What are `primary_ramp` and `secondary_ramp`?

The columns `primary_ramp` and `secondary_ramp` don't match everything I would expect from the codebook: https://tims.berkeley.edu/help/SWITRS.php

```SQL
SELECT primary_ramp, secondary_ramp, count(1) as total
FROM collisions
GROUP BY 1,2
ORDER BY 3 DESC
LIMIT 500;
```

Yields:

| | primary_ramp | secondary_ramp | total |
|----:|:---------------|:-----------------|--------:|
| 0 | | | 5352014 |
| 1 | TO | | 60595 |
| 2 | FR | | 25290 |
| 3 | | SF | 8424 |
| 4 | | NF | 8168 |
| 5 | | NO | 5380 |
| 6 | | EF | 5345 |
| 7 | | SO | 5231 |
| 8 | | WF | 4971 |
| 9 | | EO | 3656 |
| 10 | | WO | 3255 |
| 11 | | TO | 2014 |
| 12 | SF | | 1357 |
| 13 | NF | | 1237 |
| 14 | TR | | 994 |
| 15 | EF | | 864 |
| 16 | NO | | 774 |
| 17 | WF | | 761 |
| 18 | | FR | 732 |
| 19 | SO | | 643 |
| 20 | WO | | 472 |
| 21 | EO | | 471 |
| 22 | | CN | 300 |
| 23 | | TR | 240 |
| 24 | CO | | 193 |
| 25 | CN | | 100 |
| 26 | TO | TO | 86 |
| 27 | | CO | 73 |
| 28 | TO | TR | 71 |
| 29 | TO | SF | 43 |
| 30 | TO | NF | 41 |
| 31 | TO | NO | 34 |
| 32 | TO | WF | 29 |
| 33 | TO | CN | 24 |
| 34 | TO | SO | 23 |
| 35 | TO | FR | 22 |
| 36 | TO | EF | 20 |
| 37 | TO | EO | 20 |
| 38 | FR | TO | 19 |
| 39 | FR | SO | 17 |
| 40 | FR | EO | 16 |
| 41 | TO | WO | 16 |
| 42 | FR | FR | 15 |
| 43 | FR | NO | 15 |
| 44 | FR | NF | 10 |
| 45 | FR | SF | 9 |
| 46 | FR | WO | 9 |
| 47 | FR | WF | 8 |
| 48 | WO | FR | 8 |
| 49 | NO | FR | 7 |
| 50 | NO | NO | 6 |
| 51 | NO | TO | 5 |
| 52 | SF | TO | 5 |
| 53 | TO | CO | 5 |
| 54 | WO | WO | 5 |
| 55 | NF | TO | 4 |
| 56 | SO | FR | 4 |
| 57 | EF | TO | 3 |
| 58 | EO | FR | 3 |
| 59 | FR | TR | 3 |
| 60 | NF | NF | 3 |
| 61 | NF | SF | 3 |
| 62 | NO | SO | 3 |
| 63 | SF | SF | 3 |
| 64 | SF | SO | 3 |
| 65 | TR | TR | 3 |
| 66 | CN | TO | 2 |
| 67 | EO | EF | 2 |
| 68 | EO | EO | 2 |
| 69 | FR | CN | 2 |
| 70 | FR | EF | 2 |
| 71 | SF | FR | 2 |
| 72 | SF | NF | 2 |
| 73 | SO | SO | 2 |
| 74 | SO | TO | 2 |
| 75 | TR | TO | 2 |
| 76 | 0 | | 1 |
| 77 | CN | WF | 1 |
| 78 | CO | SO | 1 |
| 79 | EF | FR | 1 |
| 80 | EF | WF | 1 |
| 81 | EO | TO | 1 |
| 82 | EO | WO | 1 |
| 83 | NF | NO | 1 |
| 84 | NO | EO | 1 |
| 85 | NO | SF | 1 |
| 86 | SF | NO | 1 |
| 87 | SF | WF | 1 |
| 88 | SO | EF | 1 |
| 89 | SO | NF | 1 |
| 90 | SO | NO | 1 |
| 91 | SO | SF | 1 |
| 92 | TR | CN | 1 |
| 93 | TR | EO | 1 |
| 94 | TR | FR | 1 |
| 95 | TR | NF | 1 |
| 96 | TR | NO | 1 |
| 97 | TR | SF | 1 |
| 98 | TR | SO | 1 |
| 99 | WF | SO | 1 |
| 100 | WO | EF | 1 |
| 101 | WO | EO | 1 |
| 102 | WO | SF | 1 |
| 103 | WO | SO | 1 |
| 104 | WO | TO | 1 |