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Collision Data on ATM Corridors

The following graphs show traffic collisions in the I-5 ATM corridor. SR 520 and I-90 data will be added as it becomes available.

On the graphs below, the vertical line in the third-quarter of 2010 marks when ATM started in the corridor (August 10, 2010.)

Collisions in the I-5 ATM corridor were trending upward prior to activation with the number of collisions varying up and down from quarter to quarter and year to year. The number of  reported collisions after turning on ATM is down compared to 2009 and about the same as 2008. It is on average higher than 2007 but trending down. The collisions in the fourth quarter of 2010 increased which is typical as darkness comes earlier and weather changes to winter conditions. Collisions then drop in the next two quarters and increase in the third quarter of 2011 before dropping again in the fourth quarter. The fourth quarter of 2011 has one of the fewest rates of reported collisions than any quarter in the last four years.
 




The graph below compares collision data from the I-5 ATM corridor and four other similar I-5 corridors. The thick blue and black line is the number of collisions in the I-5 ATM corridor. It is important to note that collisions in these corridors, as in most urban corridors, are extremely variable.

Of the four other I-5 corridors examined, collisions on two corridors stayed relatively flat, collisions on one increased and one decreased. The corridor with decreasing collisions is northbound I-5 in the area just north of where ATM is deployed. This corridor tends to follow the same general pattern as the ATM corridor so it is difficult to determine if people driving differently in the ATM corridor continue to carry these driving patterns north of the corridor.





When using the number of collisions as a measure of performance it is important to consider the number of vehicles in the study period. The number of collisions per million vehicle miles traveled (VMT) is the standard performance measure. This figure accounts for the fact that as the number of vehicles increases or decreases and number of miles traveled increases or decreases, the number of collisions can change.

The graph shows collision rates for the ATM and four other comparison I-5 corridors taking vehicle miles traveled into acount. The collision rates follow a similar patter to the graphs of the number of collisions. Collision rates are lower after the start of ATM than the previous year.

The downward trend is a favorable indicator that ATM is helping to reduce collisions, but because of the high variability of collision data, the trend is not statistically significant at this time. Several more years of data need to be collected before a statistically significant conclusion can be derived from collision data.