Date: 201810
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were [Aircraft X] - our first flight of the day from ZZZ. We taxied to runway XXR for departure; and were eventually given our takeoff clearance. Our V speeds for the aircraft weight; flap configuration; and airport conditions were V1 of 155; Vr of 155; and V2 of 160 + 5kts - fairly typical speeds for [this model aircraft]; particularly when departing a higher altitude airport like ZZZ.As we were accelerating for takeoff; the tower controller issued a takeoff clearance to Aircraft Y behind us on the same runway well prior to our reaching V1.The clearance to Aircraft Y was given in such a way as to attempt to time the actual clearance with our commitment to the takeoff. Every line pilot has heard this type of clearance ('Aircraft Y traffic on the upwind is diverging from you... maintain visual separation with Aircraft X... wind 350 at 7 knots... runway XXR.... cleared for takeoff.')However; the actual words 'Cleared for takeoff' to Aircraft Y came as we were accelerating - a full 22kts below our V1.The risk is obvious. We may have still needed to abort our takeoff. And if we had - including making the required call to the tower - our communication would have likely been blocked by Aircraft Y's readback of their takeoff clearance. So they could be accelerating down our runway as we were aborting on it; without hearing our call.I have witnessed this scenario personally; on my flights; at numerous airports throughout the US. I have had planes cleared for takeoff behind me when we are below V1 at ZZZ (as described); [and at several other major hubs around the US]. I probably average one time per year of hearing a plane behind us receiving their takeoff clearance when we are still on the runway; slightly to significantly below our V1 speed.I think controller complacency is the issue here - along with a great disparity in current 737 performance. [If] we have 737s departing [on the west coast]; for example; on flights to [halfway across the country]. They will rotate very early. Then when we have a flight on the same runway departing full for [destinations on the east coast]; it may use twice the takeoff distance. In the past; controllers may have been able to use their experience to accurately guess when a 737 would commit to the takeoff and rotate. But as the plane has gotten bigger and bigger; flying longer segments; with more flap configuration settings available; they are guessing the commitment point incorrectly more often; compromising the ability to safely discontinue a takeoff.




Synopsis: B737 First Officer reported during their takeoff roll; tower issuing a takeoff clearance to a flight behind them on the same runway well prior to the report's aircraft reaching V1.