The 1984 NYTimes article nl shared in this thread has more details.

Excerpt:

When the capsule was breached it held about 400 curies of radioactive cobalt in the 6,010 pellets. Each pellet, according to Joel Lubenau, a health physicist at the Nuclear Regulatory Agency, produced a radiation dose of 25 rads per hour two inches from the pellet. One to 50 rads per hour is considered a significant radiation dose. In comparison, the highest exposure a bystander could have received from the nuclear accident at Three Mile Island was 100 millirads, or about one-tenth of a rad.

A rad is a unit of absorbed radiation. An average chest X-ray produces 20 to 30 millirads instantaneously. A lethal dose for half the population is 450 rads received instantaneously over the whole body.

Radiation exposure measurement is complicated. [0] has a decent description.

This story[1] about another cobalt-60 incident in Mexico (!) says: "With cobalt-60, a single-curie source will give you a dose of a little more than 1 rem in an hour if kept at arm's length". [0] says that "Eventual death for 50% if exposure is above 450 rem; others recover in about six months" and that 600 rem will kill almost 100% of people.

So yes, if you drive for 8-12 hours with a bunch of cobalt-60 in your truck it is going to be pretty harmful.

[0] http://www.3rd1000.com/chem101/chem105e.htm

[1] https://www.popularmechanics.com/science/energy/a9840/mexica...

One more thing to know about Co-60: its half-life of 5.27 years, combined with the radiation it emits, makes it particularly dangerous. The half life being so long means it's a 30 year problem (after 5 half lives you're at about 3% of original activity), while its being so short (many metals are thousands of years) means that a small mass can have a high activity. Its gammas that it radiates are fairly energetic compared to most (a problem not encapsulated by the Ci unit) and it bioaccumulates so that small amounts inhaled or ingested can provide huge doses over the subsequent decades.