1. A system, comprising: one or more computing devices and one or more storage devices storing instructions that when executed by the one or more computing devices cause the one or more computing devices to perform operations comprising: receiving an input of a free text query, wherein the free text query is received as an input into a search interface for a general search engine presented on a client computing device; using the free text query for the general search engine to derive terms corresponding to particular travel constraints including terms corresponding to an origin location, a destination location, and a departure date, each having a respective probability of being related to a query for travel information; determining whether a combined probability of the terms being related to a query for travel information exceeds a threshold value; in response to determining that the combined probability of the terms being related to a query for travel information exceeds the threshold value: generating a travel query from the terms derived from the free text query, wherein the generated travel query expressly specifies the derived origin location, destination location, and departure date as travel constraints, and wherein each of the origin location, the destination location, and the departure date travel constraints are expressly identified as such in the travel query; and submitting the travel query to a travel search engine distinct from the general search engine to obtain travel search results based on the travel search query and to present the travel search results based on the travel search query via the client computing device; and in response to the combined probability of the terms being related to a query for travel information not exceeding the threshold value, generating, by the general search engine, search results that satisfy the free text query input into the general search engine without using the travel search engine and presenting via the client computing device the search results that satisfy the free text query input into the general search engine.