There are a few things you can do to prevent a crash before hand, even fewer once you have a problem and luck will probably have more to do with the final outcome than anything else.

A flat is probably one of the more rare things that will bite you on a high speed descent unless you hit something that causes a flat or have a somewhat rare tire blow-off caused by excessive rim brake heating. Instead, it's the other types of unexpected things that can cause the biggest problems, such as debris on the road, on coming cars crossing the centerline into your lane, cars pulling out or turning in front of you or, as we experienced just two weeks ago, another cyclist doing something stupid and unexpected that puts you at risk. In short, knowing 'when, where, and how' it's safe to bomb the hills, keeping your wits about you, looking for an escape route, and maintaining control while keeping relaxed is your best bet for avoiding a crash.

That said, here's something I wrote about 7 years ago at Hobbes in response to a similar question.


1) What can happen that can cause a crash at higher speeds?

Just about anything you don't expect!!! But not very often.... Knock on
wood, in about 10,000mi of tandem riding over the past couple of years I
can only recall one flat at 45mph (thank goodness it was the rear), one
48mph heeled over as far as you dare go turn that scared the chamois off
of Debbie (the single bike guys were way impressed with that one), and the
usual cars pulling out, bugs in the face, etc... none of which resulted in
a crash. Single bike accidents, uhhhh - lost count of the minor ones long
ago and they almost all involved contact with another bike.

Seriously, based on what I've seen with all types of two wheeled machines,
the #1 cause of non-car related single bike accidents on high speed
descents is pilot error - riding too fast for conditions, on unfamilar
roads, and beyond your abilities (and usually a combination of all three).
These types of accidents usually occur when a tire looses adhesion with
the road resulting in a low-side crash (i.e., the bike slides out from
underneath the rider(s) vs a high sider where the rider(s) are thrown over
the bike) or "run-off-the-road-into-whatever-is-there" and pray that no
cars are coming in the on-coming lane. Primary culprit - aside from the
aforementioned pilot error - is sand and loose soil that wash into curves
during rain fall. Front flats (or rolling-off sew-ups in a turn for some -
been there done that on a hot day racing) are probably the second most
common cause of "there's nothing you can do about it" downhill or high
speed single bike crashes.

Truth is, you're more likely to have an accident on a bicycle if you ride
in large groups (can you say, Mass Start at a charity ride event) or in a
pace line. Multi-bike accidents are easily account for the majority of
road bike accidents - far more so than single bike accidents on fast
descents. Moreover, wheel to wheel contact at 15 - 20 mph is as likely to
cause a broken collar bone, wrist or concussion (some of the most common
non-car related bike injuries beyond road rash) as a crash at 40mph. The
higher speeds will definitely take more flesh off but the initial impact
is where my experience tells me that the bones fracture and break. I'm
sure one of the good doctors out there will quantify or correct me on this.

As far as concern over injuries, if I might be so bold, crashes are bad at
any speed and speed is only one of many factors that will dictate the
extent of your injuries. Therefore it's best to avoid them at all costs.
To that end, always take care of the things that are in your span of
control to reduce your chances of a crash: ride in control and within your
abilities; wear a helmet; wear eye protection, keep your machine in top
condition; use equipment that is strong enough for your weight and riding
style; and always expect the unexpected. Beyond that fate, luck, mental
lapses, and other riders, etc… will have more to do with a non-car related
bike accident than anything else and those things are usually out of your
span of control. Off-road riding will go a long way towards improving your
bike handling skills, as will racing and effective cycling classes which
could improve your chances of recovering from an unexpected event.
However, even the best bike handlers get hammered when the unexpected
happens and the margin for recovery is too slim.

Bottom Line: It is good to think about such things as the welfare of your
stokers when you take up tandem riding since they must place their safety
solely within your hands. Be sure of your own skills and abilities and
always have an out when riding with others whose skills and experience may
be lacking - they'll get you every time.