing on the right, overtake and then change lanes, no trucks on Sunday (this was a Sunday) and 100 mph didn’t feel that fast. 120 did, the Opel handled it fine, and I kind of wish I’d looked up the premium for renting a BMW. After 900 km we stumbled into a Michelin star restaurant (quite accidentally: it had the cheapest single rooms in Lörrach) and had a lovely meal that I could mostly taste.

Anyway, one thing I (sort of) noticed at 100 mph was that the autobahns do not have wide medians. There are some exceptions; like crossing the Swabian Jura between Stuttgart and Ulm, where the A8 splits into two roadways a mile apart, once of which runs on an old bridge under the Filstal high speed rail viaduct. with some gnarly curves, apparently this is a dangerous part of the network. But in general, medians for rural highways in Germany are only a couple of meters wide. France, too, where they sometimes get by with just a metal guardrail. Italy, Poland, Britain, Spain all seem the same.

Last year, the Wendlingen-Ulm high speed line opened along Autobahn 8, and outside of the aforementioned area (where it’s mostly tunneled) much of it follows along the side of the A8, because there’s no median. A side-running routing means that exit ramps become complex to build (such as here, in Merklingen), requiring much longer bridges and complex entry and exit ramps or, in this case, a tunnel. Putting the railroad in the median would obviate the need for any of these issues, especially retrofitting rail into an existing roadway layout, but those medians don’t exist in Europe.

They often do in the US. Newer roadways in open, rural areas often have medians which are 50 feet wide or wider. In many cases, these are grassy medians which do not require any guard rails, the assumption that if a car goes into the grass it will decelerate enough before it crosses to the other side. The medians can also be used for drainage when they slope down between the roadway.

As such, there are a number of Locations in the US where railroads already exist in highway medians. Several of these are transit lines in urban areas, where—to vehemently agree with Alon—they don’t belong (in many cases, the roadways were designed to accommodate rail lines, or even replace them). The width of these medians (between inside solid yellow lane lines) varies, and none has high speed rail:

LocationWidth (ft)TypeNotes
I-25 New Mexico100CRRetrofit, single track but designed for 2.
DC Metro65HROften in roadways designed for rail, several examples.
Chicago46-75HRWider Congress originally designed for 4 tracks
SF BART55HRDublin, other examples exist.
Albany, NY46FreightUrban area
Portland58-82LRWide inside shoulders

Many, if not most, rural interstates have wide medians. To route a rail line alongside in very rural areas, it doesn’t matter much if a median is used or not: exits are widely spaced there’s plenty of room to rebuild them, and sticking to a median may require shifting over or under the roadway for curves, so it depends on the roadway curvature. Cities rarely have wide medians and they’re decidedly bad places for transit anyway. There are, however, “happy medium” areas where median-aligned rail might make sense, especially in more built-up areas where highways have wide medians and railroads could benefit from higher—if not truly high—speeds.

Alon’s other again-valid point is that sticking to a median means sticking to the curve of the roadway. Such an alignment may require constantly varying speeds, which is not exactly good for high speeds (see, for example, the Shore Line in Connecticut). But many roadways with wide medians are straight—or close to it—and would allow higher speeds. It’s hard to figure out curve radii using just Google Maps so I’ve created a bit of a cheat sheet. If we assume that highway lanes are 12 feet wide (in general, they are), we can draw a chord across a curve tangent to the second lane of the roadway using the measure distance tool (right click to activate it):

This example, in Newton, Mass. where the Turnpike is adjacent to the railroad, shows a 600 foot chord, which, if you do the trigonometry, traverses about 18˚ of a circle. US railroad curvature is measured as “degrees of curve per 100 feet”, which in this case is about 3˚. (I selected this example because I happen to know current speed limit here—55—and a resource with the curvature, although the original railroad may have been slightly straighter.) This corresponds to about a 575 meter radius. This can be extrapolated outwards as follows (approximately, see also this conversion table, and speed information here):

Chord (feet)DegreesRadius (m)Speed
4006.8260real slow
6003575~100 km/h / 60 mph
6502.6675~120 km/h / 75 mph
8001.71020~150 km/h / 90 mph
11000.911925200 km/h / 125 mph
13000.652690250 km/h / 150 mph
16000.434070300 km/h / 186 mph

Note that this can also be done in metric, and it can also be done by drawing chords on the inside of standard gauge railroad tracks. The calculator has information for both metric and imperial for both highway (two lanes) and rail tracks. (You can probably save a sheet as your own and change the values to pretty much anything else.)

Absent steep grades, roadways with sharp curves are not really suitable for true high speed rail. Even a road like I-95 in Connecticut has several 700-foot-chord curves which would limit speeds to about 80 mph in a few places, so the median might not be suitable. Still, a route along the Connecticut Turnpike would probably be much faster than the Shore Line: some of the sharpest curves have a wide median which could allow smoother curves, tilting trains could improve speeds, and the curves on the Shore Line are far more restricting. Several other Interstate highways in the northeast have similarly restrictive curves. And older and more urban highways often have narrow medians anyway.

That said, there may be other opportunities to leverage our wide medians in the US. Back when I blogged more regularly, I proposed routing South Coast Rail via a stretch of 495, which has a 100-foot-wide median. (This somehow pissed off people in Taunton, who didn’t want a good idea to get in the way of the current iteration of SCR.) The only appreciable curve this encounters on 495 is a 1200′ chord, so … fast (getting to 495 would be trickier). Whenever I drive up I-95 to New Hampshire I think about how straight and wide the highway is and, yes, it could support high speeds (but it doesn’t really connect anything). Out in California, I-5 on the west side of the Central Valley bypasses every population center between LA and San Francisco, but if California HSR had used its median instead of the debacle of going through the Central Valley cities, it may have allowed a much easier path to construction in the valley.

Sometimes “high speed rail” is thrown about by people with no idea what they are talking about. Like as a solution to I-70 traffic in Colorado. Certainly not in the too-narrow median, but also not along the highway with sharp curves. (In Europe they’d just dig a base tunnel, of course, the mountains in Europe are narrower.) And then you have the Connecticut NIMBYs saying to build an inland route along I-84 which … has hills, curves and development. The same ones who killed I-95 high speed rail because highways adjacent to historic sites are fine, but highways and trains next to historic sites are bad. (To be fair, the Lyme portion of I-95 has a narrow median, and 900m curve radii.)

My interest was recently piqued by the continuing failure of the Cape Bridges project to garner necessary federal funding. Maybe the Feds are just not that interested in giving a couple of billion dollars to some bridges which are only at capacity a few days per year, and which MassDOT wants to widen (although they are nearly a century old). Of course, there is no mention of improving the rail link to the Cape, which currently takes 2:20 to get to Hyannis, an hour longer than driving without traffic (rare!).

In a civilized country, the Cape would be a transit-first destination. It has 220,000 year-round residents, 500,000 summer residents and significant additional short-term tourist traffic, and its 339 square miles include 100 square miles of National Seashore and military reserve. The two bridges crossing the canal create bottlenecks; when they were build in the 1930s, Barnstable County only had 32,000 residents. is a narrow peninsula which could be well-served by an arterial transit line, has many destinations with limited parking, and is most heavily-used in the summer, when bicycles should be a major transportation mode. Trips to the Cape include access to island ferry terminals: the islands have year-round populations of about 30,000, but nearly 10 times that in the summer, with the Steamship Authority providing half a million trips per month during peak season, plus other private ferries, with thousands of cars parked at mainland ferry terminals and minimal transit connections.

It isn’t. This would require good local transit, safe bicycle facilities and various ways of discouraging traffic, a fast train to Boston, Providence and beyond, and special notice given to transporting people to the island ferry terminals since many of them have to park far away and board a shuttle already (so why not shift the shuttle of the other side of the bridge?). As it is, there are commuter buses subjected to the same traffic as cars, a regional transit authority providing hourly-or-worse service 8 hours per day, 6 days per week, and three train trips per week in the summer, which run on an existing commuter line, a straight line on the mainland at reasonable speeds before crossing the old lift bridge, and then at excruciatingly slow speeds on the Cape itself. CapeFlyer, which I wrote about in 2015, is a nice alternative for some but is not making much of a dent in traffic.

Imagine a high speed link from Middleborough to Hyannis. The line would transition to the median of 495 (sharpest curve: ~1500m) and cross the Cape on a new fixed crossing where the canal splits two hills in a power line right-of-way, eliminating any need for long ramps to attain grade. From there, the power lines lead to the Midcape Highway, which has a narrow median at first (but borders miles of state-owned military reserve) and eventually has a wider one. The worst curves there are also about 1500m. So while the highest speeds might be off the table using a median alignment, averaging 100 mph—which would allow a trip from Middleborough to Hyannis in about 24 minutes and a trip from Boston in just over an hour—would be attainable.

I’m sure people would yowl about ruining the pastoral feel of the partially tree-lined highway median. And a train below a power line would harm the environment to no end. As for what to do with the pokey existing railroad? It might be a great extension of the existing rail trail, since the portion east of Hyannis is relatively sparsely-populated..

Highway medians are certainly not always the answer for rail lines. But given enough width and straight enough alignments, they can provide a good alignment for some new medium- to high-speed corridors. This requires both wide-enough medians (rare in Europe and hardly a given in the US) and straight- and flat-enough alignments. When these ingredients come together, they can provide a potential lower-cost, politically expedient means of building a railroad.