This tragedy should not surprise anyone familiar with past killer tornadoes. Just in Oklahoma and bordering towns:
5/1/1954, Cotton County: Two killed trying to escape a tornado in a pickup
7/15/1955, Sulphur: One killed, two injured in a car at a drive-in
4/2/1957, Murray County: Truck thrown from Route 77; driver killed
4/2/1957, Marshall County: Car thrown over 200 yards; one killed, one injured
5/5/1960, McIntosh County: Two killed, five injured; caught while driving to neighbor's cellar
5/5/1960, near Fort Smith, AR: Transport truck lofted from Route 64; driver killed
5/26/1973, Muskogee County: Truck thrown/rolled half a mile; four killed
4/10/1979, Wichita Falls, TX: 25 vehicle-related fatalities in F4 tornado; only five in homes
5/15/1990, Stillwater: One killed while driving to shelter at friend's house
4/26/1991, Pawnee County: Cars blown from Cimarron Turnpike; one killed
4/26/1991, Copan: Car thrown 250 yards; one killed, one injured
5/3/1999, OKC metro: Two killed, many injured at highway overpasses
5/3/1999, Payne County: Car lofted from beneath I-35 overpass; driver killed
5/10/2008, far SW Missouri: Multiple vehicle fatalities from Picher, OK tornado
2/10/2009, Carter County: Driver of truck on I-35 killed by Lone Grove tornado
5/10/2010, Moore/OKC: One killed trying to escape Lake Draper tornado
5/24/2011, Calumet: Three killed on I-40 by EF5 El Reno/Piedmont tornado
5/19/2013, near Shawnee: One killed in vehicle
5/31/2013, near El Reno: All eight tornado victims were in vehicles
Encounters between drivers and tornadoes seem to fall into two categories:
1. Locals trying to evacuate or reach safe shelter by car
This is a pure social science problem. There is a dangerous myth that some tornadoes are so strong as to be unsurvivable above ground. In reality, in the May 3, 1999 Bridge Creek-Moore-OKC tornado, 99% of people in homes sustaining F4 or F5 damage survived (Hammer and Schmidlin 2002). Mobile Doppler radar observed an unimaginably violent 318 mph peak wind speed in that tornado. While not all houses receiving high-end damage experienced winds quite that extreme, it's fair to conclude that if the May 3 tornado was easily survivable above ground, any tornado is.
Let me say that again, because it's important: In one of the most intense and destructive tornadoes ever seen, 99% of people sheltering in the hardest-hit houses survived. If a tornado is headed your way, the odds are against it being an EF5. Even if it is, most of its track will not experience the peak EF4-5 winds--the odds are still against your house sustaining EF4-5 damage. So if you're following appropriate tornado safety guidelines in a reasonably well-built home, you have reason to believe your chances of riding out an approaching tornado above ground are actually much better than that 99%.
That's not to say that nobody should drive to safer shelter. Not everyone is in a well-built house, office, or large public building. Mobile home residents, people working in areas without sturdy buildings (like oilfields), and others may have no choice. As the following graphic from NWS Norman stresses, it's critical for people in such situations to closely monitor weather information and put their plans into action before a tornado warning is in effect at their location. Lead time with a tornado warning is typically 20 minutes or less. That's too late to take to the road.
2. Motorists unaware of the danger
This describes a couple of the El Reno fatalities in 2013, and probably at least some of those in Garland. Anyone who's driven around the DFW metroplex can attest that trips often take much longer than the time scale of a tornado warning. The area is a perpetual traffic jam even in fair weather. Highway signs with up-to-the-minute messages can be of some use in alerting drivers to the existence of the threat, but can't convey details or maps. Even if drivers receive a tornado warning, around population centers it may be impossible to get them all off the road and into sturdy shelter in a matter of minutes. Being on a bumper-to-bumper highway when a warning is issued is a worst-case scenario. Making sure it doesn't happen calls for weather awareness well before the warning.
For both categories, the underlying issue is that many people don't act until they become aware of a tornado warning for their location.
Surviving a severe weather event begins long before a warning comes out. There are a number of products issued by the NWS, like convective outlooks and tornado watches, that provide severe weather awareness days to hours in advance. These products can't pinpoint locations down to the town and times down to the minute. What they can do is help people in the risk area avoid desperate situations. As long as occupied vehicles are being lofted by tornadoes, those products aren't achieving their goal.
A tornado watch means that the broader environment will be favorable for tornadoes over the coming hours. It's your cue to make sure you can quickly receive warnings and seek shelter if need be. A tornado warning means that a specific storm moving into your specific location either is producing a confirmed tornado or has a radar signature that strongly suggests one. A warning doesn't mean "Drive to a place where you can seek shelter." It means "Shelter right now!" In the current state of the science, there is no official product between the watch, which has a lead time of hours, and the warning, which has a lead time of minutes. That means that when you're in a watch box, you never want to be without means of receiving tornado warnings instantly, or more than a few minutes from some kind of shelter. Being on the road makes it harder to get warnings and other information, harder to be sure exactly where you are in relation to the threat, and harder to get to safe shelter quickly.
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