Recent blogroll addition the Austin Bike Blog points us to a study on cyclist behavior in bike lanes and wide curb lanes. Years ago, pre-blog and pre-cycling-killing-arthritis, I wrote the following on passing behavior in both facilities which still has some relevance today. Dragging this into the blog so it can be archived and whatnot; original is here. Done with HTML tables, the way God intended! Unfortunately, that doesn’t translate so well inside the blog. Any HTML/Movable Type geniuses want to suggest a formatting fix for me here?
One of the most common arguments in bicycle transportation circles stems from the disagreement over whether bike lanes or wide outside lanes provide "better passing distance". Foresterites claim that wide outside lanes are better for a variety of reasons; bike lane advocates come back with the "dedicated space" argument; which Foresterites then attempt to rebut by saying passing distance is "better" in wide curb lanes.
I have direct experience in this matter: my commutes to work generally take me along Shoal Creek Boulevard in north central Austin; which had fairly wide (6′?) bike lanes for several years; and then very wide (19′) curb lanes for several more years. I found that a typical 10-pass scenario would go something like the table below. The "distance" given is from car’s mirror to where I was riding in approximate center of bike lane.
|Passing distance on Shoal Creek Boulevard with Bike Lane
||Passing distance on Shoal Creek Boulevard with Wide Outside
||With minor fluctuation, the typical pass
with the bike lane consisted of the driver giving about half a foot
of distance between their right mirror and the bike lane stripe; thus
providing approximately the same passing space every time. Why does
this happen? Motorists are conditioned in other traffic interactions
to respect lane stripes.
||Some motorists (perhaps even a majority)
provide better passing distance in the wide outside lane scenario
because they are thinking about how much space to give, rather than
letting the lane stripe decide for them.
||On the other hand, some other motorists provide considerably
less passing space without the lane stripe to guide them (some from
ignorance; others from antipathy towards cyclists riding in "their
|Average passing distance from centerline of my bike: 3.5 ft
||Average passing distance from centerline of my bike: 4.0 ft
|10th percentile passing distance: 3.5 ft
||10th percentile passing distance: 1 ft
In this dataset, the 30th percentile passing distance for wide outside lanes was worse than for bike lanes; meaning that 3 out of 10 times, the passing distance could be expected to be less for wide outside lanes than it was for bike lanes. (Or, to turn it around, 7 out of 10 times, the passing distance in wide outside lanes would be better than in bike lanes).
Despite the fact that this dataset shows a superior passing distance in 7 out of 10 cases for wide outside lanes, I would choose the bike lane over the wide outside lane in this scenario. I submit that the deciding factor for cyclists, if they are thinking rationally, should not be the average passing distance; since most motorists, whatever the facility, do a fairly good job of providing adequate passing distance. The deciding factor should be the likelihood that motorists who, because they either don’t know or don’t care, don’t provide adequate passing distance. Clearly, in my experience, although average passing distance can be higher in a wide outside lane scenario, the minimum passing distance can at the same time be a lot lower. In this dataset, for instance, I’d argue that the 2 ft and 1 ft passes were close enough to be dangerous (given my width).
Sorry for the long break. I’ve been on business trips to Jebusland for 3 of the last 7 weeks, and had a vacation in the middle, and very busy even when here. Although I’m still busy, I at least have a minute (not enough time to grab any good pictures; since my google-fu was too weak to get something quickly).
I took the family on a short vacation to visit family in State College, home of Penn State (where I went to school and spent the first 9 years of my life – my grandmother still lives in the same neighborhood as the Paternos). On this trip, since my wife is still recovering from Achilles surgery, we didn’t spend much time walking through campus as we normally would – we instead spent our time driving around the edges of campus. This was an interesting contrast for me, since I spend quite a bit of time driving around the edge of another major university’s campus right here in Austin. Let’s compare.
There’s a signed and marked bike route which starts on the north end of campus (which is bounded by the old residential neighborhood in which my grandmother lives). This bike route says “Campus and Downtown”. It was added shortly before my college years but has been improved since then on each end and consists mainly of off-street paths (sharrows on the street in the neighborhood north of campus, although done poorly). Automobile traffic can still enter the campus from the north in several places, but is then shunted off to the corners – you can no longer go completely through campus from north to south by automobile. Pedestrian accomodations on this side of campus haven’t changed for decades – a pleasant cool walk under tons and tons of trees.
On the south side of campus is the downtown area – the area most analogous to The Drag; fronting College Avenue, part of a one-way couplet which carries State Route 26 through the area (other half is two blocks away, called Beaver Avenue). College Avenue has two through lanes of traffic. Shops line the road at a pleasingly short pedestrian-oriented setback, except for a few places (one a church, one a surface parking lot). Pedestrians, counting both sides of the street, get a bit more space than do cars – and cars have to stop almost every block at a traffic light. The speed limit here is 25; you can rarely go that fast. There is plenty of on-street parking. Again, there’s places where cars can penetrate campus a bit, but they can’t go through campus this direction. Bicycle access from the south comes from a major bike route (with bike lanes that end short of campus) on Garner St. – which then allows bicyclists to continue while motorists have to exit by turning a corner towards the stadium. Two images of the corner of Allen and College from different angles:
East and west at Penn State aren’t as important – the west side fronts US 322 Business (and a major automobile access point was closed; a classroom building now spans the whole old highway!). The east side is primarily for access to sports facilities and the agricultural areas. Ped access from the west is mediocre unless you feel like going through that classroom building, but not very important if you don’t since there’s not much other reason to be over there. Access from the east is the main future area for improvement – although it’s still of a caliber that we would kill for here in Austin; with 2-lane roadways and 30-35 mph speed limits; traffic signals everywhere pedestrians go in reasonable numbers; etc.
Penn State and the town of State College have made it inviting to walk to and through campus, and have made it at pleasant as possible to bike there. Some students still drive, of course, but most cars are warehoused most of the time.
On UT’s west side, Guadalupe is a wide choking monstrosity (4 car lanes with 2 bike lanes – one of which functions pretty well and the other of which was a good attempt that fails in practice due to bad driver behavior). On-street parking exists but is rather difficult to use for its intended purpose; but the merchants will still defend it tooth and nail. Despite having even more students living across this road that need to walk to UT than the analogous group at Penn State, there are fewer pedestrian crossings and they are far less attractive; and there is no bicycle access from the west that indicates any desire at all to promoting this mode of transportation. Although you can’t completely get through campus from west to east, you can get a lot farther in than you can at Penn State, and the pedestrian environment suffers for it. The city won’t put any more traffic signals on Guadalupe even though there’s thousands of pedestrians; and the built environment on Guadalupe is ghastly, with far too much surface parking and far too little in the way of street trees. This shot is about as good as it gets on Guadalupe:
On the east side of campus, there’s I-35. You’d think this would be much worse than the Guadalupe side for everybody, but at least bicyclists can use Manor Road, which is pretty civilized (better than anything on the west side). Pedestrians are pretty much screwed – noisy, stinky, and hot is no way to walk through life, son.
UT’s north side is similarly ghastly. A road clearly designed for high-speed motor vehicle traffic and then gruesomely underposted at 30 mph; way too wide and lots of surface parking. For pedestrians, this edge of campus sucks – for cyclists, it’s OK to penetrate, but then UT destroyed through access for cyclists by turning Speedway into UT’s underwhelming idea of a pedestrian mall (hint: this is what one really looks like). I could write a whole post on that (and may someday), but the short version is that years ago, UT came to our commission (UTC) with a master plan that crowed about how much they were promoting cycling, yet the only actual change from current conditions was destroying the only good cycling route to and through campus. Yeah, they put up showers and lockers – but that’s not going to help if the route TO the showers and lockers is awful enough, and it is. You’ll get a lot of cyclists at almost any university just because a lot of students won’t have cars and because parking isn’t free and plentiful, but if you really want to take it to the next level, I’m pretty confident that eliminating your one good bike route isn’t the way to go about it.
Since I went to Penn State (1989-1992), access for pedestrians and bicyclists has actually gradually improved, even though it already was much better than UT, and the campus has become more and more livable. More people walk and bike; fewer people drive; and it’s a more enjoyable place than it was before. Since I moved to Austin (1996), the environment for pedestrians and bicyclists travelling to and through UT has actually gotten worse – they’re still coasting on the fact that a lot of the area was developed before everybody had a car. Almost every decision they have made since then has been hostile to bicyclists and at least indifferent to pedestrians. As a result, a much larger proportion of students in the area have cars that they use much more often. (Just comparing near-campus-but-off-campus residents here). The recent long-overdue developments in West Campus are a start, but the built environment on the edge of campus has to dramatically change for UT to be anything more than laughable compared to other major college campuses’ interfaces with business districts.
Bonus coverage: The area I was staying in in Huntsville, AL is right next to the ‘campus’ for Alabama-Huntsville. The least said about that, the better – the area in general is like US 183 before the freeway upgrades, except even uglier (if that’s possible); and their campus has literally nowhere to walk to – my guess is that every student there has a car, even though the place is clearly not a commuter school.
Good Life magazine interviewed me (one of several) for a big piece on development and transportation, and we got a nice picture on Loop 360 last month. Now, it’s finally out, and they mispelled my last name. Every single time. Argh. The content was well-done, though; one of the better representations of an interview I’ve had (except for the part about the new office being too far to bike; I’m not biking any more due to health reasons; this is actually a wonderful bike commute).
Posted in 2008 Light Rail, Austin, Bicycle Commuting, Bicycling in Austin, Don't Hurt Us Mr. Krusee, We'll Do Whatever You Want, Driving in Austin, Personal, PS: I am not a crackpot, Texas Republicans Hate Cities, Transit in Austin, Transportation, Urban Design, Walking in Austin (Pedestrian Issues)
The acronym is for “Bike Commutes I Have Known And Loved”.
I was impelled to get going again by witnessing a lady trying to keep her bike on about one inch of pavement on the uphill shoulderless windy part of Bee Caves this morning on my drive to work. Stay tuned for #3, brave soul; there’s really no need for you to ride on that ungodly stretch.
Same format as before.
Bike Commutes I Have Known And Loved #2: Central Austin (Clarksville) to Northwest Austin (183 corridor) – four different offices in four years for S3.
Timeframe: June 1998- December 2001
Rough sketch of first half of route (the common part)
Common second part of routes to first, third, fourth offices (Bull Creek/Hancock to Mesa/Hyridge)
Second part of route to second, temporary, office (Spicewood Springs)
Final part of route to first office (Jollyville/Oak Knoll)
Final part of route to third office (Riata)
Final part of route to fourth office (Centaur)
Background: This is kind of a long one – S3 had one office when I started; were in negotiations to move to a nicer newer one but got stalled out by an acquisition which ended up pushing us into a temporary sublease for six months or so; and then when Via acquired S3, many of my coworkers left and I worked from home for a year, only to return to a temporary office in a building leased by Centaur (another of their companies) until S3 closed that office in December 2001, and I had to go find work in the middle of the dot-com bust (hooray!). All three share a common first third or so, and two are virtually identical, so they’re all grouped together here. The Riata commute was the one I actually made into the slideshow you see pictures from throughout this and the previous article.
Bike used: Mostly my old touring bike (since stolen) that I acquired for $200 used from austin.forsale.
Distance/Time: 10-15 miles each way; much longer in the morning due to hills – on days I biked all the way in on the longer versions, about 90-100 minutes. Trip home was 45 minutes or so.
Showers: Only the Riata office. For the mornings, I did the bus boost sometimes, and other times relied on cooler weather and the bathroom washcloth trick.
Route and comments:
By this point, I was becoming more comfortable asserting my position on the road, which is good since Jollyville didn’t yet have bike lanes.
First segments: To Bull Creek/Hancock: See first commute.
Second segment: Either up Shoal Creek or cross Mopac: The trick on all these commutes is where you shift from one good corridor (Bull Creek / Shoal Creek) to another (Mesa). There’s four crossings of Mopac which are accessible from here; I’ll briefly touch on them and talk about where I used them.
Segment #3: (commute #2 only): I rode up Balcones (ignore the map where it says it’s part of Mopac; I picked the wrong segment on the map) – you can actually ride up high on a nice shoulder looking down at the traffic below; nice in the mornings. Then you get to go up a pretty bad but short hill on North Hills (where northbound traffic on Balcones ends), then follow North Hills parallel to Far West all the way up to Mesa. Commute #2 is basically done here – just head up Mesa in the hilly bumpy bike lanes, hop on Spicewood and head west.
- Hancock: No on-ramps, which is nice, but a lot of debris, and requires a lot more hills if you are going particularly far north on the Mesa corridor. I used this crossing for the 2nd commute, at our temporary sublease on Spicewood Springs west of Mesa.
- Far West: A lot of novice cyclists take this one because the crossing TO Mopac is on a bike/ped bridge over the railroad, but then you’re dumped right into on-ramp traffic. I didn’t like this one as either a novice or an experienced cyclist.
- Spicewood Springs: Great downhill, but awful uphill – big hill, lots of traffic, ramps. Not recommended outbound. I used this one on the way home almost all the time.
- Steck: Best choice for uphill – least hill; most shade; least traffic (still have onramps to deal with, but they’re less busy than the other two choices). Downhill not so great – lose momentum at a 4-way stop.
Segment #3: Shoal Creek to Steck (other 3 commutes): see last chapter.
Segment #4: Shoal Creek to Mesa via Steck: Steck looks scary the first time but is actually very civilized – you can keep up with traffic on the downhill heading west, and by the time you slow down on the uphill, the light’s almost always red anyways. Crossing the bridge is the most stressful part – pump hard until you get to the other side to let the cars by, and then enjoy the shade on the short sharp uphill as the right lane turns into a bike lane. Then relax and go slow for a while and catch your breath. It’s a niice ride all the way up to Mesa – shade opportunities, little traffic, bike lane.
Segment #5: Up Mesa. Mesa has bike lanes up here, still. Fought various battles with high school over cars parked in the bike lane for years – probably still happening now. Look for Hyridge (my last commute just went straight to the end of Mesa). Left on Hyridge.
Segment #6: Across Loop 360. Two choices here; be a pedestrian and avoid a big hill, or be a cyclist and be tough. The pedestrian route takes you all the way to Old Jollyville, then left, then walk your bike across Loop 360 into the Arboretum. The less said the better (although if I got to this point and had no energy left, I did it once in a while). The bike route goes like this: Down Hyridge, split off at Mountain Ridge, BIG downhill, short uphill, and out to 360. Ride on shoulder for about 100 feet, then cut across traffic into the left turn lane for Arboretum Blvd (the cutout with no traffic light). Take your time here – no rush! Huge hill coming up. Turn across the southbound lanes onto Arboretum Blvd and then get ready for my least favorite hill – all the way up to the thing that looks like a roundabout but really isn’t at the Jollyville entrance to the Arboretum. I occasionally had to walk up this hill in the early days. The trip home is a bit different: Go through the uphill (183 side) of the Arboretum, hop on the 183 frontage for about 100 feet to get through the 360 light, then off on Old Jollyville. This is stressful at first but once you get used to it is no big deal, and you avoid some big hills.
Segment #7: Up Jollyville: When I did these commutes, there were no bike lanes on Jollyville – but I was experienced enough not to need them (although I liked them when they showed up later). Nice flat (in comparison) ride – pick up some speed here and get a breeze going. Brutal the other way in the afternoon against the inevitable summer headwind out of the south. Very little traffic in the mornings by the late end of rush hour. On the Riata commute, I’d turn at Duval and head over to the 183 frontage; for the first office I’d head straight on to almost Oak Knoll and be done. (note my comment about high gas prices – zoom into the picture).
Segment #8: Riata – luckily by this point I was pretty fearless as most people shy away from the frontage road. Not much traffic on this part – just quick hop from Duval to Riata Trace Parkway.
Modifications for trip home: On all of these commutes, I’d cross Mopac on Spicewood Springs – a nice downhill from Mesa to Mopac with no stops; could easily keep up with the cars going 35. The light at Mopac was the only stressful bit; just pump hard to get over the railroad tracks and down the hill to Shoal Creek and then rejoin the outbound route.
Bus boost possibility: Very high. The 183-corridor express buses drop off at Jollyville across from Riata (Riata actually got credit for being close to this park-and-ride, even though the road connecting Riata to it was cut in half by the freeway, requiring far too long a walk for anybody to really use the bus from there except as a cyclist). These buses are fast enough that you lose very little time compared to the drive, if you time your arrival correctly. (This applied to the two commutes out here; the other two had bus boost possibilities on the #19 in both cases and the #3 in the Centaur case – but those are slow in comparison). I used this express bus boost quite often – especially on days where I wanted to bike some but couldn’t afford to spend an extra 2 hours on it.
||Big hills in spots in the morning. Afternoon is mostly easy except for the headwind stretch on Jollyville heading south
||Steck and 360 crossings scary – but there are less scary (although more hilly) alternatives.
||9 out of 10
||Large time investment required in morning but very strenuous exercise; afternoon commute took about 45 minutes compared to 35-40 in car.
||5 out of 10
||Nice and shady in spots; lots of waiting at lights.
||9 out of 10
||Plenty of opportunities to hop on a bus with a flat tire, which I had to do many times on these commutes. Plenty of convenience stores. A bike shop or two up north.
Overall conclusion: A good medium commute – a novice would be advised to consider the pedestrian approach at 360 for a bit at the start or use the bus boost to avoid that altogether.
Just sent to the Statesman in response to Ben Wear’s article this morning
There are a few key facts that Ben Wear left out of his article on the South Mopac bicycle/pedestrian bridge which paint a very different picture:
1. There used to be a shoulder (available for use by commuting and recreational cyclists) on the Mopac bridge until a few years ago (when it was restriped to provide a longer exit lane). When the shoulder existed, it was frequently used.
2. The 15% figure cited by Wear is misleading – when you run the same comparison on total transportation funding in our area, about 1% (last time I ran the figures) went to bike/ped projects.
3. Urban residents, even those who don’t drive, are subsidizing suburban commuters through the toll-road ‘donations’ he mentioned (remember; the city has to repay those bonds from sources like sales and property taxes; not the gas tax) and in many other ways. When you add up the flows of dollars, it would take a couple of bridges like this every single year just to begin to make up for the money flowing out of Austin towards the suburbs, from drivers and non-drivers alike. Perhaps THAT would be a better focus for an article in the future. I’d be happy to help.
Urban Transportation Commission, 2000-2005
I spoke on this exact same 15% issue a few years ago on KLBJ’s morning news show but it keeps popping up as if we’re in a bad game of Whack-A-Mole. In this case, the 15% applies only to city funding, and includes pedestrian infrastructure which was never built back when saner cities would have done it (i.e. when the road was constructed in the first place). When I ran the numbers a few years ago, bike/ped funding for the whole area ended up at something like 1%.
I understand your retreat into pandering given the difficulties you’re currently facing, and I even sympathize a bit, but let’s be clear: big retail and employment destinations do NOT NOT NOT NOT belong on frontage roads.
This talking point works well with people who drive everywhere – like most folks in Allandale. It doesn’t work so well with people who actually have some experience with alternate modes of transportation, like yours truly. I used to occasionally ride the bus in the morning and get off at the stop on one side of 183 between Oak Knoll and Duval and have to go to exactly the other side – and the presence of frontage roads (destroyed an old road which used to cross) made a 2-minute walk into a 10-minute bike ride (30-minute walk). No wonder nobody else does it.
Despite past experience, I’ve once again gotten suckered into arguing with a sub-group of zealot mostly counter-culture exclusive-cyclists at Michael Bluejay’s list that cyclists do, in fact, disobey traffic signals much more often than do motorists, a position which is commonly understood by the 99.5% of the population that is not clinically insane.
I was somewhat enheartened (?) to see that there are guys like me all over the country as well as in other countries making this same case: running red lights and stop signs hurts the cause of transportation bicyclists.
Want to maintain the reasonable right to ride without a bicycle helmet? Want to get bicycle facilities? Want to be taken seriously when you try to get the cops to enforce the laws against bad motorists? BEHAVE LIKE A GROWN-UP FIRST.
PS: Every time this comes up on Michael’s e-mail list, I’m alone out there fighting the good fight. This has allowed the conventional wisdom among these folks to be: “car drivers run red lights more than bicyclists do; and you’re making up all this stuff about how drivers see so many cyclists breaking the law that it causes them to lose respect for cycling as transportation”. If you’re reading this, and you’re on that list, and you don’t chime in, you’re part of the problem.
Another quick hit:
So Elizabeth Christian has gone berserk defending her husband’s new proposal for a study of cyclists who end up at the hospital with injuries (correlating to helmet use). This is exactly how the original Thompson/Rivera study went wrong. Short summary:
- Voluntary helmet-wearers and non-wearers are quite different groups, as it turns out. The helmeted cyclists were more likely to be yuppie recreational riders (like Ms. Christian’s husband) while the un-helmeted cyclists were more likely to be poor and/or just trying to get around (in which case a helmet is enough of a pain in the ass that most rational people leave it at home).
- Later analyses of the Seattle study showed that in addition to behavioral and locational differences, helmet-wearers were also far more likely to go to the hospital for a given injury than non-wearers (probably due to the above socioeconomic differences).
- This means that the doctor in the emergency room is only going to see a non-helmeted cyclist when the injury was very serious; but he in fact sees the helmeted cyclist for minor injuries.
- Surprise! Helmet use seems to correlate with less severe injuries!
- As it turned out, though, you were also able to use the same data from this study to ‘prove’ that wearing a bicycle helmet reduced your likelihood of getting a leg injury by a similarly high percentage. Again, the guys with broken legs went to the hospital no matter what; but the non-helmeted guys with cuts and bruises just went home and sprayed Bactine while the helmet-wearers were more likely to go to the hospital; and the helmet-wearers were more likely to be leisurely riding through a park and suffer their falls in the grass rather than be hit by a motor vehicle on the roadway.
This is a clear study error. The “control” group in this case-control study is not similar enough to the “case” group to make these conclusions. Statistics 101; and don’t believe the typical bullshit response about lies, liars, and statistics – this example is pretty damn clear-cut. The study was flawed; and this new study will be equally flawed.
Of course, the Chronicle didn’t bother going into this level of detail, despite the fact that I’m sitting right here, and am no stranger to those guys. It’s as if they’re not even interested in trying anything more strenuous than reporting on press releases these days…
More on the Thompson/Rivera study from a slightly different angle.
Just sent a moment ago. Links added for reference.
Dear mayor and council members:
My name is Mike Dahmus; I served on the Urban Transportation Commission from 2000 to 2005, and still write on the subject of transportation from time to time. Until a medical condition forced me to stop, I was a frequent bicycle commuter (but, unlike some others you probably hear from, also continued to own and drive a car as well).
I can’t emphasize enough the points previously made by Jen Duthie from UT that this ordinance may seem like much ado about nothing if you’re used to thinking about bicycling as simply a sporting activity – like the ride Bruce Todd was on when he hurt himself. If you’re going out to ride for fun, a helmet doesn’t make a lot of difference – you’ll probably still ride, and even if forcing a helmet makes you delay your ride until a cooler day, for instance, the overall public health is not significantly harmed.
But for transportation bicyclists, mandating a helmet be used for what is essentially a safer activity overall than driving is a critical error – many marginal cyclists will simply stop riding their bikes and return to their cars. You certainly see this effect at play among children – hardly any of whom ride their bikes to school any more, partly because of the inconvenience and discomfort of the helmet, but also due to their parents belief that cycling must be a very dangerous activity if it requires a helmet.
Every adult cyclist you convince not to ride is one more driver. Every driver is that much more traffic and pollution; making Austin less healthy not only for themselves but for the rest of us as well.
Since the evidence in the real world has shown that there has been no actual benefit from dramatic increases in helmet usage in this and other countries, there ought to be no justification whatsoever for a mandatory helmet law (or even, I’d argue, excessive promotion of helmets compared to more effective measures such as traffic enforcement and education).
Please take this in mind when voting. No serious transportation cyclist (i.e. one who actually uses their bike to get around) has signed on to this effort as far as I’m aware.
Michael E. Dahmus
Update: Austin group fighting the mandatory helmet law is at http://www.nohelmetlaw.org/
Since the mandatory bicycle helmet law is rearing its ugly head here in Austin again thanks to the efforts of former mayor Bruce Todd, the following analysis of actual real-world results of increased bicycle helmet use in other countries is particularly relevant now.
The New York Times covered this for the USA in 2001. In short: Bicycle helmet usage went way up, but head injuries and fatalities didn’t go down. This matches the observations in Australia, the UK, and many other countries.
Ride with a helmet if you want. But don’t get smug about those who don’t – they’re NOT “organ donors”, they’re NOT stupid, and they’re NOT irresponsible. THEY’RE actually the smart ones, given the apparent lack of benefit to wearing bicycle helmets.
And, please, stop the bullshit analogies with regards to seat belts. Nobody ever stopped driving because of seat belts, and even if they did, why would we care? Bicycle helmets are hot, uncomfortable, and inconvenient – and results in country after country show that many people simply stop cycling when their use is mandated. You don’t have to carry your seat-belt around with you when you park your car; your car likely has air-conditioning; you’re not actually exercising when you drive; seat belts are built in to the car; etc. Oh, and don’t forget: seat belts, unlike bike helmets, actually WORK. The analogy couldn’t be any worse if they tried.
If it’s so damn obvious that people with “something up there to protect” would naturally choose to wear bike helmets, then why is it also not obvious that the same people would do so when driving their car? You get the same impact protection; but you’re not sweating and you have an easy place to stow the helmet when you’re done (inside the car itself).
Wikipedia has outstanding, heavily footnoted, coverage of bicycle helmets, if you don’t like the “cyclehelmets.org” people.