1. A system for validating the accuracy of a sensor, the system comprising: one or more computing devices having one or more processors configured to: receive first sensor information from a first sensor configured to detect objects in a vehicle's environment; receive second sensor information from a second sensor configured to detect objects in a vehicle's environment; identify a first object and a first characteristic value of the first object using the first sensor information; identify the first object and a second characteristic value of the first object using the second sensor information, wherein the first characteristic and the second characteristic are of a same type; determine a deviation value based on the first characteristic value with the second characteristic value; and use the deviation value to determine whether the first sensor is accurately detecting the object by comparing the deviation value to a deviation threshold value identifying an acceptable amount of difference between the first and second characteristic values; and when the comparison indicates that the deviation value does not meet the deviation threshold value, determine that the first sensor has had an operational failure.